Java Function接口实现分组

1. Function接口介绍

详细介绍：http://t.csdn.cn/3U1Yx
简单测试：

package ClassLearn;
import java.util.function.Function;
public class MyFunction {
    public static void main(String[] args) {
        /*lambda表达式*/
        Function<Integer,Integer> test1= (i)->i+1;
        Function<Integer,Integer> test2=(i)->i*i;
        System.out.println(calculate(test1,5));
        System.out.println(calculate((i)->i*i,5));

    }
    /*T:是输入数据类型
    * R:是返回数据类型
    * */
    //相当于返回 y=f(x)
    public static <T,R> Integer calculate(Function<T,R> test,T number){
        return new Integer((int)test.apply(number));
    }
}

2 . 分组实现

package test;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
public class FunctionTest {
     static class Person{
        private String name;
        private String country;
        private int age;

        public Person(String name,String country, int age) {
            this.name = name;
            this.age = age;
            this.country =country;
        }

        public Person() {
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public String getCountry() {
            return country;
        }

        public void setCountry(String country) {
            this.country = country;
        }

        public int getAge() {
            return age;
        }

        public void setAge(int age) {
            this.age = age;
        }

        @Override
        public String toString() {
            return "person{" +
                    "name='" + name + '\'' +
                    ", conuntry='" + country + '\'' +
                    ", age=" + age +
                    '}';
        }
    }
      /*要实现按国家分组*/
    public static void main(String[] args) {
        Person p1=new Person("小林","中国",25);
        Person p2=new Person("小P","中国",23);
        Person p3=new Person("小F","美国",24);
        Person p4=new Person("小明","美国",22);
        List<Person> list=new ArrayList<>();
        list.add(p1);list.add(p2);list.add(p3);list.add(p4);
        /*lambda表达式*/
        Function<Person,String> function=(country)-> country.getCountry();
        Map<Object,List<Person>> splitMap=splitByCountry(list,function);
        splitMap.forEach((key,person)-> System.out.println(key+":"+person));

    }
     /*分组*/
    public static <T,R> Map<Object, List<T>> splitByCountry(List<T> datas, Function<T,R> function){
        Map<Object,List<T>> map=new HashMap<>();
        /*取出list中每一个person（data）*/
        for(T data: datas){
            /*获取键值（country）*/
            List<T> key=map.get(function.apply(data));
            /*map中已经存放了该键值*/
            if(key!=null)
            {
                /*同一类键值person放一起*/
                key.add(data);
            }
            else {
                /*map中还没有存放该键值*/
                List<T> group=new ArrayList<>();
                group.add(data);
                map.put(function.apply(data),group);
            }
        }
        return map;
    }
}

运行结果！