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Chapter 3 – Object-Based Programming 2

Initializing Class Objects: Constructors

• Class constructor is a specical method to initialise instance variables of a class object.

– Same name as class– No return type (don’t declar void) or return statement – Cannot be called directly– Called when instantiating object of that class·– If no constructor is defined by the programmer, a default "do-nothing"

constructor is created for you. – Java does not provide a default constructor if the class define a constructor

of its own.

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Constructor Exampleclass Employee1 { private int num;

public Employee1(int newNum) { num = newNum; // call setNum() is better, why? }

public int getNum() { return num; } public void setNum(int newNum) { num = newNum; }}

class PrintEmployee5 { public static void main(String[] args) { Employee1 emp; emp = new Employee1(71623); System.out.println(emp.getNum()); }}

>java PrintEmployee5

71623

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Another Constructor Example class Student { private String name; // Student's name. private double test1, test2, test3; // Grades on three tests.

public Student(String inName, double inTest1, double inTest2, double inTest3) { setName(inName); setTest1(inTest1); setTest2(inTest2); setTest3(inTest3); }

// Getter and Setter methods .... public void setName(String inName) { name = inName; }

public String getName() { return name; }

public void setTest1(double inTest1) { test1 = inTest1; }

public double getTest1() { return test1; }

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Another Constructor Example public void setTest2(double inTest2) { test2 = inTest2; }

public double getTest2() { return test1; }

public void setTest3(double inTest3) { test3 = inTest3; }

public double getTest3() { return test3; }

double getAverage() { // compute average test grade return (test1 + test2 + test3) / 3; }

} // end of class Student

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Test Programpublic class TS { public static void main(String s[]) { Student stud1, stud2; stud1 = new Student("John", 80.5, 78.5, 83.5); stud2 = new Student("Peter", 30, 0, 0);

System.out.println(stud1.getName() + ":" + stud1.getAverage()); System.out.println(stud2.getName() + ":" + stud2.getAverage()); }}

OutputJohn:80.83333333333333Peter:10.0

Old Version stud1 = new Student(); stud1.setName("John"); stud1.setTest1(80.5); stud1.setTest2(78.5); stud1.setTest3(83.5);

stud2 = new Student(); stud2.setName("Peter"); stud2.setTest1(30);

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Using Overloaded Constructors

• Overloaded constructors– Methods (in same class) may have same name– Must have different parameter lists

• Examples in Java– String– JButton– .......

• The following example illustrate the use of overloaded constructor. Setting a time value by providing hour, minute (optional) and second (optional).

Outline7

Time2.java

Lines 16-19Default constructor has no arguments

Lines 23-26Overloaded constructor has one int argument

Lines 30-33Second overloaded constructor has two int arguments

1 // Fig. 8.6: Time2.java2 // Time2 class definition with overloaded constructors.3 4 5 // Java core packages6 import java.text.DecimalFormat; 7 8 public class Time2 {9 private int hour; // 0 - 2310 private int minute; // 0 - 5911 private int second; // 0 - 5912 13 // Time2 constructor initializes each instance variable14 // to zero. Ensures that Time object starts in a 15 // consistent state.16 public Time2() 17 { 18 setTime( 0, 0, 0 ); 19 }20 21 // Time2 constructor: hour supplied, minute and second22 // defaulted to 023 public Time2( int h ) 24 { 25 setTime( h, 0, 0 ); 26 }27 28 // Time2 constructor: hour and minute supplied, second29 // defaulted to 030 public Time2( int h, int m ) 31 { 32 setTime( h, m, 0 );33 }34

