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JAVA BASICS

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VEERA RAGAVAN

This document contains slides from a lecture on Java basics. It covers: - A brief history of Java and how it works by using bytecode and a Java Virtual Machine for platform independence. - Java's data types including primitives and classes/objects. - How to write a simple Java program, compile and run it, and basic program structure. - Other basics like variables, methods, control structures, and exceptions. The slides provide examples and explanations of core Java concepts to teach students the fundamentals of the language.

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CS 307 Fundamentals of Computer Science Java Basics 1 Topic 2 Java Basics "On the other hand, Java has already been a big win in academic circles, where it has taken the place of Pascal as the preferred tool for teaching the basics of good programming…" -The New Hacker's Dictionary version 4.3.1 www.tuxedo.org/~esr/jargon/html/The-Jargon-Lexicon-framed.html
CS 307 Fundamentals of Computer Science Java Basics 2 Agenda  Brief History of Java and overview of language  Solve a problem to demonstrate Java syntax  Discuss coding issues and style via example  Slides include more details on syntax – may not cover everything in class, but you are expected to know these
CS 307 Fundamentals of Computer Science Java Basics 3 Brief History of Java and Overview of Langauge java.sun.com/features/1998/05/birthday.html
CS 307 Fundamentals of Computer Science Java Basics 4 A brief history of Java – "Java, whose original name was Oak, was developed as a part of the Green project at Sun. It was started in December '90 by Patrick Naughton, Mike Sheridan and James Gosling and was chartered to spend time trying to figure out what would be the "next wave" of computing and how we might catch it. They came to the conclusion that at least one of the waves was going to be the convergence of digitally controlled consumer devices and computers. "  Applets and Applications – "The team returned to work up a Java technology-based clone of Mosaic they named "WebRunner" (after the movie Blade Runner), later to become officially known as the HotJavaTM browser. It was 1994. WebRunner was just a demo, but an impressive one: It brought to life, for the first time, animated, moving objects and dynamic executable content inside a Web browser. That had never been done. [At the TED conference.]"
CS 307 Fundamentals of Computer Science Java Basics 5 How Java Works  Java's platform independence is achieved by the use of the Java Virtual Machine  A Java program consists of one or more files with a .java extension – these are plain old text files  When a Java program is compiled the .java files are fed to a compiler which produces a .class file for each .java file  The .class file contains Java bytecode.  Bytecode is like machine language, but it is intended for the Java Virtual Machine not a specific chip such as a Pentium or PowerPC chip
CS 307 Fundamentals of Computer Science Java Basics 6 More on How Java Works  To run a Java program the bytecode in a .class file is fed to an interpreter which converts the byte code to machine code for a specific chip (IA-32, PowerPC)  Some people refer to the interpreter as "The Java Virtual Machine" (JVM)  The interpreter is platform specific because it takes the platform independent bytecode and produces machine language instructions for a particular chip  So a Java program could be run an any type of computer that has a JVM written for it. – PC, Mac, Unix, Linux, BeaOS, Sparc
CS 307 Fundamentals of Computer Science Java Basics 7 A Picture is Worth… The Interpreter's are sometimes referred to as the Java Virtual Machines The output of the compiler is .class file
CS 307 Fundamentals of Computer Science Java Basics 8 So What!  The platform independence of Java may be a huge marketing tool, but is actually of little use to people learning Object Oriented Programming and Abstract Data Types  What is of use is the simplicity of the Java syntax and programming concepts  Java is a "pure" Object Oriented Language – encapsulation, inheritance, and polymorphism – all code must be contained in a class – no free functions (functions that do not belong to some class) like C++, although someone who wants to write messy Java code certainly can – Is OO the best programming paradigm?
CS 307 Fundamentals of Computer Science Java Basics 9 HelloWorld.java /** * A simple program */ public class HelloWorld { public static void main(String[] args) { System.out.println("HELLO CS307!"); } }
CS 307 Fundamentals of Computer Science Java Basics 10 More on Java Programs  All code part of some class public class Foo { //start of class Foo /*all code in here!*/ } // end of class Foo  The code for class Foo will be in a file named Foo.java – just a text file with the .java extension – a class is a programmer defined data type  A complete program will normally consist of many different classes and thus many different files
CS 307 Fundamentals of Computer Science Java Basics 11 Attendance Question 1 What does 6,967 * 7,793 equal? A. 10,000 B. 23,756,201 C. 54,293,831 D. 2,147,483,647 E. - 2,147,483,648
CS 307 Fundamentals of Computer Science Java Basics 12 Attendance Question 2 How many factors does 54,161,329 have? A. 2 B. 3 C. 4 D. 6 E. more than 6 Bonus question. What are they?
CS 307 Fundamentals of Computer Science Java Basics 13 Example Problem  Determine if a given integer is prime – problem definition – really naïve algorithm – implementation – testing – a small improvement – another improvement – yet another improvement – always another way ... – what about really big numbers? (Discover AKS Primality Testing)
CS 307 Fundamentals of Computer Science Java Basics 14 Error Types  Syntax error / Compile errors – caught at compile time. – compiler did not understand or compiler does not allow  Runtime error – something “Bad” happens at runtime. Java breaks these into Errors and Exceptions  Logic Error – program compiles and runs, but does not do what you intended or want
CS 307 Fundamentals of Computer Science Java Basics 15 Java Language Review of Basic Features
CS 307 Fundamentals of Computer Science Java Basics 16 Basic Features  Data Types – primitives – classes / objects  Expressions and operators  Control Structures  Arrays  Methods  Programming for correctness – pre and post conditions – assertions
CS 307 Fundamentals of Computer Science Java Basics 17 Java Data Types
CS 307 Fundamentals of Computer Science Java Basics 18 Identifiers in Java  letters, digits, _, and $ (don't use $. Can confuse the runtime system)  start with letter, _, or $  by convention: 1. start with a letter 2. variables and method names, lowercase with internal words capitalized e.g. honkingBigVariableName 3. constants all caps with _ between internal words e.g. ANOTHER_HONKING_BIG_INDENTIFIER 4. classes start with capital letter, internal words capitalized, all other lowercase e.g HonkingLongClassName  Why? To differentiate identifiers that refer to classes from those that refer to variables
