Foreword:

The stream() method has been around since Java 8. It is a product based on Lamada expressions. You can also simply view the stram stream as a foreach. More convenient for manipulating arrays or collections. And more secure. Because it is equivalent to doing a layer of SQL operations after fetching our data source, but it does not change the data source, but creates a new container to store the operated data.

Environment construction:

In order to be more in line with the business. We create an entity.

package org.java.demo;

import java.io.Serializable;

public class User implements Serializable {
    //用户ID
    private Long id;

    //用户姓名
    private String name;

    //用户年龄
    private Integer age;

    //用户邮箱
    private String email;

    //↓get set 无参、有参构造器
    //.....
}

public static void main(String[] args) {
        User one = new User(1L,"one",12,"[email protected]");
        User two = new User(2L,"two",17,"[email protected]");
        User three = new User(3L,"three",25,"[email protected]");
        User four = new User(4L,"four",43,"[email protected]");
        List<User> list = new ArrayList<>();
        list.add(one);
        list.add(two);
        list.add(three);
        list.add(four);
    }

Business practice

1. Take out an attribute of the collection object and save it in the new List collection.

list.stream().map(User::getName).collect(Collectors.toList());

2. Take out two attributes in the object and become a map collection in the form of key-value.

Note that the key value is the only attribute in the data source.

//id为key，name为value
list.stream().collect(Collectors.toMap(User::getId, User::getName));
//id为key，User对象为value  业务中比较常见。一般避免双重for循环时使用
list.stream().collect(Collectors.toMap(User::getId, User -> User));

3. Take out a certain value type attribute of the collection object and add it

list.stream().mapToInt(User::getAge).sum();

4. Take out a certain attribute in the collection object and remove it to form a new collection.

list.stream().map(User::getEmail).distinct().collect(Collectors.toList());

5. Take out an attribute in the collection object for string splicing

//     使用流      取属性         操作        工具类的拼接方法  拼接符
list.stream().map(User::getName).collect(Collectors.joining("-"));
//输出：one-two-three-four

6. Determine whether an attribute in the collection object meets the requirements

//判断所有属性是否满足要求，全部满足返回true
list.stream().allMatch(k -> k.getAge() > 18);
//输出:flase

//判断所有属性中只要有满足要求的便会返回true
list.stream().anyMatch(k -> k.getAge() > 18);
//输出：true

7. Filter a certain attribute value in the combined object

//过滤结合对象中，年龄大于20的封装成新集合
//              流    Lamda          判断        组装成新集合
list.stream().filter(k -> k.getAge() > 20).collect(Collectors.toList());

Summarize:

These are a few of the more commonly used methods in my work. There are still many methods that need to be supplemented.

Common usage of Stream stream