Java allows defining classes within other classes or methods, known as inner classes. These include member, local, anonymous, and static inner classes.

Member Inner Classes

A member inner class is defined with in another class and has access to all members of the outer class, including private and static members.

class Shape {
    private double size = 0;
    
    public Shape(double size) {
        this.size = size;
    }
    
    class Renderer {
        public void display() {
            System.out.println(size);
        }
    }
}

To access outer class members when names conflict:

OuterClass.this.member

Creating an instance requires an outer class instance:

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

Local Inner Classes

Defined within a method or scope, accessible only within that context:

class Person {
    public Person getEmployee() {
        class Employee extends Person {
            int id = 0;
        }
        return new Employee();
    }
}

Cannot use public, protected, private, or static modifiers.

Anonymous Inner Classes

Commonly used for event listeners, implementing interfaces without explicit class definition:

button.addActionListener(new ActionListener() {
    @Override
    public void actionPerformed(ActionEvent e) {
        // Implementation
    }
});

Static Inner Classes

Independent of outer class instances, cannot access non-static outer members:

class Container {
    static class Box {
        public Box() {}
    }
}

// Instantiation
Container.Box box = new Container.Box();

Key Points

• Member inner classes maintain a reference to the outer class
• Local and anonymous classes can only access final local variables
• Static inner classes don't hold outer class references
• Inner classes enable better encapsulation and multipel inheritance simulation