Java CS Point: More topics on Method Overriding (Not For Beginners)

1) private access modifier

The private access modifier is accessible only within class.

Simple example of private access modifier

In this 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 compile time error.

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);//Compile Time Error
obj.msg();//Compile Time Error
}
}

Role of Private Constructor

If you make any class constructor private, you cannot create the instance of that class from outside the class. For example:

class A{
private A(){}//private constructor
void msg(){System.out.println("Hello java");}
}
public class Simple{
public static void main(String args[]){
A obj=new A();//Compile Time Error
}
}

Note: A class cannot be private or protected except nested class.

2) default access modifier

If you don't use any modifier, it is treated as default bydefault. The default modifier is accessible only within package.

Example of default access modifier

In this example, we have created two packages pack and mypack. We are accessing the A class from outside its package, since A class is not public, so it cannot be accessed from outside the package.

//save by A.java
package pack;
class A{
void msg(){System.out.println("Hello");}
}

//save by B.java
package mypack;
import pack.*;
class B{
public static void main(String args[]){
A obj = new A();//Compile Time Error
obj.msg();//Compile Time Error
}
}

In the above example, the scope of class A and its method msg() is default so it cannot be accessed from outside the package.

3) protected access modifier

The protected access modifier is accessible within package and outside the package but through inheritance only.

The protected access modifier can be applied on the data member, method and constructor. It can't be applied on the class.

Example of protected access modifier

In this example, we have created the two packages pack and mypack. The A class of pack package is public, so can be accessed from outside the package. But msg method of this package is declared as protected, so it can be accessed from outside the class only through inheritance.

//save by A.java
package pack;
public class A{
protected void msg(){System.out.println("Hello");}
}

//save by B.java
package mypack;
import pack.*;
class B extends A{
public static void main(String args[]){
B obj = new B();
obj.msg();
}
}

Output:Hello

4) public access modifier

The public access modifier is accessible everywhere. It has the widest scope among all other modifiers.

Example of public access modifier

//save by A.java
package pack;
public class A{
public void msg(){System.out.println("Hello");}
}

//save by B.java
package mypack;
import pack.*;
class B{
public static void main(String args[]){
A obj = new A();
obj.msg();
}
}

Output:Hello

Understanding all java access modifiers

Let's understand the access modifiers by a simple table.

Access Modifier	within class	within package	outside package by subclass only	outside package
Private	Y	N	N	N
Default	Y	Y	N	N
Protected	Y	Y	Y	N
Public	Y	Y	Y	Y

Java access modifiers with method overriding

If you are overriding any method, overridden method (i.e. declared in subclass) must not be more restrictive.

class A{
protected void msg(){System.out.println("Hello java");}
}
public class Simple extends A{
void msg(){System.out.println("Hello java");}//C.T.Error
public static void main(String args[]){
Simple obj=new Simple();
obj.msg();
}
}

The default modifier is more restrictive than protected. That is why there is compile time error.

Java CS Point

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Friday 27 March 2015

More topics on Method Overriding (Not For Beginners)