Outline8

Time2.java

Lines 36-39Third overloaded constructor has three int arguments

Lines 42-45Fourth overloaded constructor has Time2 argument

35 // Time2 constructor: hour, minute and second supplied36 public Time2( int h, int m, int s ) 37 { 38 setTime( h, m, s ); 39 }40 41 // Time2 constructor: copy constructor42 public Time2( Time2 time )43 {44 setTime( time.hour, time.minute, time.second );45 }46 47 // Set a new time value using universal time. Perform 48 // validity checks on data. Set invalid values to zero.49 public void setTime( int h, int m, int s )50 {51 hour = ( ( h >= 0 && h < 24 ) ? h : 0 );52 minute = ( ( m >= 0 && m < 60 ) ? m : 0 );53 second = ( ( s >= 0 && s < 60 ) ? s : 0 );54 }55 56 // convert to String in universal-time format57 public String toUniversalString()58 {59 DecimalFormat twoDigits = new DecimalFormat( "00" );60 61 return twoDigits.format( hour ) + ":" +62 twoDigits.format( minute ) + ":" +63 twoDigits.format( second );64 }65 66 // convert to String in standard-time format67 public String toString()68 {69 DecimalFormat twoDigits = new DecimalFormat( "00" );

Outline9

Time2.java

70 71 return ( (hour == 12 || hour == 0) ? 12 : hour % 12 ) +72 ":" + twoDigits.format( minute ) +73 ":" + twoDigits.format( second ) +74 ( hour < 12 ? " AM" : " PM" );75 }76 77 } // end class Time2

Outline10

TimeTest4.java

Line 15Declare six references to Time2 objects

Lines 17-22Instantiate each Time2 reference using a different constructor

1 // Fig. 8.7: TimeTest4.java2 // Using overloaded constructors3 4 // Java extension packages5 import javax.swing.*;6 7 8 9 10 public class TimeTest4 {11 12 // test constructors of class Time213 public static void main( String args[] )14 {15 Time2 t1, t2, t3, t4, t5, t6;16 17 t1 = new Time2(); // 00:00:0018 t2 = new Time2( 2 ); // 02:00:0019 t3 = new Time2( 21, 34 ); // 21:34:0020 t4 = new Time2( 12, 25, 42 ); // 12:25:4221 t5 = new Time2( 27, 74, 99 ); // 00:00:0022 t6 = new Time2( t4 ); // 12:25:4223 24 String output = "Constructed with: " +25 "\nt1: all arguments defaulted" +26 "\n " + t1.toUniversalString() +27 "\n " + t1.toString();28 29 output += "\nt2: hour specified; minute and " +30 "second defaulted" +31 "\n " + t2.toUniversalString() +32 "\n " + t2.toString();33

Outline11

TimeTest4.java

34 output += "\nt3: hour and minute specified; " +35 "second defaulted" +36 "\n " + t3.toUniversalString() +37 "\n " + t3.toString();38 39 output += "\nt4: hour, minute, and second specified" +40 "\n " + t4.toUniversalString() +41 "\n " + t4.toString();42 43 output += "\nt5: all invalid values specified" +44 "\n " + t5.toUniversalString() +45 "\n " + t5.toString();46 47 output += "\nt6: Time2 object t4 specified" +48 "\n " + t6.toUniversalString() +49 "\n " + t6.toString();50 51 JOptionPane.showMessageDialog( null, output,52 "Demonstrating Overloaded Constructors",53 JOptionPane.INFORMATION_MESSAGE );54 55 System.exit( 0 );56 }57 58 } // end class TimeTest4

Outline12

TimeTest4.java

Different outputs, because each Time2 reference was instantiated with a different constructor

Different outputs, because each Time2 reference was instantiated

with a different constructor

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More Examples• Explain why the following programs are incorrect.

class Employee { private int num;

public void Employee(int newNum) { num = newNum; }

public int getNum() { return num; } public void setNum(int newNum) { num = newNum; }}

class TE { public static void main(String[] args) { Employee emp; emp = new Employee(71623); System.out.println(emp.getNum()); }}

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More Examples

class Employee { private int num;

public Employee(int newNum) { num = newNum; }

public int getNum() { return num; } public void setNum(int newNum) { num = newNum; }}

class TE { public static void main(String[] args) { Employee emp = new Employee(); emp.setNum(71623); System.out.println(emp.getNum()); }}

