Java8 Stream流的常用方法汇总

public class User {
	public User(String name, String age, int height) {
		this.name = name;
		this.age = age;
		this.height = height;
	}
	private String name;
	private String age;
	private int height;
	// setter、getter方法我就不写了
}

// 创建三个user
User user1 = new User("111", "18", 180);
User user2 = new User("222", "18", 175);
User user3 = new User("333", "19", 170);

List<User> userList = new ArrayList<>();
userList.add(user1);
userList.add(user2);
userList.add(user3);

// stream流，创建的是动态数组，可以添加元素
List<User> userList = Stream.of(user1, user2, user3).collect(Collectors.toList());

// 本质是将一个数组转成list，数组的大小是固定的，所以此list不能添加元素
// 如果调用add方法增加新的元素，会报异常：java.lang.UnsupportedOperationException
List<String> s = Arrays.asList("1","2","3") 

// 遍历
List<String> userNameList = new ArrayList<>();
for (User user : userList) {
    userNameList.add(user.getName());
}

// Stream流
List<String> userNameList = userList.stream().map(User::getName).collect(Collectors.toList());

List<User> newUserList = new ArrayList<>();
// if判断
for (User user : userList) {
    if(user.getName() != null) {
        newUserList.add(user); 
    }
}

// 获取userName不为空的user的List
List<User> userList = userList.stream().filter(user-> user.getName() != null).collect(Collectors.toList());

Map<String, List<User>> map = new HashMap<>();
// if判断
for (User user : userList) {
    if (map.get(user.getAge()) == null) {
        map.put(user.getAge(), new ArrayList());
    }
    map.get(user.getAge()).add(user);
}

Map<String, List<User>> map =userList.stream().collect( Collectors.groupingBy(User::getAge, Collectors.toList()));

// int、double、long:
double max = userList.stream().mapToDouble(User::getHeight).sum();

    Map<String, User> userMap = new Map<>();
    for (User user : userList) {
        userMap.put(user.getName(), user);
    }

    Map<String, User> userMap= userList.stream().collect(Collectors.toMap(User::getName, Function.identity(),(key1, key2)->key2));

    List<User> userList = new List<>();
    for (String userName : userMap.keySet()) {
        userList.add(userMap.get(userName));
    }

   List<User> userList = userMap.entrySet().stream().map(e ->e.getValue()).collect(Collectors.toList());

    userList.stream().anyMatch(user -> user.getHeight() > 175);

    userList.stream().allMatch(user -> user.getHeight() > 175);

    userList.stream().noneMatch(user -> user.getHeight() > 175);

    userList.stream().filter(user -> user.getHeight() > 175).count();