Java căn bản - Chapter3

Objectives After you have read and studied this chapter, you should be able to Select proper types for numerical data. Write arithmetic expressions in Java. Evaluate arithmetic expressions using the precedence rules. Describe how the memory allocation works for objects and primitive data values. Write mathematical expressions, using methods in the Math class. Use the GregorianCalendar class in manipulating date information such as year, month, and day. Use the DecimalFormat class to format numerical data Convert input string values to numerical data Perform input and output by using System.in and System.out

Manipulating Numbers In Java, to add two numbers x and y, we write x + y But before the actual addition of the two numbers takes place, we must declare their data type. If x and y are integers, we write int x, y; or int x; int y;

Variables When the declaration is made, memory space is allocated to store the values of x and y. x and y are called variables . A variable has three properties: A memory location to store the value, The type of data stored in the memory location, and The name used to refer to the memory location. Sample variable declarations: int x; int v, w, y;

Numerical Data Types There are six numerical data types: byte , short , int , long , float , and double . Sample variable declarations: int i, j, k; float numberOne, numberTwo; long bigInteger; double bigNumber; At the time a variable is declared, it also can be initialized. For example, we may initialize the integer variables count and height to 10 and 34 as int count = 10, height = 34;

Data Type Precisions The six data types differ in the precision of values they can store in memory.

Assignment Statements We assign a value to a variable using an assignment statements . The syntax is <variable> = <expression> ; Examples: sum = firstNumber + secondNumber; avg = (one + two + three) / 3.0;

Arithmetic Operators The following table summarizes the arithmetic operators available in Java. This is an integer division where the fractional part is truncated.

Arithmetic Expression How does the expression x + 3 * y get evaluated? Answer: x is added to 3*y. We determine the order of evaluation by following the precedence rules . A higher precedence operator is evaluated before the lower one. If two operators are the same precedence, then they are evaluated left to right for most operators.

Type Casting If x is a float and y is an int , what will be the data type of the following expression? x * y The answer is float . The above expression is called a mixed expression . The data types of the operands in mixed expressions are converted based on the promotion rules . The promotion rules ensure that the data type of the expression will be the same as the data type of an operand whose type has the highest precision.

Explicit Type Casting Instead of relying on the promotion rules, we can make an explicit type cast by prefixing the operand with the data type using the following syntax: ( <data type> ) <expression> Example (float) x / 3 (int) (x / y * 3.0) Type case x to float and then divide it by 3. Type cast the result of the expression x / y * 3.0 to int .

Implicit Type Casting Consider the following expression: double x = 3 + 5; The result of 3 + 5 is of type int . However, since the variable x is double , the value 8 (type int ) is promoted to 8.0 (type double ) before being assigned to x . Notice that it is a promotion. Demotion is not allowed. int x = 3.5; A higher precision value cannot be assigned to a lower precision variable.

Constants We can change the value of a variable. If we want the value to remain the same, we use a constant . final double PI = 3.14159; final int MONTH_IN_YEAR = 12; final short FARADAY_CONSTANT = 23060; These are constants, also called named constant . The reserved word final is used to declare constants. These are called literal constant.

Primitive vs. Reference Numerical data are called primitive data types . Objects are called reference data types , because the contents are addresses that refer to memory locations where the objects are actually stored.

Primitive Data Declaration and Assignments Code State of Memory int firstNumber, secondNumber; firstNumber = 234; secondNumber = 87; A B firstNumber secondNumber A. Variables are allocated in memory. B. Values are assigned to variables. 234 87

Assigning Numerical Data Code State of Memory int number; number = 237; number A. The variable is allocated in memory. B. The value 237 is assigned to number . 237 A B C number = 35; C. The value 35 overwrites the previous value 237. 35

Assigning Objects Code State of Memory Customer customer; customer = new Customer( ); customer = new Customer( ); customer A. The variable is allocated in memory. A B C B. The reference to the new object is assigned to customer . Customer C. The reference to another object overwrites the reference in customer. Customer

Having Two References to a Single Object Code State of Memory Customer clemens, twain, clemens = new Customer( ); twain = clemens; A B C A. Variables are allocated in memory. clemens twain B. The reference to the new object is assigned to clemens . Customer C. The reference in clemens is assigned to customer.

Type Mismatch Suppose we want to input an age. Will this work? No. String value cannot be assigned directly to an int variable. int age; age = JOptionPane.showInputDialog( null , “Enter your age”);

Type Conversion Wrapper classes are used to perform necessary type conversions, such as converting a String object to a numerical value. int age; String inputStr; inputStr = JOptionPane.showInputDialog( null , “Enter your age”); age = Integer.parseInt(inputStr);

Sample Code Fragment //code fragment to input radius and output //area and circumference double radius, area, circumference; radiusStr = JOptionPane.showInputDialog( null , "Enter radius: " ); radius = Double.parseDouble(radiusStr); //compute area and circumference area = PI * radius * radius; circumference = 2.0 * PI * radius; JOptionPane.showMessageDialog( null , "Given Radius: " + radius + "\n" + "Area: " + area + "\n" + "Circumference: " + circumference);

Overloaded Operator + The plus operator + can mean two different operations, depending on the context. <val1> + <val2> is an addition if both are numbers. If either one of them is a String, the it is a concatenation. Evaluation goes from left to right. output = “test” + 1 + 2; output = 1 + 2 + “test”;

The DecimalFormat Class Use a DecimalFormat object to format the numerical output. double num = 123.45789345; DecimalFormat df = new DecimalFormat(“0.000”); //three decimal places System.out.print ( num ) ; System.out.print ( df.format ( num )) ; 123.45789345 123.458

3.5 Standard Output The showMessageDialog method is intended for displaying short one-line messages, not for a general-purpose output mechanism. Using System.out, we can output multiple lines of text to the standard output window.