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More Examples

class Employee { private int num;

public Employee1(int newNum) { num = newNum; } public int getNum() { return num; } public void setNum(int newNum) { num = newNum; }

}

class TE { public static void main(String[] args) { Employee emp = new Employee1(71623); System.out.println(emp.getNum()); }}

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Read Only Property

• Consider the name attributes of the following Student class:

public class Student { private String name; // Student's name. public double test1, test2, test3; // Grades on three tests. Student(String theName) { name = theName; } public String getName() { return name; } public double getAverage() { return (test1 + test2 + test3) / 3; } } // end of class Student

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Read Only Property

• The name attribute cannot be modified once the object is created.

std1 = new Student("John Smith");

std2 = new Student("Mary Jones");

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Final Instance Variables

• final keyword– Indicates that variable is not modifiable

• Any attempt to modify final variable results in error

Declares variable INCREMENT as a constant

– Enforces principle of least privilege

private final int INCREMENT = 5;

Outline19

Increment.java

Line 15final keyword declares INCREMENT as constant

Line 32final variable INCREMENT must be initialized before using it

1 // Fig. 8.11: Increment.java2 // Initializing a final variable3 4 // Java core packages5 import java.awt.*;6 import java.awt.event.*;7 8 // Java extension packages9 import javax.swing.*;10 11 public class Increment extends JApplet12 implements ActionListener {13 14 private int count = 0, total = 0;15 private final int INCREMENT = 5; // constant variable16 17 private JButton button; 18 19 // set up GUI20 public void init() 21 { 22 Container container = getContentPane();23 24 button = new JButton( "Click to increment" );25 button.addActionListener( this );26 container.add( button );27 }28 29 // add INCREMENT to total when user clicks button30 public void actionPerformed( ActionEvent actionEvent )31 {32 total += INCREMENT;33 count++;34 showStatus( "After increment " + count +35 ": total = " + total );

Outline20

Increment.java

36 }37 38 } // end class Increment

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Constants (Static)

• More often constants will be defined as public and static:

• public - Other classes can read the value of the constant

• static - only one copy shared by all objects, hence save memory (see next slide)

• Examples in Java - JOptionPane, String, Math, .....

public static final int INCREMENT = 5;

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Static Class Members

• Most properties, like the balance in bank account, are unique to the object, which should be stored as instance variable (attribute or data member).

• But some properties are shared among all objects of a given class. For example, the interest rate is a property shared by all saving accounts in the same bank.

• Such properties are called class member.

• Class members are defined using the keyword static. So class members are also called static members.

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Static Class Members

• static class variable– Class-wide information

• All class objects share same data

– can be accessed via class name or an object reference

• static method– can be accessed via class name or an object reference

– can access static members only (i.e. CANNOT read non-static instance variables or call non-static methods.)

• non-static method– can access both static and non-static variables

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Static Class Members

• When should a variable be declared as static?– data will be shared by all classes (e.g. Employee Counter,

Account Interest, constants)

– it saves memory and is easy to maintain

• When should a method be declared as static?– the method will be used as an utility, not operates on an

object (e.g. main, Math.sqrt, Math.sin, .....)

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// TestStatic.java

class Employee { static int maxNum = 20000; private int num; public int getEmpNum() { return num; } public void setEmpNum(int newNum) { if (newNum<=maxNum && newNum >1) num = newNum; }}

class TestStatic { public static void main(String[] args) { Employee emp1 = new Employee(); Employee emp2 = new Employee(); System.out.println(“maxNum of emp1 : " + emp1.maxNum); System.out.println(“maxNum of emp2 : " + emp2.maxNum); emp1.maxNum = 99999; System.out.println("\nmaxNum of emp2 : " + emp2.maxNum); }}

>java TestStaticmaxNum of emp1 : 20000maxNum of emp2 : 20000

maxNum of emp2 : 99999

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Class (Static) Members

Object1

Object2

Object3

num

num

num

MaxNum