CS 307 Fundamentals of Computer Science Java Basics 19 Data Types  Primitive Data Types – byte short int long float double boolean char – stick with int for integers, double for real numbers  Classes and Objects – pre defined or user defined data types consisting of constructors, methods, and fields (constants and fields (variables) which may be primitives or objects.) //dataType identifier; int x; int y = 10; int z, zz; double a = 12.0; boolean done = false, prime = true; char mi = 'D';
CS 307 Fundamentals of Computer Science Java Basics 20 Java Primitive Data Types Data Type Characteristics Range byte 8 bit signed integer -128 to 127 short 16 bit signed integer -32768 to 32767 int 32 bit signed integer -2,147,483,648 to 2,147,483,647 long 64 bit signed integer -9,223,372,036,854,775,808 to- 9,223,372,036,854,775,807 float 32 bit floating point number + 1.4E-45 to + 3.4028235E+38 double 64 bit floating point number + 4.9E-324 to + 1.7976931348623157E+308 boolean true or false NA, note Java booleans cannot be converted to or from other types char 16 bit, Unicode Unicode character, u0000 to uFFFF Can mix with integer types
CS 307 Fundamentals of Computer Science Java Basics 21 What are Classes and Objects?  Class is synonymous with data type  Object is like a variable – The data type of the Object is some Class – referred to as an instance of a Class  Classes contain: – the implementation details of the data type – and the interface for programmers who just want to use the data type  Objects are complex variables – usually multiple pieces of internal data – various behaviors carried out via methods
CS 307 Fundamentals of Computer Science Java Basics 22 Creating and Using Objects  Declaration - DataType identifier Rectangle r1;  Creation - new operator and specified constructor r1 = new Rectangle(); Rectangle r2 = new Rectangle();  Behavior - via the dot operator r2.setSize(10, 20); String s2 = r2.toString();  Refer to documentation for available behaviors (methods)
CS 307 Fundamentals of Computer Science Java Basics 23 Built in Classes  Java has a large built in library of classes with lots of useful methods  Ones you should become familiar with quickly  String  Math  Integer, Character, Double  System  Arrays  Scanner  File  Object  Random  Look at the Java API page
CS 307 Fundamentals of Computer Science Java Basics 24 import  import is a reserved word  packages and classes can be imported to another class  does not actually import the code (unlike the C++ include preprocessor command)  statement outside the class block import java.util.ArrayList; import java.awt.Rectangle; public class Foo{ // code for class Foo }
CS 307 Fundamentals of Computer Science Java Basics 25 More on import  can include a whole package – import java.util.*;  or list a given class – import java.util.Random;  instructs the compiler to look in the package for types it can't find defined locally  the java.lang.* package is automatically imported to all other classes.  Not required to import classes that are part of the same project in Eclipse
CS 307 Fundamentals of Computer Science Java Basics 26 The String Class  String is a standard Java class – a whole host of behaviors via methods  also special (because it used so much) – String literals exist (no other class has literals) String name = "Mike D."; – String concatenation through the + operator String firstName = "Mike"; String lastName = "Scott"; String wholeName = firstName + lastName; – Any primitive or object on other side of + operator from a String automatically converted to String
CS 307 Fundamentals of Computer Science Java Basics 27 Standard Output  To print to standard output use System.out.print( expression ); // no newline System.out.println( expression ); // newline System.out.println( ); // just a newline common idiom is to build up expression to be printed out System.out.println( "x is: " + x + " y is: " + y );
CS 307 Fundamentals of Computer Science Java Basics 28 Constants  Literal constants - "the way you specify values that are not computed and recomputed, but remain, well, constant for the life of a program." – true, false, null, 'c', "C++", 12, -12, 12.12345  Named constants – use the keyword final to specify a constant – scope may be local to a method or to a class  By convention any numerical constant besides -1, 0, 1, or 2 requires a named constant final int NUM_SECTIONS = 3;
CS 307 Fundamentals of Computer Science Java Basics 29 Expressions and Operators
CS 307 Fundamentals of Computer Science Java Basics 30 Operators  Basic Assignment: =  Arithmetic Operators: +, -, *, /, %(remainder) – integer, floating point, and mixed arithmetic and expressions  Assignment Operators: +=, -=, *=, /=, %=  increment and decrement operators: ++, -- – prefix and postfix. – avoid use inside expressions. int x = 3; x++;
CS 307 Fundamentals of Computer Science Java Basics 31 Expressions  Expressions are evaluated based on the precedence of operators  Java will automatically convert numerical primitive data types but results are sometimes surprising – take care when mixing integer and floating point numbers in expressions  The meaning of an operator is determined by its operands / is it integer division or floating point division?
CS 307 Fundamentals of Computer Science Java Basics 32 Casting  Casting is the temporary conversion of a variable from its original data type to some other data type. – Like being cast for a part in a play or movie  With primitive data types if a cast is necessary from a less inclusive data type to a more inclusive data type it is done automatically. int x = 5; double a = 3.5; double b = a * x + a / x; double c = x / 2;  if a cast is necessary from a more inclusive to a less inclusive data type the class must be done explicitly by the programmer – failure to do so results in a compile error. double a = 3.5, b = 2.7; int y = (int) a / (int) b; y = (int)( a / b ); y = (int) a / b; //syntax error
CS 307 Fundamentals of Computer Science Java Basics 33 Primitive Casting double float long int short, char byte Outer ring is most inclusive data type. Inner ring is least inclusive. In expressions variables and sub expressions of less inclusive data types are automatically cast to more inclusive. If trying to place expression that is more inclusive into variable that is less inclusive, explicit cast must be performed. From MORE to LESS
CS 307 Fundamentals of Computer Science Java Basics 34 Java Control Structures
