Question

I design a class:

public class CustomEvent< P, T >
{
  /** Facade interface used for adopting user interfaces to our generic class. */
  public interface ICaller< P, T >
  {
    /** callback facade method. */
    void call( P parent, T callback, Object... objects );
  }

  /** Abstract class for simplifying naming in constructor.  */
  public abstract class Caller implements ICaller< P, T >{}

  /** Constructor. */
  public CustomEvent( final String name, final P parent, final Caller caller ){}
}

Now I want to create a instance of such class:

  public class TestClass
  {
    private final TestEvent mEventOnLoad;

    public TestClass()
    {   
      // ERROR here: No enclosing instance of type CustomEvent<P,T> is accessible. 
      // Must qualify the allocation with an enclosing instance of type 
      // CustomEvent<P,T> (e.g. x.new A() where x is an instance of CustomEvent<P,T>).
      mEventOnLoad = new TestEvent( "onLoad", this, new TestEvent.Caller() {
        public void call( TestClass parent, ITestCallbacks callback, Object... objects )
        {
          // some code here
        }
      } );
    }

    private class TestEvent extends CustomEvent< TestClass, ITestCallbacks >
    {
      public TestEvent( String name, TestClass parent, TestEvent.Caller caller )
      {
        super( name, parent, caller );
      }
    };
  }

Is it possible to somehow to workaround that? I want to simplify naming of the classes, instead of long generic declaration use short abstract class name which contains all needed type definitions?

I have a filling that it is possible… but I’m not very comfortable with Java yet…

Solution

    private class TestEvent extends CustomEvent< TestClass, ITestCallbacks >
    { 
      public final static TestEvent Instance = new TestEvent( null, null, null );

      public TestEvent( String name, TestClass parent, TestEvent.Caller caller )
      {
        super( name, parent, caller );
      }
    };

    public TestClass()
    {   
      mEventOnLoad = new TestEvent( "onLoad", this, TestEvent.Instance.new Caller() {
        public void call( TestClass parent, ITestCallbacks callback, Object... objects )
        {
          // some code here
        }
      } );
    }

Answer 1

If you’re just trying to get your code to compile (mind you, you’d need to fix several issues with type safety and parameterising), try something like this:

public TestClass() {
    CustomEvent bla = new CustomEvent("name", null, null); // TODO: parameterise
    mEventOnLoad = new TestEvent("onLoad", this, bla.new Caller() {
        @Override
        public void call(Object parent, Object callback, Object... objects) {
            // some code here
        }
    });
}

What the error message was trying to say is that you need an instance (bla) to call new on. I’d suggest reading up on inner classes a bit 😉