Thursday, 4 August 2011

Java Access Specifiers

The access to classes, constructors, methods and fields are regulated using access modifiers i.e. a class can control what information or data can be accessible by other classes. To take advantage of encapsulation, you should minimize access whenever possible.
Java provides a number of access modifiers to help you set the level of access you want for classes as well as the fields, methods and constructors in your classes. A member has package or default accessibility when no accessibility modifier is specified.

Access Modifiers

1. private
2. protected
3. default
4. public

public access modifier

Fields, methods and constructors declared public (least restrictive) within a public class are visible to any class in the Java program, whether these classes are in the same package or in another package.

private access modifier

The private (most restrictive) fields or methods cannot be used for classes and Interfaces. It also cannot be used for fields and methods within an interface. Fields, methods or constructors declared private are strictly controlled, which means they cannot be accesses by anywhere outside the enclosing class. A standard design strategy is to make all fields private and provide public getter methods for them.

protected access modifier

The protected fields or methods cannot be used for classes and Interfaces. It also cannot be used for fields and methods within an interface. Fields, methods and constructors declared protected in a superclass can be accessed only by subclasses in other packages. Classes in the same package can also access protected fields, methods and constructors as well, even if they are not a subclass of the protected member’s class.

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default access modifier

Java provides a default specifier which is used when no access modifier is present. Any class, field, method or constructor that has no declared access modifier is accessible only by classes in the same package. The default modifier is not used for fields and methods within an interface.
Below is a program to demonstrate the use of public, private, protected and default access modifiers while accessing fields and methods. The output of each of these java files depict the Java access specifiers.
The first class is SubclassInSamePackage.java which is present in pckage1 package. This java file contains the Base class and a subclass within the enclosing class that belongs to the same class as shown below.

package pckage1;

class BaseClass {

 public int x = 10;
 private int y = 10;
 protected int z = 10;
 int a = 10; //Implicit Default Access Modifier
 public int getX() {
  return x;
 }
 public void setX(int x) {
  this.x = x;
 }
 private int getY() {
  return y;
 }
 private void setY(int y) {
  this.y = y;
 }
 protected int getZ() {
  return z;
 }
 protected void setZ(int z) {
  this.z = z;
 }
 int getA() {
  return a;
 }
 void setA(int a) {
  this.a = a;
 }
}

public class SubclassInSamePackage extends BaseClass {

 public static void main(String args[]) {
  BaseClass rr = new BaseClass();
  rr.z = 0;
  SubclassInSamePackage subClassObj = new SubclassInSamePackage();
  //Access Modifiers - Public
  System.out.println("Value of x is : " + subClassObj.x);
  subClassObj.setX(20);
  System.out.println("Value of x is : " + subClassObj.x);
  //Access Modifiers - Public
  //     If we remove the comments it would result in a compilaton
  //     error as the fields and methods being accessed are private
  /*     System.out.println("Value of y is : "+subClassObj.y);

   subClassObj.setY(20);

   System.out.println("Value of y is : "+subClassObj.y);*/
  //Access Modifiers - Protected
  System.out.println("Value of z is : " + subClassObj.z);
  subClassObj.setZ(30);
  System.out.println("Value of z is : " + subClassObj.z);
  //Access Modifiers - Default
  System.out.println("Value of x is : " + subClassObj.a);
  subClassObj.setA(20);
  System.out.println("Value of x is : " + subClassObj.a);
 }
}

Output

Value of x is : 10
Value of x is : 20
Value of z is : 10
Value of z is : 30
Value of x is : 10
Value of x is : 20
The second class is SubClassInDifferentPackage.java which is present in a different package then the first one. This java class extends First class (SubclassInSamePackage.java).

import pckage1.*;

public class SubClassInDifferentPackage extends SubclassInSamePackage {

 public int getZZZ() {
  return z;
 }

 public static void main(String args[]) {
  SubClassInDifferentPackage subClassDiffObj = new SubClassInDifferentPackage();
  SubclassInSamePackage subClassObj = new SubclassInSamePackage();
  //Access specifiers - Public
  System.out.println("Value of x is : " + subClassObj.x);
  subClassObj.setX(30);
  System.out.println("Value of x is : " + subClassObj.x);
  //Access specifiers - Private
  //      if we remove the comments it would result in a compilaton
  //      error as the fields and methods being accessed are private
  /*      System.out.println("Value of y is : "+subClassObj.y);

   subClassObj.setY(20);

   System.out.println("Value of y is : "+subClassObj.y);*/
  //Access specifiers - Protected
  //      If we remove the comments it would result in a compilaton
  //      error as the fields and methods being accessed are protected.
  /*      System.out.println("Value of z is : "+subClassObj.z);

   subClassObj.setZ(30);

   System.out.println("Value of z is : "+subClassObj.z);*/
  System.out.println("Value of z is : " + subClassDiffObj.getZZZ());
  //Access Modifiers - Default
  //      If we remove the comments it would result in a compilaton
  //      error as the fields and methods being accessed are default.
  /*

   System.out.println("Value of a is : "+subClassObj.a);

   subClassObj.setA(20);

   System.out.println("Value of a is : "+subClassObj.a);*/
 }
}

Output

Value of x is : 10
Value of x is : 30
Value of z is : 10
The third class is ClassInDifferentPackage.java which is present in a different package then the first one.

import pckage1.*;

public class ClassInDifferentPackage {

 public static void main(String args[]) {
  SubclassInSamePackage subClassObj = new SubclassInSamePackage();
  //Access Modifiers - Public
  System.out.println("Value of x is : " + subClassObj.x);
  subClassObj.setX(30);
  System.out.println("Value of x is : " + subClassObj.x);
  //Access Modifiers - Private
  //      If we remove the comments it would result in a compilaton
  //      error as the fields and methods being accessed are private
  /*      System.out.println("Value of y is : "+subClassObj.y);

   subClassObj.setY(20);

   System.out.println("Value of y is : "+subClassObj.y);*/
  //Access Modifiers - Protected
  //      If we remove the comments it would result in a compilaton
  //      error as the fields and methods being accessed are protected.
  /*      System.out.println("Value of z is : "+subClassObj.z);

   subClassObj.setZ(30);

   System.out.println("Value of z is : "+subClassObj.z);*/
  //Access Modifiers - Default
  //      If we remove the comments it would result in a compilaton
  //      error as the fields and methods being accessed are default.
  /*      System.out.println("Value of a is : "+subClassObj.a);

   subClassObj.setA(20);

   System.out.println("Value of a is : "+subClassObj.a);*/
 }
}

Output

Value of x is : 10
Value of x is : 30
Download all the 3 source files

Java program codes

Thursday, 4 August 2011

Java Access Specifiers

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