CS 307 Fundamentals of Computer Science Java Basics 35 Control Structures  linear flow of control – statements executed in consecutive order  Decision making with if - else statements if(boolean-expression) statement; if(boolean-expression) { statement1; statement2; statement3; } A single statement could be replaced by a statement block, braces with 0 or more statements inside
CS 307 Fundamentals of Computer Science Java Basics 36 Boolean Expressions  boolean expressions evaluate to true or false  Relational Operators: >, >=, <, <=, ==, !=  Logical Operators: &&, ||, ! – && and || cause short circuit evaluation – if the first part of p && q is false then q is not evaluated – if the first part of p || q is true then q is not evaluated //example if( x <= X_LIMIT && y <= Y_LIMIT) //do something
CS 307 Fundamentals of Computer Science Java Basics 37 More Flow of Control if-else: if(boolean-expression) statement1; else statement2;  multiway selection: if(boolean-expression1) statement1; else if(boolean-expression2) statement2; else statement3;  individual statements could be replaced by a statement block, a set of braces with 0 or more statements  Java also has the switch statement, but not part of our subset
CS 307 Fundamentals of Computer Science Java Basics 38 for Loops  for loops for(init-expr;boolean-expr;incr-expr) statement;  init-expr and incr-expr can be more zero or more expressions or statements separated by commas  statement could be replaced by a statement block execute init-expr evaluate boolean-expr false skip to 1st statement after body of loop true execute body of loop execute incr-expr
CS 307 Fundamentals of Computer Science Java Basics 39 while loops  while loops while(boolean-expression) statement; //or statement block  do-while loop part of language do statement; while(boolean-expression);  Again, could use a statement block  break, continue, and labeled breaks – referred to in the Java tutorial as branching statements – keywords to override normal loop logic – use them judiciously (which means not much)
CS 307 Fundamentals of Computer Science Java Basics 40 Attendance Question 3 True or false: Strings are a primitive data type in Java. A. TRUE B. FALSE
CS 307 Fundamentals of Computer Science Java Basics 41 Attendance Question 4 What is output by the following Java code? int x = 3; double a = x / 2 + 3.5; System.out.println(a); A. a B. 5 C. 4.5 D. 4 E. 5.0
CS 307 Fundamentals of Computer Science Java Basics 42 Arrays
CS 307 Fundamentals of Computer Science Java Basics 43 Arrays in Java  "Should array indices start at 0 or 1? My compromise of 0.5 was rejected without, I thought, proper consideration. " – S. Kelly-Bootle  Java has built in arrays. a.k.a. native arrays  arrays hold elements of the same type – primitive data types or classes – space for array must be dynamically allocated with new operator. (Size is any integer expression. Due to dynamic allocation does not have to be constant.) public void arrayExamples() { int[] intList = new int[10]; for(int i = 0; i < intList.length; i++) { assert 0 >= i && i < intList.length; intList[i] = i * i * i; } intList[3] = intList[4] * intList[3]; }
CS 307 Fundamentals of Computer Science Java Basics 44 Array Details  all arrays must be dynamically allocated  arrays have a public, final field called length – built in size field, no separate variable needed – don't confuse length (capacity) with elements in use  elements start with an index of zero, last index is length - 1  trying to access a non existent element results in an ArrayIndexOutOfBoundsException (AIOBE)
CS 307 Fundamentals of Computer Science Java Basics 45 Array Initialization  Array variables are object variables  They hold the memory address of an array object  The array must be dynamically allocated  All values in the array are initialized (0, 0.0, char 0, false, or null)  Arrays may be initialized with an initializer list: int[] intList = {2, 3, 5, 7, 11, 13}; double[] dList = {12.12, 0.12, 45.3}; String[] sList = {"Olivia", "Kelly", "Isabelle"};
CS 307 Fundamentals of Computer Science Java Basics 46 Arrays of objects  A native array of objects is actually a native array of object variables – all object variables in Java are really what? – Pointers! public void objectArrayExamples() { Rectangle[] rectList = new Rectangle[10]; // How many Rectangle objects exist? rectList[5].setSize(5,10); //uh oh! for(int i = 0; i < rectList.length; i++) { rectList[i] = new Rectangle(); } rectList[3].setSize(100,200); }
CS 307 Fundamentals of Computer Science Java Basics 47 Array Utilities  In the Arrays class, static methods  binarySearch, equals, fill, and sort methods for arrays of all primitive types (except boolean) and arrays of Objects – overloaded versions of these methods for various data types  In the System class there is an arraycopy method to copy elements from a specified part of one array to another – can be used for arrays of primitives or arrays of objects
CS 307 Fundamentals of Computer Science Java Basics 48 The arraycopy method  static voidarraycopy(Object src, int srcPos, Object dest, int destPos, int length) Copies an array from the specified source array, beginning at the specified position, to the specified position of the destination array. int[] list = new int[10]; // code to fill list // list needs to be resized int[] temp = new int[list.length * 2]; System.arraycopy(list, 0, temp, 0, list.length); list = temp;
CS 307 Fundamentals of Computer Science 2D Arrays 49 2D Arrays in Java  Arrays with multiple dimensions may be declared and used int[][] mat = new int[3][4];  the number of pairs of square brackets indicates the dimension of the array.  by convention, in a 2D array the first number indicates the row and the second the column  Java multiple dimensional arrays are handles differently than in many other programming languages.
CS 307 Fundamentals of Computer Science 2D Arrays 50 Two Dimensional Arrays 0 1 2 3 column 0 1 2 row 0 0 0 0 0 0 0 0 0 0 0 0 This is our abstract picture of the 2D array and treating it this way is fine. mat[2][1] = 12;
CS 307 Fundamentals of Computer Science 2D Arrays 51 The Real Picture mat 0 1 2 0 1 2 3 0 1 2 3 0 1 2 3 mat holds the memory address of an array with 3 elements. Each element holds the memory address of an array of 4 ints 0 0 0 0 0 0 0 0 0 0 0 0
CS 307 Fundamentals of Computer Science 2D Arrays 52 Arrays of Multiple Dimension  because multiple dimensional arrays are treated as arrays of arrays of arrays…… multiple dimensional arrays can be ragged – each row does not have to have the same number of columns – each row array has its own length field int[][] raggedMat = new int[5][]; for(int i = 0; i < raggedMat.length; i++) raggedMat[i] = new int[i + 1];
CS 307 Fundamentals of Computer Science 2D Arrays 53 Ragged Arrays  Ragged arrays are sometime useful, but normally we deal with rectangular matrices – each row has the same number of columns as every other row – use this a lot as precondition to methods that work on matrices  working on matrices normally requires nested loops – why is this so hard?
