Java language uses access modifiers to set the level of the visibility of a class to another class. A class has access to another class if it can:

• Initialize that class
• Extend that class
• Access methods and instance variables defined in that class

If a class Class1 has access to the class Class2, Class2 is visible to Class1. In the same way, if a method or instance variable of Class2 can be accessed by Class1, then this method or instance variable is said to be visible to Class1.

A class has access to all the methods and variables that it defines.

Now let’s see the rules of class access in Java.

For classes:

1- A public class is visible to all classes. It can be initialized by any class if it has a public constructor.

2- A protected class is visible to its subclasses and the other classes define in the same package.

A class cannot be declared private.

A class A’s methods and variables can be visible to class B only if the class A is visible to class B.

For methods and variables:

1- Public instance variables and methods are visible to any other class.

2- Protected instance variables and methods are visible to only classes inside the same package and to the subclasses.

3- Instance variables and methods with default access are visible to only the classes inside the same package.

4- Private methods and fields can be accessed only by methods inside the same class, including the main method.

For class A’s x method to be visible to class B

1- Class A should have access to class B

2- Method x should be visible to class B

Now let’s see this with examples.

package com.mycompany;

public class Person

{

public String firstName;

String middleName;

protected String lastName;

private int age;

public void sayName(){

System.out.println(“My name is + firstName + “ “ + lastName);

}

private void sayAge(){

System.out.println(“I am “ + age + “ years old.”);

}

}

package com.mycompany;

//PersonInfo class is defined inside the same package.

public class PersonInfo {

public static void main(String [] args) {

Person person = new Person();

person.firstName = “James”; // public instance variable is accessible by all the other classes.

person.middleName = “Stanley”; // instance variable with default access is accessible by the classes inside the same package.

person.lastName = “West”;//protected instance variable is accesible by the classes inside the same package.

//This would cause a compile error because age is private.

//person.age = 48;

//This would again cause a compile error because age is private and not visible to this class.

//System.out.println(p.firstName + “ “ + p.lastName + “ is “ + p.age + “ years old.”);

}

package com.mycompany.examples;

//PersonInfo class is defined inside the same package.

public class Example {

public static void main(String [] args) {

Person person = new Person();

person.firstName = “James”; // public instance variable is accessible by all the other classes.

//This would cause a compile error since instance variable with default access is not accessible by the classes outside the same package.

//person.middleName = “Stanley”;

//This would cause a compile error since protected instance variable is not accessible by the classes outside the same package.

//person.lastName = “West”;

//This would cause a compile error because age is private.

//person.age = 48;

//This would again cause a compile error because age is private and not visible to this class.

//System.out.println(p.firstName + “ “ + p.lastName + “ is “ + p.age + “ years old.”);

}

package com.mycompany.examples;

public SuperClass extends Person {

public static void main(String [] args) {

Person person = new Person();

person.firstName = “James”; // public instance variable is accessible by all the other classes.

//This would cause a compile error since instance variable with default access is not accessible by child classes.

//person.middleName = “Stanley”;

//This would cause a compile error since protected instance variable is not accessible by the subclasses.

//person.lastName = “West”;

//This would cause a compile error because age is private.

//person.age = 48;

//This would again cause a compile error because age is private and not visible to this class.

//System.out.println(p.firstName + “ “ + p.lastName + “ is “ + p.age + “ years old.”);

}

}

Final classes

final is one of the three modifiers allowed in class declarations along with public and protected. A final class cannot be extended. In other words, a final class does not have subclasses. A final class can extend other classes.

public class MainClass {

}

public final class FinalClass extends MainClass{

}

//This would cause a compile error since final classes cannot be extended.
//public SubClass extends FinalClass {

//}

For example, String class in Java is a final class and cannot be extended.