一、合并出共有部分

package com.xu.demo.test;

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class ListMergeTest1 {

    public static void main(String[] args) {
        List<User> list1 = Arrays.asList(new User(1, "Alice"), new User(2, "Bob"), new User(4, "Ronnie"));
        List<User> list2 = Arrays.asList(new User(2, "Bobby"), new User(3, "Charlie"), new User(5, "Trump"));

        List<User> mergedList = list1.stream()
                .flatMap(user1 -> list2.stream()
                        .filter(user2 -> user1.getId() == user2.getId())
                        .map(user2 -> {
                            User newUser = new User();
                            newUser.setId(user2.getId());
                            newUser.setName(user2.getName());
                            return newUser;
                        })
                )
                .collect(Collectors.toList());

        mergedList.forEach(System.out::println);
    }

}

class User {
    private int id;
    private String name;

    public User(int id, String name) {
        this.id = id;
        this.name = name;
    }

    // Getter and Setter methods
    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public void setId(int id) {
        this.id = id;
    }

    public void setName(String name) {
        this.name = name;
    }

    public User() {
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
    }
}

运行结果：注意流处理中return的位置

二、合并到某一List（假如保留到第一个List）

package com.xu.demo.test;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class ListMergeTest2 {

    public static void main(String[] args) {

        List<Map<String,Object>> list1 = new ArrayList<>();
        Map<String,Object> data=new HashMap<>();
        data.put("userId","100001");
        data.put("userName","唐僧");
        list1.add(data);
        data=new HashMap<>();
        data.put("userId","100002");
        data.put("userName","八戒");
        list1.add(data);

        List<Map<String,Object>> list2 = new ArrayList<>();
        data=new HashMap<>();
        data.put("userId","100001");
        data.put("gender","男");
        data.put("age",20);
        list2.add(data);
        data=new HashMap<>();
        data.put("userId","100002");
        data.put("gender","雄");
        data.put("age",1000);
        list2.add(data);

        System.out.println(list1);
        System.out.println(list2);

        //使用stream流把list1和list2根据属性userId合并一个list集合
        list1.stream().map(m1 -> {
            list2.stream().filter(m2 -> m1.get("userId").equals(m2.get("userId"))).forEach( m2 -> {
                m1.put("gender", m2.get("gender"));
                m1.put("age", m2.get("age"));
            });
            return m1;
        }).collect(Collectors.toList());

        System.out.println(list1);
        System.out.println(list2);
    }

}

运行结果如下：注意流处理中return的位置

三、合并成新对象列表

package com.xu.demo.test;

import com.alibaba.fastjson.JSON;
import lombok.Data;
import org.apache.commons.lang3.StringUtils;

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class Test {

    public static void main(String[] args) {
        new Test().test();
    }

    public void test(){
        List<Person> userListA = Arrays.asList(
                new Person("关羽", 926),
                new Person("赵云", 916),
                new Person("张飞", 906),
                new Person("许褚", 911));
        List<Rule> userListB = Arrays.asList(
                new Rule("关羽", "字·云长-关公-武财神-汉寿亭侯"),
                new Rule("张飞", "字·益德-勇武过人-西乡侯"),
                new Rule("刘备", "字·玄德-百折不挠-汉昭烈帝"),
                new Rule("赵云", "字·子龙-忠义-永昌亭侯"),
                new Rule("周瑜", "字·公瑾-文武兼备-永昌亭侯"),
                new Rule("许褚", "字·仲康-勇力绝人-虎侯"));

        List<Hero> commonList = userListA.stream()
                .map(uA -> userListB.stream()
                        .filter(uB1 -> StringUtils.equals(uB1.getUserName(), uA.getUserName()))
                        .map(uB2 -> new Hero(uB2.getUserName(), uA.getBloodVolume(), uB2.getAlign()))
                        .collect(Collectors.toList())) // 结果类型 Steam<List<Hero>>
                .flatMap(List::stream) // 结果类型 Steam<Hero>
                .collect(Collectors.toList()); // 结果类型 List<Hero>

        System.out.println(JSON.toJSONString(commonList));
        // [{"align":"字·云长-关公-武财神-汉寿亭侯","bloodVolume":926,"userName":"关羽"},
        // {"align":"字·子龙-忠义-永昌亭侯","bloodVolume":916,"userName":"赵云"},
        // {"align":"字·益德-勇武过人-西乡侯","bloodVolume":906,"userName":"张飞"},
        // {"align":"字·仲康-勇力绝人-虎侯","bloodVolume":911,"userName":"许褚"}]
    }
}

@Data
class Person {
    private String userName;
    private int bloodVolume;
    public Person(String userName, int bloodVolume) {
        this.userName = userName;
        this.bloodVolume = bloodVolume;
    }
}

@Data
class Rule {
    private String userName;
    private String align;
    public Rule(String userName, String align) {
        this.userName = userName;
        this.align = align;
    }
}

@Data
class Hero {
    private String userName;
    private int bloodVolume;
    private String align;

    public Hero(String userName, int bloodVolume, String align) {
        this.userName = userName;
        this.bloodVolume = bloodVolume;
        this.align = align;
    }
}

Java8的流式处理提供了大量强大而且简单的数据（结构）处理，一定要好好总结学习，对你的编程思维和能能力的提高大有好处。