CS 307 Fundamentals of Computer Science Java Basics 54 Enhanced for loop  New in Java 5.0  a.k.a. the for-each loop  useful short hand for accessing all elements in an array (or other types of structures) if no need to alter values  alternative for iterating through a set of values for(Type loop-variable : set-expression) statement  logic error (not a syntax error) if try to modify an element in array via enhanced for loop
CS 307 Fundamentals of Computer Science Java Basics 55 Enhanced for loop public static int sumListEnhanced(int[] list) { int total = 0; for(int val : list) { total += val; System.out.println( val ); } return total; } public static int sumListOld(int[] list) { int total = 0; for(int i = 0; i < list.length; i++) { total += list[i]; System.out.println( list[i] ); } return total; }
CS 307 Fundamentals of Computer Science Java Basics 56 Attendance Question 5 What is output by the code to the right when method d1 is called? A. 322 B. 323 C. 363 D. 366 E. 399 public void d2(int x){ x *= 2; System.out.print(x); } public void d1(){ int x = 3; System.out.print(x); d2(x); System.out.print(x); }
CS 307 Fundamentals of Computer Science Java Basics 57 Attendance Question 6 What is output by the code to the right? A. Output will vary from one run of program to next B. 00 C. 363 D. 7 then a runtime error E. No output due to syntax error int[] list = {5, 1, 7, 3}; System.out.print( list[2] ); System.out.print( list[4] );
CS 307 Fundamentals of Computer Science Java Basics 58 Methods
CS 307 Fundamentals of Computer Science Java Basics 59 Methods methods are analogous to procedures and functions in other languages – local variables, parameters, instance variables – must be comfortable with variable scope: where is a variable defined?  methods are the means by which objects are manipulated (objects state is changed) - much more on this later  method header consists of – access modifier(public, package, protected, private) – static keyword (optional, class method) – return type (void or any data type, primitive or class) – method name – parameter signature
CS 307 Fundamentals of Computer Science Java Basics 60 More on Methods  local variables can be declared within methods. – Their scope is from the point of declaration until the end of the methods, unless declared inside a smaller block like a loop  methods contain statements  methods can call other methods – in the same class: foo(); – methods to perform an operation on an object that is in scope within the method: obj.foo(); – static methods in other classes: double x = Math.sqrt(1000);
CS 307 Fundamentals of Computer Science Java Basics 61 static methods  the main method is where a stand alone Java program normally begins execution  common compile error, trying to call a non static method from a static one public class StaticExample { public static void main(String[] args) { //starting point of execution System.out.println("In main method"); method1(); method2(); //compile error; } public static void method1() { System.out.println( "method 1"); } public void method2() { System.out.println( "method 2"); } }
CS 307 Fundamentals of Computer Science Java Basics 62 Method Overloading and Return  a class may have multiple methods with the same name as long as the parameter signature is unique – may not overload on return type  methods in different classes may have same name and signature – this is a type of polymorphism, not method overloading  if a method has a return value other than void it must have a return statement with a variable or expression of the proper type  multiple return statements allowed, the first one encountered is executed and method ends – style considerations
CS 307 Fundamentals of Computer Science Java Basics 63 Method Parameters  a method may have any number of parameters  each parameter listed separately  no VAR (Pascal), &, or const & (C++)  final can be applied, but special meaning  all parameters are pass by value  Implications of pass by value???
CS 307 Fundamentals of Computer Science Java Basics 64 Value Parameters vs. Reference Parameters  A value parameter makes a copy of the argument it is sent. – Changes to parameter do not affect the argument.  A reference parameter is just another name for the argument it is sent. – changes to the parameter are really changes to the argument and thus are permanent
CS 307 Fundamentals of Computer Science Java Basics 65 Value vs. Reference // value void add10(int x) { x += 10; } void calls() { int y = 12; add10(y); // y = ? } // C++, reference void add10(int& x) { x += 10; } void calls() { int y = 12; add10(y); // y = ? } 12 y 12 x 12 y x
CS 307 Fundamentals of Computer Science Java Basics 66 Programming for Correctness
CS 307 Fundamentals of Computer Science Java Basics 67 Creating Correct Programs  methods should include pre conditions and post conditions  Preconditions are things that must be true before a method is called  Postconditions are things that will be true after a method is complete if the preconditions were met  it is the responsibility of the caller of a method to ensure the preconditions are met – the class must provide a way of ensuring the precondition is true – the preconditions must be stated in terms of the interface, not the implementation  it is the responsibility of the class (supplier, server) to ensure the postconditions are met
CS 307 Fundamentals of Computer Science Java Basics 68 Programming by Contract  preconditions and postconditions create a contract between the client (class or object user) and a supplier (the class or object itself) – example of a contract between you and me for a test   Obligations Benefits Client (Student) (Must ensure preconditions) Be at test on time, bring pencil and eraser, write legibly, answer questions in space provided (May benefit from postcondition) Receive fair and accurate evaluation of test to help formulate progress in course Supplier (Mike) (Must ensure postcondition) Fairly and accurately grade test based on universal guidelines applied to all tests (May assume preconditions) No need to grade test or questions that are illegible, on wrong part of exam, or give makeup exams for unexcused absences
CS 307 Fundamentals of Computer Science Java Basics 69 Thinking about pre and postconditions  pre and postconditions are part of design  coming up with pre and postconditions at the time of implementation is too late  the pre and post conditions drive the implementation and so must exist before the implementation can start – The sooner you start to code, the longer your program will take. -Roy Carlson, U Wisconsin  You must spend time on design
CS 307 Fundamentals of Computer Science Java Basics 70 Precondition Example /** * Find all indices in <tt>source</tt> that are the start of a complete * match of <tt>target</tt>. * @param source != null, source.length() > 0 * @param target != null, target.length() > 0 * @return an ArrayList that contains the indices in source that are the * start of a complete match of target. The indices are stored in * ascending order in the ArrayList */ public static ArrayList<Integer> matches(String source, String target) { // check preconditions assert (source != null) && (source.length() > 0) && (target != null) && (target.length() > 0) : "matches: violation of precondition";
CS 307 Fundamentals of Computer Science Java Basics 71 Creating Correct Programs  Java features has a mechanism to check the correctness of your program called assertions  Assertions are statements that are executed as normal statements if assertion checking is on – you should always have assertion checking on when writing and running your programs  Assertions are boolean expressions that are evaluated when reached. If they evaluate to true the program continues, if they evaluate to false then the program halts  logical statements about the condition or state of your program
CS 307 Fundamentals of Computer Science Java Basics 72 Assertions  Assertions have the form assert boolean expression : what to output if assertion is false  Example if ( (x < 0) || (y < 0) ) { // we know either x or y is < 0 assert x < 0 || y < 0 : x + " " + y; x += y; } else { // we know both x and y are not less than zero assert x >= 0 && y >= 0 : x + " " + y; y += x; }  Use assertion liberally in your code – part of style guide
CS 307 Fundamentals of Computer Science Java Basics 73 Assertions Uncover Errors in Your Logic if ( a < b ) { // we a is less than b assert a < b : a + " " + b; System.out.println(a + " is smaller than " + b); } else { // we know b is less than a assert b < a : a + " " + b; System.out.println(b + " is smaller than " + a); }  Use assertions in code that other programmers are going to use.  In the real world this is the majority of your code!