Standard Output Window A sample standard output window for displaying multiple lines of text. The exact style of standard output window depends on the Java tool you use.

The print Method We use the print method to output a value to the standard output window. The print method will continue printing from the end of the currently displayed output. Example System.out.print ( “Hello, Dr. Caffeine.” );

The println Method We use println instead of print to skip a line. int x = 123, y = x + x; System.out.println( "Hello, Dr. Caffeine.“ ); System.out.print( " x = “ ); System.out.println( x ); System.out.print( " x + x = “ ); System.out.println( y ); System.out.println( " THE END“ );

Standard Input The technique of using System.in to input data is called standard input. We can only input a single byte using System.in directly. To input primitive data values, we use the Scanner class (from Java 5.0). Scanner scanner; scanner = Scanner.create(System.in); int num = scanner.nextInt();

Method Example nextByte( ) byte b = scanner.nextByte( ); nextDouble( ) double d = scanner.nextDouble( ); nextFloat( ) float f = scanner.nextFloat( ); nextInt( ) int i = scanner.nextInt( ); nextLong( ) long l = scanner.nextLong( ); nextShort( ) short s = scanner.nextShort( ); next() String str = scanner.next(); Common Scanner Methods:

The Math class The Math class in the java.lang package contains class methods for commonly used mathematical functions. double num, x, y; x = …; y = …; num = Math.sqrt(Math.max(x, y) + 12.4);

Some Math Class Methods Table 3.8 page 113 in the textbook contains a list of class methods defined in the Math class. The sine of a . (Note: all trigonometric functions are computed in radians) sin(a) The square root of a . sqrt(a) The larger of a and b. max(a,b) Natural logarithm (base e ) of a . log(a) The number a raised to the power of b . pow(a,b) Natural number e raised to the power of a . exp(a) The largest whole number less than or equal to a . floor(a) Description Method

Computing the Height of a Pole alphaRad = Math.toRadians ( alpha ) ; betaRad = Math.toRadians ( beta ) ; height = ( distance * Math.sin ( alphaRad ) * Math.sin ( betaRad ) ) / Math.sqrt ( Math.sin ( alphaRad + betaRad ) * Math.sin ( alphaRad - betaRad ) ) ;

The GregorianCalendar Class Use a GregorianCalendar object to manipulate calendar information GregorianCalendar today, independenceDay; today = new GregorianCalendar () ; independenceDay = new GregorianCalendar ( 1776, 6, 4 ) ; //month 6 means July; 0 means January

Retrieving Calendar Information This table shows the class constants for retrieving different pieces of calendar information from Date .

Sample Calendar Retrieval GregorianCalendar cal = new GregorianCalendar () ; //Assume today is Nov 9, 2003 System.out.print(“Today is ” + (cal.get(Calendar.MONTH)+1) + “/” + cal.get(Calendar.DATE) + “/” + cal.get(Calendar.YEAR)); Today is 11/9/2003 Output

Problem Statement Problem statement: Write a loan calculator program that computes both monthly and total payments for a given loan amount, annual interest rate, and loan period.

Overall Plan Tasks: Get three input values: loanAmount , interestRate , and loanPeriod . Compute the monthly and total payments. Output the results.

Development Steps We will develop this program in four steps: Start with code to accept three input values. Add code to output the results. Add code to compute the monthly and total payments. Update or modify code and tie up any loose ends.

Step 1 Design Call the showInputDialog method to accept three input values: loan amount, annual interest rate, loan period. Data types are int in years loan period double in percent (e.g.,12.5) annual interest rate double dollars and cents loan amount Data Type Format Input

Step 1 Code Directory: Chapter3/Step1 Source File: Ch3LoanCalculator.java Program source file is too big to list here. From now on, we ask you to view the source files using your Java IDE.

Step 1 Test In the testing phase, we run the program multiple times and verify that we can enter three input values we see the entered values echo-printed correctly on the standard output window

Step 2 Design We will consider the display format for out. Two possibilities are (among many others)

Step 2 Code Directory: Chapter3/Step2 Source File: Ch3LoanCalculator.java

Step 2 Test We run the program numerous times with different types of input values and check the output display format. Adjust the formatting as appropriate

Step 3 Design The formula to compute the geometric progression is the one we can use to compute the monthly payment. The formula requires the loan period in months and interest rate as monthly interest rate. So we must convert the annual interest rate (input value) to a monthly interest rate (per the formula), and the loan period to the number of monthly payments.

Step 3 Code Directory: Chapter3/Step3 Source File: Ch3LoanCalculator.java

Step 3 Test We run the program numerous times with different types of input values and check the results.

Step 4: Finalize We will add a program description We will format the monthly and total payments to two decimal places using DecimalFormat. Directory: Chapter3/Step4 Source File: Ch3LoanCalculator.java

Java căn bản - Chapter3

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Java căn bản - Chapter3