???

JDK 7 way

public interface Mapper<T, V> {
  public V map(T t) ;
}

public interface Predicate<T> {
  public boolean filter(T t) ;
}

public interface Reducer<T> {
  public T reduce(T t1, T t2) ;
}

List<Person> persons = ... ;
int sum =
  persons.map(
    new Mapper<Person, Integer>() {
    public Integer map(Person p) {
      return p.getAge() ;
    }
  })
.filter(
  new Filter<Integer>() {
    public boolean filter(Integer age) {
      return age > 20 ;
    }
  })
.reduce(0,
  new Reducer<Integer>() {
    public Integer reduce(Integer i1, Integer i2) {
      return i1 + i2 ;
    }
  }
}) ;