CS 307 Fundamentals of Computer Science Java Basics 74 javadoc  javadoc is a program that takes the comments in Java source code and creates the html documentation pages  Open up Java source code. (Found in the src.zip file when you download the Java sdk.)  Basic Format /** Summary sentence for method foo. More details. More details. pre: list preconditions post: list postconditions @param x describe what x is @param y describe what y is @return describe what the method returns */ public int foo(int x, double y)  Comments interpreted as html

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JAVA BASICS

  • 1. CS 307 Fundamentals of Computer Science Java Basics 1 Topic 2 Java Basics "On the other hand, Java has already been a big win in academic circles, where it has taken the place of Pascal as the preferred tool for teaching the basics of good programming…" -The New Hacker's Dictionary version 4.3.1 www.tuxedo.org/~esr/jargon/html/The-Jargon-Lexicon-framed.html
  • 2. CS 307 Fundamentals of Computer Science Java Basics 2 Agenda  Brief History of Java and overview of language  Solve a problem to demonstrate Java syntax  Discuss coding issues and style via example  Slides include more details on syntax – may not cover everything in class, but you are expected to know these
  • 3. CS 307 Fundamentals of Computer Science Java Basics 3 Brief History of Java and Overview of Langauge java.sun.com/features/1998/05/birthday.html
  • 4. CS 307 Fundamentals of Computer Science Java Basics 4 A brief history of Java – "Java, whose original name was Oak, was developed as a part of the Green project at Sun. It was started in December '90 by Patrick Naughton, Mike Sheridan and James Gosling and was chartered to spend time trying to figure out what would be the "next wave" of computing and how we might catch it. They came to the conclusion that at least one of the waves was going to be the convergence of digitally controlled consumer devices and computers. "  Applets and Applications – "The team returned to work up a Java technology-based clone of Mosaic they named "WebRunner" (after the movie Blade Runner), later to become officially known as the HotJavaTM browser. It was 1994. WebRunner was just a demo, but an impressive one: It brought to life, for the first time, animated, moving objects and dynamic executable content inside a Web browser. That had never been done. [At the TED conference.]"
  • 5. CS 307 Fundamentals of Computer Science Java Basics 5 How Java Works  Java's platform independence is achieved by the use of the Java Virtual Machine  A Java program consists of one or more files with a .java extension – these are plain old text files  When a Java program is compiled the .java files are fed to a compiler which produces a .class file for each .java file  The .class file contains Java bytecode.  Bytecode is like machine language, but it is intended for the Java Virtual Machine not a specific chip such as a Pentium or PowerPC chip
  • 6. CS 307 Fundamentals of Computer Science Java Basics 6 More on How Java Works  To run a Java program the bytecode in a .class file is fed to an interpreter which converts the byte code to machine code for a specific chip (IA-32, PowerPC)  Some people refer to the interpreter as "The Java Virtual Machine" (JVM)  The interpreter is platform specific because it takes the platform independent bytecode and produces machine language instructions for a particular chip  So a Java program could be run an any type of computer that has a JVM written for it. – PC, Mac, Unix, Linux, BeaOS, Sparc
  • 7. CS 307 Fundamentals of Computer Science Java Basics 7 A Picture is Worth… The Interpreter's are sometimes referred to as the Java Virtual Machines The output of the compiler is .class file
  • 8. CS 307 Fundamentals of Computer Science Java Basics 8 So What!  The platform independence of Java may be a huge marketing tool, but is actually of little use to people learning Object Oriented Programming and Abstract Data Types  What is of use is the simplicity of the Java syntax and programming concepts  Java is a "pure" Object Oriented Language – encapsulation, inheritance, and polymorphism – all code must be contained in a class – no free functions (functions that do not belong to some class) like C++, although someone who wants to write messy Java code certainly can – Is OO the best programming paradigm?
  • 9. CS 307 Fundamentals of Computer Science Java Basics 9 HelloWorld.java /** * A simple program */ public class HelloWorld { public static void main(String[] args) { System.out.println("HELLO CS307!"); } }
  • 10. CS 307 Fundamentals of Computer Science Java Basics 10 More on Java Programs  All code part of some class public class Foo { //start of class Foo /*all code in here!*/ } // end of class Foo  The code for class Foo will be in a file named Foo.java – just a text file with the .java extension – a class is a programmer defined data type  A complete program will normally consist of many different classes and thus many different files
  • 11. CS 307 Fundamentals of Computer Science Java Basics 11 Attendance Question 1 What does 6,967 * 7,793 equal? A. 10,000 B. 23,756,201 C. 54,293,831 D. 2,147,483,647 E. - 2,147,483,648
  • 12. CS 307 Fundamentals of Computer Science Java Basics 12 Attendance Question 2 How many factors does 54,161,329 have? A. 2 B. 3 C. 4 D. 6 E. more than 6 Bonus question. What are they?
  • 13. CS 307 Fundamentals of Computer Science Java Basics 13 Example Problem  Determine if a given integer is prime – problem definition – really naïve algorithm – implementation – testing – a small improvement – another improvement – yet another improvement – always another way ... – what about really big numbers? (Discover AKS Primality Testing)
  • 14. CS 307 Fundamentals of Computer Science Java Basics 14 Error Types  Syntax error / Compile errors – caught at compile time. – compiler did not understand or compiler does not allow  Runtime error – something “Bad” happens at runtime. Java breaks these into Errors and Exceptions  Logic Error – program compiles and runs, but does not do what you intended or want
  • 15. CS 307 Fundamentals of Computer Science Java Basics 15 Java Language Review of Basic Features
  • 16. CS 307 Fundamentals of Computer Science Java Basics 16 Basic Features  Data Types – primitives – classes / objects  Expressions and operators  Control Structures  Arrays  Methods  Programming for correctness – pre and post conditions – assertions
  • 17. CS 307 Fundamentals of Computer Science Java Basics 17 Java Data Types
  • 18. CS 307 Fundamentals of Computer Science Java Basics 18 Identifiers in Java  letters, digits, _, and $ (don't use $. Can confuse the runtime system)  start with letter, _, or $  by convention: 1. start with a letter 2. variables and method names, lowercase with internal words capitalized e.g. honkingBigVariableName 3. constants all caps with _ between internal words e.g. ANOTHER_HONKING_BIG_INDENTIFIER 4. classes start with capital letter, internal words capitalized, all other lowercase e.g HonkingLongClassName  Why? To differentiate identifiers that refer to classes from those that refer to variables
