内部类对象作为外部类构造函数参数

Question

I have an abstract wrapping class Foo which get functionality defined by providing it with the Interface Reader. All works well when I'm implementing a seperate Reader and provide it. It goes wrong when I'm, trying to do this via a inner class. Having the Reader implementation in an inner class is a requirement for me.

public abstract class Foo
    {
        private Reader reader;

        public Foo(Reader reader)
        {
            this.reader = reader;
        }

        public void read()
        {
            this.reader.doit();
        }
    }

"No enclosing instance of type MapLink is available due to some intermediate constructor invocation"

public class ReaderFoo extends Foo
{
    public class FooReader implements Reader
    {
        @Override
        public void doit()
        {
            // functionality
        }
    }   

    public ReaderFoo ()
    {
        super(new FooReader());
    }
}

What am I doing wrong?

Answer 1

Try making FooReader static . Inner classes in Java are bound to an instance of the outer rather than the class unless they're static .

public class ReaderFoo extends Foo
{
    public static class FooReader implements Reader
    {
        @Override
        public void doit()
        {
            // functionality
        }
    }   

    public ReaderFoo ()
    {
        super(new FooReader());
    }
}

You cannot use an instance inner class before actually having an instance, as the actual type of the Reader is going to be like myInstance.Reader .

Answer 2

The issue is that the constructor to FooReader requires the outer class ( ReaderFoo ) to be instantiated before it (because inner classes store a reference to their containing instance), but you're making one in the constructor to ReaderFoo . It's a chicken-and-egg problem.