Lec 8 03_sept [compatibility mode]
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This document discusses Java data types, variables, and methods. It covers primitive data types like int, float, and boolean. It describes rules for identifiers and conventions for naming variables and classes. The document defines parameter types, implicit and explicit parameters, and how to call methods. It explains constructing objects with the new operator and accessor and mutator methods. The summary concludes with a discussion of static methods and how to call them without an object.
![Example
import java.awt.Rectangle;
public class Move
{
public static void main(String[] args)
{
Rectangle box = new Rectangle(5, 10, 20, 30);
// Move the rectangle
box.translate(15, 25);
// Print information about the moved rectangle
System.out.println("After moving, the top-left corner is:");
System.out.println(box.getX());
System.out.println(box.getY());
}
}](https://image.slidesharecdn.com/lec803septcompatibilitymode-130917013815-phpapp02/85/Lec-8-03_sept-compatibility-mode-14-320.jpg)
![Self Check
Which are the most commonly used number types in Java?
When does the cast (long) x [double x] yield a different result from
the call Math.round(x)?
How do you round the double value x to the nearest int value?
Answers
int and double
When the fractional part of x is ≥ 0.5
By using a cast: (int) Math.round(x)
22](https://image.slidesharecdn.com/lec803septcompatibilitymode-130917013815-phpapp02/85/Lec-8-03_sept-compatibility-mode-22-320.jpg)
Lec 8 03_sept [compatibility mode]
- 1. Java Data types and Variables Lecture 8 Naveen Kumar
- 2. Primitive Data Types Integer Type Bit Size byte 8 short 16 (-32,768, 32,767) int 32 long 64 Float float 32 double 64 Character 16 (65,535) Boolean 1/0
- 3. Identifiers Identifier: name of a variable, method, or class Rules for identifiers in Java: – Can be made up of letters, digits, and the underscore (_) character – Cannot start with a digit – Cannot use other symbols such as ? or % – Spaces are not permitted inside identifiers – You cannot use reserved words – They are case sensitive By convention, variable names start with a lowercase letter By convention, class names start with an uppercase letter
- 4. Implicit and Explicit Parameters Parameter (explicit parameter): – Input to a method. – Not all methods have explicit parameters. System.out.println(greeting) greeting.length() // has no explicit parameter Implicit parameter: The object on which a method is invoked greeting.length()
- 5. Replace Method Let, String river=“Mississippi”; replace method carries out a search-and-replace operation river.replace("issipp", "our") // constructs a new string ("Missouri") This method call has – one implicit parameter: the string "Mississippi" – two explicit parameters: the strings "issipp" and "our" – a return value: the string "Missouri"
- 6. Method Overloading A method name is overloaded if a class has more than one method with the same name (but different parameter types) public void println(String output) public void println(int output)
- 7. Rectangular Shapes and Rectangle Objects Objects of type Rectangle describe rectangular shapes A Rectangle object isn't a rectangular shape– it is an object that contains a set of numbers that describe the rectangle
- 8. Constructing Objects new Rectangle(5, 10, 20, 30) Detail: – The new operator makes a Rectangle object – It uses the parameters (in this case, 5, 10, 20, and 30) to initialize the data of the object – It returns the object Usually the output of the new operator is stored in a variable Rectangle box = new Rectangle(5, 10, 20, 30);
- 9. Constructing Objects The process of creating a new object is called construction The four values 5, 10, 20, and 30 are called the construction parameters new Rectangle() // constructs a rectangle with its top-left corner // at the origin (0, 0), width 0, and height 0
- 10. Self Check How do you construct a square with center (100, 100) and side length 20? What does the following statement print? System.out.println(new Rectangle().getWidth()); Answers new Rectangle(90, 90, 20, 20) 0
- 11. Accessor and Mutator Methods Accessor method: does not change the state of its implicit parameter double width = box.getWidth(); Mutator method: changes the state of its implicit parameter box.translate(15, 25);
- 12. Self Check Is the toUpperCase method of the String class an accessor or a mutator? Which call to translate is needed to move the box rectangle so that its top-left corner is the origin (0,0)? Answers An accessor–it doesn't modify the original string but returns a new string with uppercase letters box.translate(-5, -10), provided the method is called immediately after storing the new rectangle into box Rectangle box = new Rectangle(5, 10, 20, 30);
- 13. Importing Packages Don't forget to include appropriate packages: Java classes are grouped into packages Import library classes by specifying the package and class name: import java.awt.Rectangle;
- 14. Example import java.awt.Rectangle; public class Move { public static void main(String[] args) { Rectangle box = new Rectangle(5, 10, 20, 30); // Move the rectangle box.translate(15, 25); // Print information about the moved rectangle System.out.println("After moving, the top-left corner is:"); System.out.println(box.getX()); System.out.println(box.getY()); } }
- 15. Output After moving, the top-left corner is: 20 35
- 16. Self Check The Random class is defined in the java.util package. What do you need to do in order to use that class in your program? Answers Add the statement import java.util.Random; at the top of your program
- 17. Random class Random r = new Random(); int i = r.nextInt(int n) Returns random int ≥ 0 and < n int i = r.nextInt() Returns random int (full range) long i = r.nextLong() Returns random long (full range) float f = r.nextFloat() Returns random float ≥0.0 and <1.0 double d = r.nextDouble() Returns random double ≥ 0.0 and < 1.0 boolean b = r.nextBoolean() Returns random double (true ,false) double x; x = Math.random(); // assigns random number to x 17
- 18. Object References Describe the location of objects The new operator returns a reference to a new object Rectangle box = new Rectangle(); Multiple object variables can refer to the same object Rectangle box = new Rectangle(5, 10, 20, 30); Rectangle box2 = box; box2.translate(15, 25); Primitive type variables ≠ object variables
- 19. Self Check What is the effect of the assignment greeting2 = greeting? After calling greeting2.toUpperCase(), what are the contents of greeting and greeting2? Answers Now greeting and greeting2 both refer to the same String object. Both variables still refer to the same string, and the string has not been modified. Recall that the toUpperCase method constructs a new string that contains uppercase characters, leaving the original string unchanged.