  • 19. CS 307 Fundamentals of Computer Science Java Basics 19 Data Types  Primitive Data Types – byte short int long float double boolean char – stick with int for integers, double for real numbers  Classes and Objects – pre defined or user defined data types consisting of constructors, methods, and fields (constants and fields (variables) which may be primitives or objects.) //dataType identifier; int x; int y = 10; int z, zz; double a = 12.0; boolean done = false, prime = true; char mi = 'D';
  • 20. CS 307 Fundamentals of Computer Science Java Basics 20 Java Primitive Data Types Data Type Characteristics Range byte 8 bit signed integer -128 to 127 short 16 bit signed integer -32768 to 32767 int 32 bit signed integer -2,147,483,648 to 2,147,483,647 long 64 bit signed integer -9,223,372,036,854,775,808 to- 9,223,372,036,854,775,807 float 32 bit floating point number + 1.4E-45 to + 3.4028235E+38 double 64 bit floating point number + 4.9E-324 to + 1.7976931348623157E+308 boolean true or false NA, note Java booleans cannot be converted to or from other types char 16 bit, Unicode Unicode character, u0000 to uFFFF Can mix with integer types
  • 21. CS 307 Fundamentals of Computer Science Java Basics 21 What are Classes and Objects?  Class is synonymous with data type  Object is like a variable – The data type of the Object is some Class – referred to as an instance of a Class  Classes contain: – the implementation details of the data type – and the interface for programmers who just want to use the data type  Objects are complex variables – usually multiple pieces of internal data – various behaviors carried out via methods
  • 22. CS 307 Fundamentals of Computer Science Java Basics 22 Creating and Using Objects  Declaration - DataType identifier Rectangle r1;  Creation - new operator and specified constructor r1 = new Rectangle(); Rectangle r2 = new Rectangle();  Behavior - via the dot operator r2.setSize(10, 20); String s2 = r2.toString();  Refer to documentation for available behaviors (methods)
  • 23. CS 307 Fundamentals of Computer Science Java Basics 23 Built in Classes  Java has a large built in library of classes with lots of useful methods  Ones you should become familiar with quickly  String  Math  Integer, Character, Double  System  Arrays  Scanner  File  Object  Random  Look at the Java API page
  • 24. CS 307 Fundamentals of Computer Science Java Basics 24 import  import is a reserved word  packages and classes can be imported to another class  does not actually import the code (unlike the C++ include preprocessor command)  statement outside the class block import java.util.ArrayList; import java.awt.Rectangle; public class Foo{ // code for class Foo }
  • 25. CS 307 Fundamentals of Computer Science Java Basics 25 More on import  can include a whole package – import java.util.*;  or list a given class – import java.util.Random;  instructs the compiler to look in the package for types it can't find defined locally  the java.lang.* package is automatically imported to all other classes.  Not required to import classes that are part of the same project in Eclipse
  • 26. CS 307 Fundamentals of Computer Science Java Basics 26 The String Class  String is a standard Java class – a whole host of behaviors via methods  also special (because it used so much) – String literals exist (no other class has literals) String name = "Mike D."; – String concatenation through the + operator String firstName = "Mike"; String lastName = "Scott"; String wholeName = firstName + lastName; – Any primitive or object on other side of + operator from a String automatically converted to String
  • 27. CS 307 Fundamentals of Computer Science Java Basics 27 Standard Output  To print to standard output use System.out.print( expression ); // no newline System.out.println( expression ); // newline System.out.println( ); // just a newline common idiom is to build up expression to be printed out System.out.println( "x is: " + x + " y is: " + y );
  • 28. CS 307 Fundamentals of Computer Science Java Basics 28 Constants  Literal constants - "the way you specify values that are not computed and recomputed, but remain, well, constant for the life of a program." – true, false, null, 'c', "C++", 12, -12, 12.12345  Named constants – use the keyword final to specify a constant – scope may be local to a method or to a class  By convention any numerical constant besides -1, 0, 1, or 2 requires a named constant final int NUM_SECTIONS = 3;
  • 29. CS 307 Fundamentals of Computer Science Java Basics 29 Expressions and Operators
  • 30. CS 307 Fundamentals of Computer Science Java Basics 30 Operators  Basic Assignment: =  Arithmetic Operators: +, -, *, /, %(remainder) – integer, floating point, and mixed arithmetic and expressions  Assignment Operators: +=, -=, *=, /=, %=  increment and decrement operators: ++, -- – prefix and postfix. – avoid use inside expressions. int x = 3; x++;
  • 31. CS 307 Fundamentals of Computer Science Java Basics 31 Expressions  Expressions are evaluated based on the precedence of operators  Java will automatically convert numerical primitive data types but results are sometimes surprising – take care when mixing integer and floating point numbers in expressions  The meaning of an operator is determined by its operands / is it integer division or floating point division?
  • 32. CS 307 Fundamentals of Computer Science Java Basics 32 Casting  Casting is the temporary conversion of a variable from its original data type to some other data type. – Like being cast for a part in a play or movie  With primitive data types if a cast is necessary from a less inclusive data type to a more inclusive data type it is done automatically. int x = 5; double a = 3.5; double b = a * x + a / x; double c = x / 2;  if a cast is necessary from a more inclusive to a less inclusive data type the class must be done explicitly by the programmer – failure to do so results in a compile error. double a = 3.5, b = 2.7; int y = (int) a / (int) b; y = (int)( a / b ); y = (int) a / b; //syntax error
  • 33. CS 307 Fundamentals of Computer Science Java Basics 33 Primitive Casting double float long int short, char byte Outer ring is most inclusive data type. Inner ring is least inclusive. In expressions variables and sub expressions of less inclusive data types are automatically cast to more inclusive. If trying to place expression that is more inclusive into variable that is less inclusive, explicit cast must be performed. From MORE to LESS
  • 34. CS 307 Fundamentals of Computer Science Java Basics 34 Java Control Structures
  • 35. CS 307 Fundamentals of Computer Science Java Basics 35 Control Structures  linear flow of control – statements executed in consecutive order  Decision making with if - else statements if(boolean-expression) statement; if(boolean-expression) { statement1; statement2; statement3; } A single statement could be replaced by a statement block, braces with 0 or more statements inside
