Java 8允许在接口中加入具体方法。接口中的具体方法有两种，default方法和static方法，identity()就是Function接口的一个静态方法。
Function.identity()返回一个输出跟输入一样的Lambda表达式对象，等价于形如t -> t形式的Lambda表达式

private static void identity() {  
    Stream<String> stream = Stream.of("I", "love", "you", "too");  
    Map<String, Integer> map = stream.collect(Collectors.toMap(Function.identity(), String::length));  
    System.out.println(map);  
}

输出结果为：

{love=4, too=3, I=1, you=3}

@Test public void test() {
List personList = new ArrayList<>();
personList.add(new Person("hepengju", 28, 20000.0));
personList.add(new Person("lisi" , 44, 40000.0));
personList.add(new Person("wangwu" , 55, 50000.0));
personList.add(new Person("zhaoliu" , 66, 60000.0));
personList.add(new Person("zhangsan", 33, 33333.0));
personList.add(new Person("wgr", 23, 10000.0));
Map collect = personList.stream().collect(Collectors.toMap(Person::getName, Function.identity()));
collect.forEach((name,p) ->{
System.out.println(name + ":"+p);
});

}