如何实例化抽象类的内部类-Java

Question

Consider

public abstract class Foo{

    ///some code

    class Foo1 extends Foo
    {
        //// some code
    }

    class Foo2 extends Foo
    {
        ///// some code 
    }
}

How can I create an object of the inner classes?

Answer 1

To instantiate nested non-static (inner) class, you need an instance of enclosing class.

But your enclosing class is abstract, so you can't instantiate it. What should you do?

If you really need non-static nested class, then the only option is to extend the abstract class and instantiate the inner class using instance of extending class.

public abstract class Foo {
    public class Foo1 extends Foo {
    }

    public static class FooImpl extends Foo {
    }

    public static void main(String[] args) {
        Foo foo = new FooImpl();
        Foo1 foo1 = foo.new Foo1();
    }
}

Answer 2

You need another top level class extending Foo:

class Foo3 extends Foo {
  Foo foo;

  Foo3(){
   foo= new this.Foo2();
  }
}

or declare a static inner class:

public abstract class Foo{

    ///some code

    class Foo1 extends Foo
    {
        //// some code
    }

    class Foo2 extends Foo
    {
        ///// some code 
    }

    static class Foo3 extends Foo {
      Foo foo;

      Foo3(){
       foo= new this.Foo2();
      }
    }
}