NestedClass.java

package com.interview.basics.java;

public class NestedClass {

	public static void main(String[] args){
		OuterClass.StaticNestedClass staticNestedClassInstance = new OuterClass.StaticNestedClass();
		staticNestedClassInstance.printOuterClassFields();

        //OuterClass.InnerClass innerClassInstance = new OuterClass.InnerClass(); <- Compiling Error
		OuterClass outerClassInstance = new OuterClass();
		OuterClass.InnerClass innerClassInstance = outerClassInstance.new InnerClass();
		innerClassInstance.printOuterClassFields();
	}
}

class OuterClass {
	private int a = 0;
	int b = 1;
	protected int c = 2;
	public int d = 3;
	private static int e = 4;
	
	private static void invokeStaticMethod(){
		System.out.println("\t Static Method of OuterClass is invoked");
	}
	
	private void invokeInstanceMethod() {
		System.out.println("\t Instance Method of OuterClass is invoked");
	}
	
	// If a nested class is static, it's called static nested class
	static class StaticNestedClass {		
		public void printOuterClassFields(){
			System.out.println("Static Nested Class");
			// System.out.println("\t a is " + a); <- Can't access instance fields
			// invokeInstanceMethod(); <- Can't access instance method
			System.out.println("\t Cannot access instance fields of OuterClass from StaticNestedClass");
			System.out.println("\t Static field e is " + e);
			invokeStaticMethod();
		}
	}
	
	// If a nested class is NOT static, it's called inner class
	class InnerClass{
		public void printOuterClassFields(){
			System.out.println("Inner Class");
			System.out.println("\t Instance Field a is " + a);
			System.out.println("\t Instance Field b is " + b);
			System.out.println("\t Instance Field c is " + c);
			System.out.println("\t Instance Field d is " + d);
			System.out.println("\t Static field e is " + e);
			invokeStaticMethod();
			invokeInstanceMethod();
		}
	}
}