Java default visitor

DefaultFooVisitor.java

class DefaultFooVisitor<T> implements Foo.Visitor<T> {
  T defaultValue;

  private DefaultFooVisitor(T defaultValue) {
    this.defaultValue = defaultValue;
  }

  @Override
  public T caseBar(Bar b) {
    return defaultValue;
  }

  @Override
  public T caseBaz(Baz b) {
    return defaultValue;
  }

  static int countBars(Foo f) {
    return f.match(new DefaultFooVisitor<Integer>(0) {
      @Override
      public Integer caseBar(Bar b) {
        return b.bar;
      }
    });
  }
}