Inner Classes

A. Inner Classes in Java

1. What is an Inner Class?

An inner class is a class that is defined within another class. It is logically associated with its enclosing class and can access its private members.

Java supports several types of inner classes:

• Non-static (regular) inner classes
• Static nested classes
• Local classes (defined within a method)
• Anonymous inner classes

2. Why Use Inner Classes?

• To logically group classes that are used only in one place
• To encapsulate helper functionality
• To improve readability and maintainability
• To have access to members of the enclosing class

3. Types of Inner Classes

a. Member Inner Class (Non-static)

• Defined inside a class but outside any method, and not static.

  class Outer {
    private String message = "Hello from Outer";
  
    class Inner {
        void showMessage() {
            System.out.println(message); // can access private field
        }
    }
  }
  

• Usage:

  Outer outer = new Outer();
  Outer.Inner inner = outer.new Inner();
  inner.showMessage();
  

b. Static Nested Class

Declared as static, meaning it does not require an instance of the outer class.

class Outer {
    static class StaticInner {
        void display() {
            System.out.println("Inside static nested class");
        }
    }
}

Usage:

Outer.StaticInner inner = new Outer.StaticInner();
inner.display();

c. Local Inner Class

Declared within a method and can access local variables if they are effectively final.

class Outer {
    void display() {
        int number = 10;

        class LocalInner {
            void show() {
                System.out.println("Local number: " + number);
            }
        }

        LocalInner inner = new LocalInner();
        inner.show();
    }
}

d. Anonymous Inner Class A class without a name, used to create one-time implementations, typically of interfaces or abstract classes.

abstract class Animal {
    abstract void speak();
}

public class Test {
    public static void main(String[] args) {
        Animal dog = new Animal() {
            void speak() {
                System.out.println("Bark!");
            }
        };
        dog.speak();
    }
}

Used widely in event handling, threading, and GUI programming.

4. Access Rules

• Non-static inner classes can access all members (even private) of the outer class.
• Static nested classes can only access static members of the outer class.
• Inner classes can shadow variables from outer classes (though not recommended).