记录Java Steam流的一些用法

基础示例类：
Use.java

public class User {

    private String userId;

    private String userName;

    private Integer age;

    private String region;
}

 List userList = new ArrayList<>();
 userList.add(new User("1", "张三", 20, "上海"));
 userList.add(new User("2", "李四", 20, "北京"));
 userList.add(new User("3", "王五", 25, "上海"));

 userList.stream().forEach(user -> {
     // 操作
     System.out.println(user.getUserName() + ":" + user.getAge() + ":" + user.getRegion());
 });

 // 与上面方法等同
 userList.forEach(user -> {
     // 操作
     System.out.println(user.getUserName() + ":" + user.getAge() + ":" + user.getRegion());
 });

例如需要对User里统一地区的人进行分组

 List userList = new ArrayList<>();
 userList.add(new User("1", "张三", 20, "上海"));
 userList.add(new User("2", "李四", 20, "北京"));
 userList.add(new User("3", "王五", 25, "上海"));

 Map> collect = userList.stream().collect(Collectors.groupingBy(User::getRegion));
 System.out.println(collect);

结果：

{
	上海=[
		User(userId=1, userName=张三, age=20, region=上海),
		User(userId=3, userName=王五, age=25, region=上海)
		], 
	北京=[
		User(userId=2, userName=李四, age=20, region=北京)
		]
}

例如获取User列表所有用户姓名

 List userList = new ArrayList<>();
 userList.add(new User("1", "张三", 20, "上海"));
 userList.add(new User("2", "李四", 20, "北京"));
 userList.add(new User("3", "王五", 25, "上海"));

 String userNameStr = userList.stream().map(User::getUserName).collect(Collectors.joining(","));
 System.out.println(userNameStr);

结果：

张三,李四,王五