- 20. Primitive Types Type Description Size int The integer type, with range -2,147,483,648 . . . 2,147,483,647 4 bytes byte The type describing a single byte, with range -128 . . . 127 1 byte short The short integer type, with range -32768 . . . 32767 2 bytes long The long integer type, with range -9,223,372,036,854,775,808 . . . - 9,223,372,036,854,775,807 8 bytes double The double-precision floating-point type, with a range of about ±10 308 and about 15 significant decimal digits 8 bytes float The single-precision floating-point type, with a range of about ±10 38 and about 7 significant decimal digits 4 bytes char The character type, representing code units in the Unicode encoding scheme 2 bytes boolean The type with the two truth values false and true 1 bit 20
- 21. Cast (typeName) expression Example: (int) (balance * 100) Purpose: To convert an expression to a different type 21
- 22. Self Check Which are the most commonly used number types in Java? When does the cast (long) x [double x] yield a different result from the call Math.round(x)? How do you round the double value x to the nearest int value? Answers int and double When the fractional part of x is ≥ 0.5 By using a cast: (int) Math.round(x) 22
- 23. Constants: final A final variable is a constant Once its value has been set, it cannot be changed Named constants make programs easier to read and maintain Convention: use all-uppercase names for constants final double QUARTER_VALUE = 0.25; final double DIME_VALUE = 0.1; 23
- 24. Constants: static final static is used with class members If constant values are needed in several methods, declare them together and tag them as static and final Give static final constants public access to enable other classes to use them public class Math { . . . public static final double E = 2.7182818284590452354; public static final double PI = 3.14159265358979323846; } double circumference = Math.PI * diameter; (call without object) 24
- 25. Syntax : Constant Definition In a method: final typeName variableName = expression ; In a class: accessSpecifier static final typeName variableName = expression; Example: final double NICKEL_VALUE = 0.05; public static final double LITERS_PER_GALLON = 3.785; Purpose: To define a constant in a method or a class 25
- 26. Self Check What is the difference between the following two statements? final double CM_PER_INCH = 2.54; and public static final double CM_PER_INCH = 2.54; What is wrong with the following statement? double circumference = 3.14 * diameter; Answers The first definition is used inside a method, the second inside a class (1) You should use a named constant, not 3.14 (2) 3.14 is not an accurate representation of π 26
- 27. Increment, and Decrement items++ is the same as items = items + 1 items-- subtracts 1 from items ++a; --a; a++; a--; 27
- 28. Arithmetic Operations / is the division operator If both arguments are integers, the result is an integer. The remainder is discarded – 7.0 / 4 yields 1.75 – 7 / 4 yields 1 Get the remainder with % (pronounced "modulo") 7 % 4 is 328
- 29. The Math class Math class: contains methods like sqrt and pow To compute xn, you write Math.pow(x, n) However, to compute x2 it is significantly more efficient simply to compute x * x To take the square root of a number x, use the Math.sqrt; for example, Math.sqrt(x) 29
- 30. Mathematical Methods in Java 30 Math.sqrt(x) square root Math.pow(x, y) power xy Math.exp(x) ex Math.log(x) natural log Math.sin(x), Math.cos(x), Math.tan(x) sine, cosine, tangent (x in radian) Math.round(x) closest integer to x Math.min(x, y), Math.max(x, y) minimum, maximum
- 31. java.lang.Math methods E, PI sin(_), cos(_), abs(_), tan(_), ceil(_), floor(_), log(_), max(_,_), min(_,_), pow(_,_), sqrt(_), round(_), random(), toDegrees(_), toRadians(_) 31
- 32. Self Check What is the value of 1729 / 100? and 1729 % 100? What does the following statement compute ? double average = s1 + s2 + s3 / 3; What is the value of Math.sqrt(Math.pow(x, 2) + Math.pow(y, 2)) in mathematical notation? Answers 17 and 29 Only s3 is divided by 3. To get the correct result, use parentheses. Moreover, if s1, s2, and s3 are integers, you must divide by 3.0 to avoid integer division: (s1 + s2 + s3) / 3.0 32
- 33. Calling Static Methods A static method does not operate on an object double x = 4; double root = x.sqrt(); // Error Static methods are defined inside classes Naming convention: Classes start with an uppercase letter; objects start with a lowercase letter Math System.out 33
- 34. Static Method Call ClassName. methodName(parameters) Example: Math.sqrt(4) Purpose: To invoke a static method (a method that does not operate on an object) and supply its parameters34
- 35. Self Check Why can't you call x.pow(y) to compute xy ? Is the call System.out.println(4) a static method call? Answers x is a number, not an object, and you cannot invoke methods on numbers No–the println method is called on the object System.out35