  • 36. CS 307 Fundamentals of Computer Science Java Basics 36 Boolean Expressions  boolean expressions evaluate to true or false  Relational Operators: >, >=, <, <=, ==, !=  Logical Operators: &&, ||, ! – && and || cause short circuit evaluation – if the first part of p && q is false then q is not evaluated – if the first part of p || q is true then q is not evaluated //example if( x <= X_LIMIT && y <= Y_LIMIT) //do something
  • 37. CS 307 Fundamentals of Computer Science Java Basics 37 More Flow of Control if-else: if(boolean-expression) statement1; else statement2;  multiway selection: if(boolean-expression1) statement1; else if(boolean-expression2) statement2; else statement3;  individual statements could be replaced by a statement block, a set of braces with 0 or more statements  Java also has the switch statement, but not part of our subset
  • 38. CS 307 Fundamentals of Computer Science Java Basics 38 for Loops  for loops for(init-expr;boolean-expr;incr-expr) statement;  init-expr and incr-expr can be more zero or more expressions or statements separated by commas  statement could be replaced by a statement block execute init-expr evaluate boolean-expr false skip to 1st statement after body of loop true execute body of loop execute incr-expr
  • 39. CS 307 Fundamentals of Computer Science Java Basics 39 while loops  while loops while(boolean-expression) statement; //or statement block  do-while loop part of language do statement; while(boolean-expression);  Again, could use a statement block  break, continue, and labeled breaks – referred to in the Java tutorial as branching statements – keywords to override normal loop logic – use them judiciously (which means not much)
  • 40. CS 307 Fundamentals of Computer Science Java Basics 40 Attendance Question 3 True or false: Strings are a primitive data type in Java. A. TRUE B. FALSE
  • 41. CS 307 Fundamentals of Computer Science Java Basics 41 Attendance Question 4 What is output by the following Java code? int x = 3; double a = x / 2 + 3.5; System.out.println(a); A. a B. 5 C. 4.5 D. 4 E. 5.0
  • 42. CS 307 Fundamentals of Computer Science Java Basics 42 Arrays
  • 43. CS 307 Fundamentals of Computer Science Java Basics 43 Arrays in Java  "Should array indices start at 0 or 1? My compromise of 0.5 was rejected without, I thought, proper consideration. " – S. Kelly-Bootle  Java has built in arrays. a.k.a. native arrays  arrays hold elements of the same type – primitive data types or classes – space for array must be dynamically allocated with new operator. (Size is any integer expression. Due to dynamic allocation does not have to be constant.) public void arrayExamples() { int[] intList = new int[10]; for(int i = 0; i < intList.length; i++) { assert 0 >= i && i < intList.length; intList[i] = i * i * i; } intList[3] = intList[4] * intList[3]; }
  • 44. CS 307 Fundamentals of Computer Science Java Basics 44 Array Details  all arrays must be dynamically allocated  arrays have a public, final field called length – built in size field, no separate variable needed – don't confuse length (capacity) with elements in use  elements start with an index of zero, last index is length - 1  trying to access a non existent element results in an ArrayIndexOutOfBoundsException (AIOBE)
  • 45. CS 307 Fundamentals of Computer Science Java Basics 45 Array Initialization  Array variables are object variables  They hold the memory address of an array object  The array must be dynamically allocated  All values in the array are initialized (0, 0.0, char 0, false, or null)  Arrays may be initialized with an initializer list: int[] intList = {2, 3, 5, 7, 11, 13}; double[] dList = {12.12, 0.12, 45.3}; String[] sList = {"Olivia", "Kelly", "Isabelle"};
  • 46. CS 307 Fundamentals of Computer Science Java Basics 46 Arrays of objects  A native array of objects is actually a native array of object variables – all object variables in Java are really what? – Pointers! public void objectArrayExamples() { Rectangle[] rectList = new Rectangle[10]; // How many Rectangle objects exist? rectList[5].setSize(5,10); //uh oh! for(int i = 0; i < rectList.length; i++) { rectList[i] = new Rectangle(); } rectList[3].setSize(100,200); }
  • 47. CS 307 Fundamentals of Computer Science Java Basics 47 Array Utilities  In the Arrays class, static methods  binarySearch, equals, fill, and sort methods for arrays of all primitive types (except boolean) and arrays of Objects – overloaded versions of these methods for various data types  In the System class there is an arraycopy method to copy elements from a specified part of one array to another – can be used for arrays of primitives or arrays of objects
  • 48. CS 307 Fundamentals of Computer Science Java Basics 48 The arraycopy method  static voidarraycopy(Object src, int srcPos, Object dest, int destPos, int length) Copies an array from the specified source array, beginning at the specified position, to the specified position of the destination array. int[] list = new int[10]; // code to fill list // list needs to be resized int[] temp = new int[list.length * 2]; System.arraycopy(list, 0, temp, 0, list.length); list = temp;
  • 49. CS 307 Fundamentals of Computer Science 2D Arrays 49 2D Arrays in Java  Arrays with multiple dimensions may be declared and used int[][] mat = new int[3][4];  the number of pairs of square brackets indicates the dimension of the array.  by convention, in a 2D array the first number indicates the row and the second the column  Java multiple dimensional arrays are handles differently than in many other programming languages.
  • 50. CS 307 Fundamentals of Computer Science 2D Arrays 50 Two Dimensional Arrays 0 1 2 3 column 0 1 2 row 0 0 0 0 0 0 0 0 0 0 0 0 This is our abstract picture of the 2D array and treating it this way is fine. mat[2][1] = 12;
  • 51. CS 307 Fundamentals of Computer Science 2D Arrays 51 The Real Picture mat 0 1 2 0 1 2 3 0 1 2 3 0 1 2 3 mat holds the memory address of an array with 3 elements. Each element holds the memory address of an array of 4 ints 0 0 0 0 0 0 0 0 0 0 0 0
  • 52. CS 307 Fundamentals of Computer Science 2D Arrays 52 Arrays of Multiple Dimension  because multiple dimensional arrays are treated as arrays of arrays of arrays…… multiple dimensional arrays can be ragged – each row does not have to have the same number of columns – each row array has its own length field int[][] raggedMat = new int[5][]; for(int i = 0; i < raggedMat.length; i++) raggedMat[i] = new int[i + 1];
  • 53. CS 307 Fundamentals of Computer Science 2D Arrays 53 Ragged Arrays  Ragged arrays are sometime useful, but normally we deal with rectangular matrices – each row has the same number of columns as every other row – use this a lot as precondition to methods that work on matrices  working on matrices normally requires nested loops – why is this so hard?
