chapter 4 - classes in java

1

Classes

• class: reserved word; collection of a fixed number of components

• Components: members of a class• Members accessed by name• Class categories/modifiers

– private– protected– public

2

Classes (continued)

• private: members of class are not accessible outside class

• public: members of class are accessible outside class

• Class members: can be methods or variables

• Variable members declared like any other variables

3

Syntax

The general syntax for defining a class is:

4

Syntax (continued)

• If a member of a class is a named constant, you declare it just like any other named constant

• If a member of a class is a variable, you declare it just like any other variable

• If a member of a class is a method, you define it just like any other method

5

Syntax (continued)

• If a member of a class is a method, it can (directly) access any member of the class—data members and methods

- Therefore, when you write the definition of a method (of the class), you can directly access any data member of the class (without passing it as a parameter)

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Syntax: Value-Returning Method

77

User-Defined Methods

• Value-returning methods– Used in expressions– Calculate and return a value– Can save value for later calculation or print value

• modifiers: public, private, protected, static, abstract, final

• returnType: type of the value that the method calculates and returns (using return statement)

• methodName: Java identifier; name of method

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Syntax

• Syntax: Formal Parameter List-The syntax of the formal parameter list is:

• Method Call-The syntax to call a value-returning method is:

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Syntax (continued)

• Syntax: return Statement -The return statement has the following syntax:

return expr;

• Syntax: Actual Parameter List-The syntax of the actual parameter list is:

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Equivalent Method Definitions

public static double larger(double x, double y){ double max;

if (x >= y) max = x; else max = y;

return max;}

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Equivalent Method Definitions (continued)

public static double larger(double x, double y){ if (x >= y) return x; else return y;}

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Equivalent Method Definitions (continued)

public static double larger(double x, double y){ if (x >= y) return x; return y;}

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Programming Example: Palindrome Number

• Palindrome: integer or string that reads the same forwards and backwards

• Input: integer or string

• Output: Boolean message indicating whether integer string is a palindrome

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Solution: isPalindrome Method

public static boolean isPalindrome(String str){ int len = str.length(); int i, j; j = len - 1;

for (i = 0; i <= (len - 1) / 2; i++) { if (str.charAt(i) != str.charAt(j)) return false; j--; } return true; }

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Sample Runs: Palindrome Number

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Sample Runs: Palindrome Number (continued)

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Flow of Execution• Execution always begins with the first statement

in the method main• User-defined methods execute only when called

• Call to method transfers control from caller to called method

• In method call statement, specify only actual parameters, not data type or method type

• Control goes back to caller when method exits

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Programming Example: Largest Number

• Input: set of 10 numbers

• Output: largest of 10 numbers

• Solution– Get numbers one at a time– Method largest number: returns the larger of 2

numbers– For loop: calls method largest number on each number

received and compares to current largest number

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Solution: Largest Numberstatic Scanner console = new Scanner(System.in);

public static void main(String[] args){ double num; double max; int count; System.out.println("Enter 10 numbers."); num = console.nextDouble(); max = num; for (count = 1; count < 10; count++) { num = console.nextDouble(); max = larger(max, num); } System.out.println("The largest number is " + max); }

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Sample Run: Largest Number

• Sample Run:

Enter 10 numbers:10.5 56.34 73.3 42 22 67 88.55 26 62 11The largest number is 88.55

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Void Methods

• Similar in structure to value-returning methods

• Call to method is always stand-alone statement

• Can use return statement to exit method early

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Void Methods: Syntax

• Method Definition-The general form (syntax) of a void method without parameters is as follows:

modifier(s) void methodName(){ statements}• Method Call (Within the Class)

-The method call has the following syntax:methodName();

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Void Methods with Parameters: Syntax

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Void Methods with Parameters: Syntax (continued)

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Primitive Data Type Variables as Parameters

• A formal parameter receives a copy of its corresponding actual parameter

• If a formal parameter is a variable of a primitive data type:– Value of actual parameter is directly stored– Cannot pass information outside the method– Provides only a one-way link between actual

parameters and formal parameters

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Reference Variables as Parameters

