Foo.java

abstract class Foo {
  abstract <T> T match(Visitor<T> visitor);

  interface Visitor<T> {
    T caseBar(Bar b);
    T caseBaz(Baz b);
  }

  static class Bar extends Foo {
    final int bar;

    private Bar(int bar) {
      this.bar = bar;
    }
    
    @Override
    <T> T match(Visitor<T> visitor) {
      return visitor.caseBar(this);
    }
  }

  static class Baz extends Foo {
    final String baz;

    private Baz(String baz) {
      this.baz = baz;
    }
    
    @Override
    <T> T match(Visitor<T> visitor) {
      return visitor.caseBaz(this);
    }
  }

  static void handle(Foo f) {
    System.out.println(f.match(new Visitor<String>() {
      @Override
      public String caseBar(Bar b) {
        return "I have " + b.bar + " bars";
      }
      @Override
      public String caseBaz(Baz b) {
        return "I have the " + b.baz + " baz";
      }
    }));
  }

  public static void main(String[] args) {
    handle(new Bar(42));
    handle(new Baz("Luhrmann"));
  }
}