public interface Foo {
  void foo(Bar bar);

  static enum Bar { BAR1, BAR2, BAR3 }
}

public interface Foo {
  void foo(Bar bar);

  static class Bar {
    private final String id;
    public Bar(Sting id) { this.id = id; }
    public String getId() { return id; }
  }
}

public interface Convertor {
  String convert(Object arg);
}

public interface Convertors {
  static class Convertor1 implements Convertor { String convert(Object arg) { /* ... */ }}
  static class Convertor2 implements Convertor { String convert(Object arg) { /* ... */ }}
  static class Convertor3 implements Convertor { String convert(Object arg) { /* ... */ }}
}

public interface Foo {
  void foo() throws BarException;

  static class BarException extends Exception {
     // ...
  }
}

public interface Foo {
  Bar foo();

  static class FooImpl implements Foo {
    @Override
    public void foo() { return null; }
  }
}

@Retention(RetentionPolicy.RUNTIME)
  public @interface Foo {
      // annotation fields

      static class FooHandler implements Handler<Foo> {
          public void handle(Foo foo, Object arg) {
            //...
          }
      }
  }

  public interface Handler<A extends Annotation> {
      void handle(A a, Object arg);
  }