  • 54. CS 307 Fundamentals of Computer Science Java Basics 54 Enhanced for loop  New in Java 5.0  a.k.a. the for-each loop  useful short hand for accessing all elements in an array (or other types of structures) if no need to alter values  alternative for iterating through a set of values for(Type loop-variable : set-expression) statement  logic error (not a syntax error) if try to modify an element in array via enhanced for loop
  • 55. CS 307 Fundamentals of Computer Science Java Basics 55 Enhanced for loop public static int sumListEnhanced(int[] list) { int total = 0; for(int val : list) { total += val; System.out.println( val ); } return total; } public static int sumListOld(int[] list) { int total = 0; for(int i = 0; i < list.length; i++) { total += list[i]; System.out.println( list[i] ); } return total; }
  • 56. CS 307 Fundamentals of Computer Science Java Basics 56 Attendance Question 5 What is output by the code to the right when method d1 is called? A. 322 B. 323 C. 363 D. 366 E. 399 public void d2(int x){ x *= 2; System.out.print(x); } public void d1(){ int x = 3; System.out.print(x); d2(x); System.out.print(x); }
  • 57. CS 307 Fundamentals of Computer Science Java Basics 57 Attendance Question 6 What is output by the code to the right? A. Output will vary from one run of program to next B. 00 C. 363 D. 7 then a runtime error E. No output due to syntax error int[] list = {5, 1, 7, 3}; System.out.print( list[2] ); System.out.print( list[4] );
  • 58. CS 307 Fundamentals of Computer Science Java Basics 58 Methods
  • 59. CS 307 Fundamentals of Computer Science Java Basics 59 Methods methods are analogous to procedures and functions in other languages – local variables, parameters, instance variables – must be comfortable with variable scope: where is a variable defined?  methods are the means by which objects are manipulated (objects state is changed) - much more on this later  method header consists of – access modifier(public, package, protected, private) – static keyword (optional, class method) – return type (void or any data type, primitive or class) – method name – parameter signature
  • 60. CS 307 Fundamentals of Computer Science Java Basics 60 More on Methods  local variables can be declared within methods. – Their scope is from the point of declaration until the end of the methods, unless declared inside a smaller block like a loop  methods contain statements  methods can call other methods – in the same class: foo(); – methods to perform an operation on an object that is in scope within the method: obj.foo(); – static methods in other classes: double x = Math.sqrt(1000);
  • 61. CS 307 Fundamentals of Computer Science Java Basics 61 static methods  the main method is where a stand alone Java program normally begins execution  common compile error, trying to call a non static method from a static one public class StaticExample { public static void main(String[] args) { //starting point of execution System.out.println("In main method"); method1(); method2(); //compile error; } public static void method1() { System.out.println( "method 1"); } public void method2() { System.out.println( "method 2"); } }
  • 62. CS 307 Fundamentals of Computer Science Java Basics 62 Method Overloading and Return  a class may have multiple methods with the same name as long as the parameter signature is unique – may not overload on return type  methods in different classes may have same name and signature – this is a type of polymorphism, not method overloading  if a method has a return value other than void it must have a return statement with a variable or expression of the proper type  multiple return statements allowed, the first one encountered is executed and method ends – style considerations
  • 63. CS 307 Fundamentals of Computer Science Java Basics 63 Method Parameters  a method may have any number of parameters  each parameter listed separately  no VAR (Pascal), &, or const & (C++)  final can be applied, but special meaning  all parameters are pass by value  Implications of pass by value???
  • 64. CS 307 Fundamentals of Computer Science Java Basics 64 Value Parameters vs. Reference Parameters  A value parameter makes a copy of the argument it is sent. – Changes to parameter do not affect the argument.  A reference parameter is just another name for the argument it is sent. – changes to the parameter are really changes to the argument and thus are permanent
  • 65. CS 307 Fundamentals of Computer Science Java Basics 65 Value vs. Reference // value void add10(int x) { x += 10; } void calls() { int y = 12; add10(y); // y = ? } // C++, reference void add10(int& x) { x += 10; } void calls() { int y = 12; add10(y); // y = ? } 12 y 12 x 12 y x
  • 66. CS 307 Fundamentals of Computer Science Java Basics 66 Programming for Correctness
  • 67. CS 307 Fundamentals of Computer Science Java Basics 67 Creating Correct Programs  methods should include pre conditions and post conditions  Preconditions are things that must be true before a method is called  Postconditions are things that will be true after a method is complete if the preconditions were met  it is the responsibility of the caller of a method to ensure the preconditions are met – the class must provide a way of ensuring the precondition is true – the preconditions must be stated in terms of the interface, not the implementation  it is the responsibility of the class (supplier, server) to ensure the postconditions are met
  • 68. CS 307 Fundamentals of Computer Science Java Basics 68 Programming by Contract  preconditions and postconditions create a contract between the client (class or object user) and a supplier (the class or object itself) – example of a contract between you and me for a test   Obligations Benefits Client (Student) (Must ensure preconditions) Be at test on time, bring pencil and eraser, write legibly, answer questions in space provided (May benefit from postcondition) Receive fair and accurate evaluation of test to help formulate progress in course Supplier (Mike) (Must ensure postcondition) Fairly and accurately grade test based on universal guidelines applied to all tests (May assume preconditions) No need to grade test or questions that are illegible, on wrong part of exam, or give makeup exams for unexcused absences
  • 69. CS 307 Fundamentals of Computer Science Java Basics 69 Thinking about pre and postconditions  pre and postconditions are part of design  coming up with pre and postconditions at the time of implementation is too late  the pre and post conditions drive the implementation and so must exist before the implementation can start – The sooner you start to code, the longer your program will take. -Roy Carlson, U Wisconsin  You must spend time on design
  • 70. CS 307 Fundamentals of Computer Science Java Basics 70 Precondition Example /** * Find all indices in <tt>source</tt> that are the start of a complete * match of <tt>target</tt>. * @param source != null, source.length() > 0 * @param target != null, target.length() > 0 * @return an ArrayList that contains the indices in source that are the * start of a complete match of target. The indices are stored in * ascending order in the ArrayList */ public static ArrayList<Integer> matches(String source, String target) { // check preconditions assert (source != null) && (source.length() > 0) && (target != null) && (target.length() > 0) : "matches: violation of precondition";
  • 71. CS 307 Fundamentals of Computer Science Java Basics 71 Creating Correct Programs  Java features has a mechanism to check the correctness of your program called assertions  Assertions are statements that are executed as normal statements if assertion checking is on – you should always have assertion checking on when writing and running your programs  Assertions are boolean expressions that are evaluated when reached. If they evaluate to true the program continues, if they evaluate to false then the program halts  logical statements about the condition or state of your program
  • 72. CS 307 Fundamentals of Computer Science Java Basics 72 Assertions  Assertions have the form assert boolean expression : what to output if assertion is false  Example if ( (x < 0) || (y < 0) ) { // we know either x or y is < 0 assert x < 0 || y < 0 : x + " " + y; x += y; } else { // we know both x and y are not less than zero assert x >= 0 && y >= 0 : x + " " + y; y += x; }  Use assertion liberally in your code – part of style guide
  • 73. CS 307 Fundamentals of Computer Science Java Basics 73 Assertions Uncover Errors in Your Logic if ( a < b ) { // we a is less than b assert a < b : a + " " + b; System.out.println(a + " is smaller than " + b); } else { // we know b is less than a assert b < a : a + " " + b; System.out.println(b + " is smaller than " + a); }  Use assertions in code that other programmers are going to use.  In the real world this is the majority of your code!
  • 74. CS 307 Fundamentals of Computer Science Java Basics 74 javadoc  javadoc is a program that takes the comments in Java source code and creates the html documentation pages  Open up Java source code. (Found in the src.zip file when you download the Java sdk.)  Basic Format /** Summary sentence for method foo. More details. More details. pre: list preconditions post: list postconditions @param x describe what x is @param y describe what y is @return describe what the method returns */ public int foo(int x, double y)  Comments interpreted as html

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