• If a formal parameter is a reference variable:– Copies value of corresponding actual parameter– Value of actual parameter is address of the object

where actual data is stored– Both formal and actual parameter refer to same

object

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Uses of Reference Variables as Parameters

• Can return more than one value from a method

• Can change the value of the actual object

• When passing address, would save memory space and time, relative to copying large amount of data

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Reference Variables as Parameters: type String

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Example 7-11public class Example7_11{ public static void main(String[] args) { int num1; //Line 1 IntClass num2 = new IntClass(); //Line 2 char ch; //Line 3 StringBuffer str; //Line 4 num1 = 10; //Line 5 num2.setNum(15); //Line 6 ch = 'A'; //Line 7 str = new StringBuffer("Sunny"); //Line 8 System.out.println("Line 9: Inside main: " + "num1 = " + num1 + ", num2 = "

+ num2.getNum() + ", ch = " + ch + ", and str = " + str); //Line 9

Reference Variables as Parameters: type String (continued)

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funcOne(num1, num2, ch, str); //Line 10 System.out.println("Line 11: After funcOne: " + "num1 = " + num1 + ", num2 = "

+ num2.getNum() + ", ch = " + ch + ", and str = " + str); //Line 11 }

Reference Variables as Parameters: type String (continued)

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public static void funcOne(int a, IntClass b, char v,

StringBuffer pStr) { int num; //Line 12 int len; //Line 13 num = b.getNum(); //Line 14 a++; //Line 15 b.addToNum(12); //Line 16 v = 'B'; //Line 17 len = pStr.length(); //Line 18 pStr.delete(0, len); //Line 19 pStr.append("Warm"); //Line 20 System.out.println("Line 21: Inside funcOne: \n" + " a = " + a + ", b = " + b.getNum() + ", v = " + v + ", pStr = " + pStr + ", len = " + len + ", and num = " + num); //Line 21 }}

Reference Variables as Parameters: type String (continued)

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Reference Variables as Parameters: type String (continued)

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num1 = 10; //Line 5 num2.setNum(15); //Line 6 ch = 'A';

//Line 7str = new StringBuffer("Sunny"); //Line 8

Reference Variables as Parameters: type String (continued)

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System.out.println("Line 9: Inside main: " + "num1 = " + num1 + ", num2 = " + num2.getNum() + ", ch = " + ch + ", and str = " + str); //Line 9

Reference Variables as Parameters: type String (continued)

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int num; //Line 12 int len; //Line 13 num = b.getNum();

//Line 14

Reference Variables as Parameters: type String (continued)

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num = b.getNum(); //Line 14

Reference Variables as Parameters: type String (continued)

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a++; //Line 15

Reference Variables as Parameters: type String (continued)

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b.addToNum(12); //Line 16

Reference Variables as Parameters: type String (continued)

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v = 'B'; //Line 17

Reference Variables as Parameters: type String (continued)

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len = pStr.length(); //Line 18

Reference Variables as Parameters: type String (continued)

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pStr.delete(0, len); //Line 19

Reference Variables as Parameters: type String (continued)

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pStr.append("Warm"); //Line 20

Reference Variables as Parameters: type String (continued)

4343Java Programming: From Problem Analysis to Program Design, 3e

System.out.println("Line 21: Inside funcOne: \n" + " a = " + a + ", b = " + b.getNum() + ", v = " + v + ", pStr = " + pStr + ", len = " + len + ", and num = " + num); //Line 21

Reference Variables as Parameters: type String (continued)

4444Java Programming: From Problem Analysis to Program Design, 3e

Reference Variables as Parameters: type String (continued)

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System.out.println("Line 11: After funcOne: " + "num1 = " + num1 + ", num2 = " + num2.getNum() + ", ch = " + ch + ", and str = " + str); //Line 11

Reference Variables as Parameters: type String (continued)

46

Accessing Objects

• Referencing the object’s data:

objectRefVar.data

e.g., myCircle.radius

• Invoking the object’s method:

objectRefVar.methodName(arguments)

e.g., myCircle.getArea()