Nested class in Java

Question

I need to have nested classes in Java. Why the following implementation doesn’t work?

public class A{
    private class B{
        public B(){
            System.out.println("class B");
        }
    }

    public static void main(String[] args){
         A a = new A();
         B b = new B();
    }
} 

On the line I try to instantiate the Inner class B, the following error occurs:

No enclosing instance of type A is accessible

Answer 1

Your code does not work because you are trying to access a non-static member of A (the nested class B) from a static method (static main); and you can only access non-static members of a class from a class instance.

So for your code to work, you have 3 options:

Instantiate B from an instance of A:

public class A{
    private class B{
        public B(){
            System.out.println("class B");
        }
    }

    public static void main(String[] args){
         A a = new A();
         B b = a.new B(); // instância de B a partir de uma instância de A
    }
} 

Change the method main for non-static, because if it is an instance method, it has access to non-static members of its class:

public class A{
     private class B{
        public B(){
            System.out.println("class B");
        }
    }
    // instância de B a partir de um método não estático de A    
    public void doSomething(String[] args){ 
        // A a = new A();
         B b = new B();
    }
} 

Or declare B as a static class, so it is accessible from static members (in this case, the method main).

public class A{
     // B declarado como classe estática
     private static class B{
        public B(){
            System.out.println("class B");
        }
    }

    public static void main(String[] args){
         A a = new A();
         B b = new B();
    }
} 

Update: In the second example, I changed the name of the method so that it would not be confused as an input method for a Java main class, as it would need to be static. This code has only the didactic role on nested classes.

Answer 2

You can’t instantiate in the direct way you did. A reference to the parent class is required so first you need to create an instance of it and in this instance access the inner class. Look at this example I got from a tutorial on internal classes:

public class InnerClassTest {
    public void foo() {
        System.out.println("Outer class");
    }

    public class ReallyInner {
        public void foo() {
            System.out.println("Inner class");
        }

        public void test() {
            this.foo();
            InnerClassTest.this.foo();
        }
    }

    public static void main(String[] args) {
        InnerClassTest o = new InnerClassTest();
        InnerClassTest.ReallyInner i = o.new ReallyInner();
        i.test();
    }
}

I put in the Github for future reference.