Why can I not access an inner class in Java?

Question

I am having a trouble using nested classes in Java, does anyone know why Java does not allow me to do it?

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();
    }
}

You should not use inner classes in first place :). Do you really need it? — libik, Oct 20 '14 at 19:50
@libik: There's nothing wrong with inner classes, but like most features of a language, you can abuse it. — Makoto, Oct 20 '14 at 19:51
Just a thought... always include the type or error that you are experiencing as well when posting questions. — cp5, Oct 20 '14 at 19:52
@Makoto - well there is nothing wrong with `instanceof` or `singleton` pattern, but it usually means bad design, if you need it. — libik, Oct 20 '14 at 19:54

Victor2748 · Answer 1 · 2014-12-30T03:18:16.070

Because you are trying to access a non-static inner-class from a static method.

The 1st solution will be to change your inner-class B to static:

public class A{
    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();
    }
}

a static inner-class is accessible from anywhere, but a non-static one, requires an instance of your container class.

Another solution will be:

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

This will give you a better understanding of how inner classes work in Java: http://www.javaworld.com/article/2077411/core-java/inner-classes.html

score 7 · Answer 2 · answered Oct 20 '14 at 19:50

7

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

can solve your problem

You used private inner class.How can you get instance outside the A class?

public class JustForShow {
    public class JustTry{

        public JustTry() {
            System.out.println("Initialized");
        }
    }
    public static void main(String[] args) {
        JustForShow jfs = new JustForShow();
        JustTry jt = jfs.new JustTry();
    }
}

answered Oct 20 '14 at 19:50

Gurkan İlleez

1,503
1
10
12

Reread your code there matey. – christopher Oct 20 '14 at 19:51
1

This is valid java code that does indeed create an instance of B. – that other guy Oct 20 '14 at 19:54

score 4 · Answer 3 · answered Oct 20 '14 at 19:52

You are trying to access a non-static membor from a static method. To solve that, you have two options:

Change your class B to be static, so add the static keyword in the class definition, like this:
```
public static class B { // ...
```
Change your main method, and use the created instance a to create B, like this:
```
B b = a.new B();
```

If B doesn't use any non-static resources of class A, I would recommend to use the first method.

score 1 · Answer 4 · answered Oct 20 '14 at 19:54

1

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

answered Oct 20 '14 at 19:54

Schroed

194
10

Why can I not access an inner class in Java?

4 Answers4