Islamic University of Gaza

Faculty of Engineering

Computer Engineering Department

Computer Programming Lab (ECOM 2114)

Lab 12

Object Oriented

Programming (II)

Eng. Mohammed Alokshiya

December 23, 2014

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Static Variables and Methods

The data field age in the Person class is known as an instance variable. An

instance variable is tied to a specific instance of the class; it is not shared

among objects of the same class. For example, suppose that you create the

following objects:

Person person1 = new Person("Mohammed", "Ahmed", 23, 120191379);

Person person2 = new Person("Ahmed", "Khaled", 19, 120140000);

The age in person1 is independent of the age in person2 and is stored in a

different memory location. Changes made to person1’s age do not affect

person2’s age, and vice versa.

If you want all the instances of a class to share data, use static variables,

also known as class variables. Static variables store values for the variables

in a common memory location. Because of this common location, if one

object changes the value of a static variable, all objects of the same class are

affected. Java supports static methods as well as static variables. Static

methods can be called without creating an instance of the class.

Let’s modify the Person class by adding a static variable counter to count the

number of person objects created. When the first object of this class is

created, counter is 1. When the second object is created, counter becomes

2. The constructor methods are guaranteed to be invoked once when

creating a new object, so we increment the counter in the constructors.

class Person class Person {

// Fields

String fName;

String lName;

int age;

int ID;

static int counter = 0;

// A Constructor

public Person() {

counter++;

}

// Another Constructor

public Person(String fName, String lName, int age, int ID) {

Static field

Increment the counter when creating

a new object using first constructor

()

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this.fName = fName;

this.lName = lName;

this.age = age;

this.ID = ID;

counter++;

}

}

Now the counter field will be incremented when you create any object of

class Person. To print the value of counter, get it with the class Name:

System.out.println(Person.count);

You can also define static methods. Static methods can be invoked with

class name, without creating objects.

An instance method can invoke an instance or static method and access an

instance or static data field. A static method can invoke a static method and

access a static data field. However, a static method cannot invoke an

instance method or access an instance data field, since static methods and

static data fields do not belong to a particular object. The relationship

between static and instance members is summarized in the following

diagram:

Static Method public static int getCounter() {

return counter;

}

Increment the counter when creating

a new object using second constructor

()

4

Visibility Modifiers

The visibility (access) modifiers in java specifies accessibility (scope) of a data

member, method, constructor or class.

There are 4 types of java access modifiers:

private

default

protected

public

Private access modifier:

The private access modifier is accessible only within class. In the following

example, we have created two classes A and Simple. A class contains private

data member and private method. We are accessing these private members

from outside the class, so there is compilation error.

Simple.java class A {

private int data = 40;

private void msg() {

System.out.println("Hello java");

}

}

public class Simple {

public static void main(String args[]) {

A obj = new A();

System.out.println(obj.data); // Compilation Error

obj.msg(); // Compilation Error

}

}

Default access modifier:

If you do not use any modifier, it is treated as default by default. The default

modifier is accessible only within package. In the following example, we have

created two packages pack and mypack. We are accessing the msg() method

in obj object, which is an instance of A class from outside its package, since

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msg() method is not public, so it cannot be accessed from outside the

package.

A.java package pack;

public class A {

void msg() {

System.out.println("Hello");

}

}

B.java package mypack;

import pack.A;

public class B {

public static void main(String args[]) {

A obj = new A();

obj.msg();// Compilation Error

}

}

In the above example, the scope of method msg() is default so it cannot be

accessed from outside the package.

Protected access modifier:

Protected access modifier will be explained later, when you start working

with inheritance and super classes.

Public access modifier:

The public access modifier is accessible everywhere. It has the widest scope

among all other modifiers. Example:

A.java package pack;

public class A {

public void msg() {

System.out.println("Hello");

}

}

B.java package mypack;

import pack.A;

public class B {

public static void main(String args[]) {

A obj = new A();

obj.msg();

}

}

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Output Hello

Summary:

Visibility Modifiers Summary Modifier | Class | Package | Subclass | World

————————————+———————+—————————+——————————+———————

public | y | y | y | y

————————————+———————+—————————+——————————+———————

protected | y | y | y | n

————————————+———————+—————————+——————————+———————

no modifier | y | y | n | n

————————————+———————+—————————+——————————+———————

private | y | n | n | n

y: accessible

n: not accessible

Subclass and protected modifier will be explained in details in Java II.

7

Encapsulation

The whole idea behind encapsulation is to hide the implementation details

from users. If a data member is private it means it can only be accessed

within the same class. No outside class can access private data member

(fields) of other class. However if we setup public getter and setter methods

to update (for e.g. void setAge(int age)) and read (for e.g. int getAge()) the

private data fields then the outside class can access those private data

fields via public methods. This way data can only be accessed by public

methods thus making the private fields and their implementation hidden for

outside classes. That’s why encapsulation is known as data hiding. Lets see

an example to understand this concept better.

Person.java class Person {

// Fields

private String fName;

private String lName;

private int age;

private int ID;

private static int counter = 0;

// Only Getter Method for static field counter

// to guarantee that there is no way to change its value

// except creating an instance

public static int getCounter() {

return counter;

}

// A Constructor

public Person() {

counter++;

}

// Another Constructor

public Person(String fName, String lName, int age, int ID) {

this.fName = fName;

this.lName = lName;

this.age = age;

this.ID = ID;

counter++;

}

// Getters and Setters Methods

public String getfName() {

return fName;

}

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public void setfName(String fName) {

this.fName = fName;

}

public String getlName() {

return lName;

}

public void setlName(String lName) {

this.lName = lName;

}

public int getAge() {

return age;

}

public void setAge(int age) {

this.age = age;

}

public int getID() {

return ID;

}

public void setID(int ID) {

this.ID = ID;

}

}

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Netbeans Tips & Tricks

Code Generation Right click on Edit pane and select Insert Code, or simply press (ALT + INSERT)

Then choose what you want to generate. For example, generate Setters and

Getters for the class fields:

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A new window will appear to let you choose the fields that you want to

generate setters and getters for them. Moreover, you can use the same

window to auto-encapsulate the fields:

After you click generate, a new methods will be generated in the class:

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You can also use Code Generator to generate other methods, like a new

constructor(s):

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Result: