Cannot Instantiate Private Class

Question

I am a first time user of Java swing and this is the first time I'm trying to use a private class.

I am trying the following code -

ActionListener listener = new AddButtonListener();

Where AddButtonListener is a private class that implements the ActionListener interface.

private class AddButtonListener implements ActionListener{
  public void actionPerformed(ActionEvent e){
  ....
  }
}

However, I am getting an eclipse error that reads

No enclosing instance of type someType is accessible. Must qualify the allocation with an enclosing instance of type someType (e.g. x.new A() where x is an instance of someType).

Note that the class is being instantiated in a static main method inside someType.

Why is this error coming up? Is it because the main method is static?

because only one specific button needs to use it- it makes no sense for anyone else to know about it — praks5432, Oct 21 '12 at 03:43
Vikdor answer is correct assuming your listener class is inside another class. — kosa, Oct 21 '12 at 03:46
Read this http://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html — kosa, Oct 21 '12 at 03:46
Possible duplicate of [Java - No enclosing instance of type Foo is accessible](http://stackoverflow.com/questions/9560600/java-no-enclosing-instance-of-type-foo-is-accessible) — fabian, Mar 04 '16 at 00:22

Vikdor · Accepted Answer · 2012-10-21T03:51:24.303

5

Since AddButtonListener is an inner class and is not static, it can be instantiated only using an object of outer class.

For example, if your AddButtonListener class is defined in SomeType, then

SomeType obj = new SomeType();

SomeType.AddButtonListener listener = obj.new AddButtonListener();

If you are in some method in SomeType, then you would create an object of this non-static inner class as

AddButtonListener listener = this.new AddButtonListener();

If you want to create an instance of AddButtonListener without using an instance of SomeType (enclosing type), then you should mark AddButtonListener as static class.

private static class AddButtonListener implementsActionListener{
    public void actionPerformed(ActionEvent e){
        ....
    }
}

So, it's not about the class being private, but about it not being static.

edited Oct 21 '12 at 03:51

answered Oct 21 '12 at 03:45

Vikdor

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well, what I'm really trying to do is have a class that sets up a GUI (adds everything properly to a bunch of frames and sets up listeners) - what's the best way to do that? – praks5432 Oct 21 '12 at 03:48
1

Since an AddButtonListener doesn't seem to make sense without the enclosing frame/window, I would still leave it as a non-static inner class and use the `this.new ` syntax to create an object of the listener and associate it with the window control (please see my update with the this.new syntax). – Vikdor Oct 21 '12 at 03:51

score 2 · Answer 2 · answered Oct 21 '12 at 03:55

Exception message pretty much explains all.

A private class require an instance of its enclosing type exist in order to instantiate. Since a private class can access members in an enclosing type of instance, if there is no such instance exist, where can a private class instance access the member in enclosing type ?

Back to your question.

If you want to create an instance without having to create other instance. You can declare your AddButtonListener as private static class, but this will make AddButtonListener lose the ability to access the member in SomeType class insatnce.

or, you have to use the syntax like this.

SomeType t = new SomeType();
AddButtonListener listener = t.new AddButtonListener();

However, I cannot determine which way is correct in your case, since you didn't provide any detail code. Because in my experience, you shouldn't have such problem when you using a private class in its enclosing class. Unless you're trying to test it in a main method or something else..

score 2 · Answer 3 · answered Oct 21 '12 at 03:59

Vikdor is right. But for such a case (the listener is used just once) you usually use a anonymous class:

...
ActionListener listener = new ActionListener(){

    @Override
    public void actionPerformed(ActionEvent e) {
        // TODO Auto-generated method stub          
    }       

};
...

Here you define the class onde inside the method by extending the needed interface or class.

Cannot Instantiate Private Class

3 Answers3