Java Stream Collectors的toList()、toSet()、toCollection()和toMap()的使用

Stream是Java 8的新特性，基于lambda表达式，是对集合对象功能的增强，它专注于对集合对象进行各种高效、方便聚合操作或者大批量的数据操作，提高了编程效率和代码可读性。Collectors通常在Stream处理后，返回转换成集合类时使用，本文主要介绍Java Stream中Collectors.toList()、Collectors.toSet()、Collectors.toCollection()和Collectors.toMap()的使用，以及相关的示例代码。

1、Collectors.toList()的使用

Collectors.toList()是将Stream转换成List,List为ArrayList，需要转换成其它，则需要使用Collectors.toCollection()，例如，

import java.util.*;import java.util.stream.Collectors;import java.util.stream.Stream;public class Main {   public static class Person {     int age;     Person (int age) {         this.age = age;     }     public int getAge() {         return age;     }     public void setAge(int age) {         this.age = age;     }  }public static void main(String[] args) {    List pList = Arrays.asList(new Person(11),new Person(22), new Person(33), new Person(44), new Person(55));    List result = pList.stream()                                .map(o -> {                            o.setAge(18);                            return o;                        }).collect(Collectors.toList());    System.out.println("stream() result ："+result.getClass());    System.exit(0); //success  }}

2、Collectors.toSet()的使用

Collectors.toSet()是将Stream转换成Set,Set为HashSet，需要转换成其它，则也需要使用Collectors.toCollection()，例如，

import java.util.*;import java.util.stream.Collectors;import java.util.stream.Stream;public class Main {   public static class Person {     int age;     Person (int age) {         this.age = age;     }     public int getAge() {         return age;     }     public void setAge(int age) {         this.age = age;     }  }public static void main(String[] args) {    List pList = Arrays.asList(new Person(11),new Person(22), new Person(33), new Person(44), new Person(55));    Set setResult = pList.stream()                                .map(o -> {                            o.setAge(18);                            return o;                        }).collect(Collectors.toSet());    System.out.println("stream() result ："+setResult.getClass());    System.exit(0); //success  }}

3、Collectors.toCollection()的使用

toList()和toSet()使用时，需要转换成指定的集合类型，则可以使用Collectors.toCollection()，如下，

import java.util.*;import java.util.stream.Collectors;import java.util.stream.Stream;public class Main {   public static class Person {     int age;     String name;     Person (int age,String name) {         this.age = age;         this.name = name;     }     public int getAge() {         return age;     }     public void setAge(int age) {         this.age = age;     }     public String getName() {         return name;     }     public void setName(String name) {         this.name = name;     }  }public static void main(String[] args) {    List pList = Arrays.asList(new Person(11,"mike"),new Person(22,"lynn"), new Person(33,"john"), new Person(44,"mickey"), new Person(55,"fiona"));    List listResult = pList.stream().collect(Collectors.toCollection(LinkedList::new));    System.out.println("stream() result ："+listResult.getClass());    System.exit(0); //success  }}

4、Collectors.toMap()的使用

Collectors.toMap()是将Stream转换成Map,Map为HashMap，需要转换成其它，则也需要使用Collectors.toCollection()，

例如，

import java.util.*;import java.util.stream.Collectors;import java.util.stream.Stream;public class Main {   public static class Person {     int age;     String name;     Person (int age,String name) {         this.age = age;         this.name = name;     }     public int getAge() {         return age;     }     public void setAge(int age) {         this.age = age;     }     public String getName() {         return name;     }     public void setName(String name) {         this.name = name;     }  }public static void main(String[] args) {    List pList = Arrays.asList(new Person(11,"mike"),new Person(22,"lynn"), new Person(33,"john"), new Person(44,"mickey"), new Person(55,"fiona"), new Person(31,"fiona"));    //(oldValue, newValue) -> newValue是重复key时，使用的值，可以指定    Map map = pList.stream().collect(Collectors.toMap(Person::getName, Person::getAge,(oldValue, newValue) -> newValue));    System.out.println("stream() result ："+map);    System.out.println("stream() result ："+map.getClass());    //指定转换成集合类型LinkedHashMap    Map map1 = pList.stream().collect(Collectors.toMap(Person::getName, Person::getAge,(oldValue, newValue) -> newValue,LinkedHashMap::new));    System.out.println("stream() result ："+map1);    System.out.println("stream() result ："+map1.getClass());    System.exit(0); //success  }}