Creating instance of inner class outside the outer class in java

Question

I'm new to Java.

My file A.java looks like this:

public class A {
    public class B {
        int k;
        public B(int a) { k=a; }
    }
    B sth;
    public A(B b) { sth = b; }
}

In another java file I'm trying to create the A object calling

anotherMethod(new A(new A.B(5)));

but for some reason I get error: No enclosing instance of type A is accessible. Must qualify the allocation with an enclosing instance of type A (e.g. x.new B() where x is an instance of A).

Can someone explain how can I do what I want to do? I mean, do I really nead to create instance of A, then set it's sth and then give the instance of A to the method, or is there another way to do this?

Solution 1

In your example you have an inner class that is always tied to an instance of the outer class.

If, what you want, is just a way of nesting classes for readability rather than instance association, then you want a static inner class.

public class A {
    public static class B {
        int k;
        public B(int a) { k=a; }
    }
    B sth;
    public A(B b) { sth = b; }
}

new A.B(4);

Solution 2

Outside the outer class, you can create instance of inner class like this

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

In your case

A a = new A();
A.B b = a.new B(5);

For more detail read Java Nested Classes Official Tutorial

Solution 3

Interesting puzzle there. Unless you make B a static class, the only way you can instantiate A is by passing null to the constructor. Otherwise you would have to get an instance of B, which can only be instantiated from an instance of A, which requires an instance of B for construction...

The null solution would look like this:

anotherMethod(new A(new A(null).new B(5)));