Stream流 - 两个list集合对象属性的合并、对象属性值运算

两个list集合对象属性的合并

合并两个 list＜map＞, 并将 userId 相同的所有属性合并到一个 map 中

list1中对象的属性：userId、userName

list2中对象的属性：userId、gender、age

最终总集合中对象的属性：userId、userName、gender、age

public class Test3 {
    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：

结果可见，userId 相同的所有属性合并到集合 list1中。

两个list集合对象属性值运算

合并两个 list＜map＞, 并将 userId 相同的某个属性值进行运算

list1中对象的属性：id、name、amount

list2中对象的属性：id、name、amount

最终总集合中对象的属性：id、name、amount（金额进行累加）

User ：

@Data
@AllArgsConstructor
public class User {
    public String id;
    public String name;
    public BigDecimal amount;
}

public class Test3 {
    public static void main(String[] args){

        List<User> userList1 = new ArrayList<>();
        User user1 = new User("1001", "lili", new BigDecimal("100.5"));
        User user2 = new User("1002", "lucy", new BigDecimal("100.66"));
        userList1.add(user1);
        userList1.add(user2);

        List<User> userList2 = new ArrayList<>();
        User user3 = new User("1001", "lili", new BigDecimal("200.5"));
        User user4 = new User("1002", "lucy", new BigDecimal("200.66"));
        User user5 = new User("1003", "jack", new BigDecimal("200.88"));
        userList2.add(user3);
        userList2.add(user4);
        userList2.add(user5);

        System.out.println("userList1：" + userList1);
        System.out.println("userList2：" + userList2);

        List<User> collect = userList1.stream().map(u1 -> {
            userList2.stream().filter(u2 -> u2.getId().equals(u1.getId())).forEach(u2 -> {
                if (u2.getAmount() != null) {
                    u1.setAmount(u1.getAmount().add(u2.getAmount()));
                }
            });
            return u1;
        }).collect(Collectors.toList());

        System.out.println("userList1：" + userList1);
        System.out.println("collect：" + collect);
        System.out.println("userList2：" + userList2);
    }
}

运行结果：

结果可见，相同 id 记录的 amount 属性值进行了累计运算。

结果可见，userList1 的值发生了改变，如果不想改变原集合内容：

public class Test3 {
    public static void main(String[] args){

        List<User> userList1 = new ArrayList<>();
        User user1 = new User("1001", "lili", new BigDecimal("100.5"));
        User user2 = new User("1002", "lucy", new BigDecimal("100.66"));
        userList1.add(user1);
        userList1.add(user2);

        List<User> userList2 = new ArrayList<>();
        User user3 = new User("1001", "lili", new BigDecimal("200.5"));
        User user4 = new User("1002", "lucy", new BigDecimal("200.66"));
        User user5 = new User("1003", "jack", new BigDecimal("200.88"));
        userList2.add(user3);
        userList2.add(user4);
        userList2.add(user5);

        ArrayList<User> result = new ArrayList<>();

        List<User> collect = userList1.stream().map(u1 -> {
            userList2.stream().filter(u2 -> u2.getId().equals(u1.getId())).forEach(u2 -> {
                if (u2.getAmount() != null) {
                    BigDecimal add = u1.getAmount().add(u2.getAmount());
                    result.add(new User(u1.getId(), u1.getName(), add));
                }
            });
            return u1;
        }).collect(Collectors.toList());

        System.out.println("userList1：" + userList1);
        System.out.println("userList2：" + userList2);
        System.out.println("collect：" + collect);
        System.out.println("result： " + result);
    }
}

运行结果：

新增集合 result 进行最终结果存储，原集合内容不变。

多个集合合并属性

有时候查询多个结果集需要进行合并，比如：

List1：[ {id:1001, value1:100}, {id:1002, value1:200} ]
List2：[ {id:1001, value2:200} ]
List3：[ {id:1001, value3:300}, {id:1002, value3:300}, {id:1003, value3:300} ]

合并后：
List：[ {id:1001, value1:100, value2:200, value3:300}, {id:1002, value1:200, value3:300}, {id:1003, value3:300} ]

Total：

@Data
@AllArgsConstructor
public class Total {
    public String id;

    public String value1;
    public String value2;
    public String value3;
}

public class MergeValue {
    public static void main(String[] args) {
        ArrayList<Total> list1 = new ArrayList<>();
        list1.add(new Total("1001","100","",""));
        list1.add(new Total("1002","200","",""));

        ArrayList<Total> list2 = new ArrayList<>();
        list2.add(new Total("1001","","200",""));

        ArrayList<Total> list3 = new ArrayList<>();
        list3.add(new Total("1001","","","300"));
        list3.add(new Total("1002","","","300"));
        list3.add(new Total("1003","","","300"));

        System.out.println(list1);
        System.out.println(list2);
        System.out.println(list3);

        List<Total> collect = list1.stream().map(v1 -> {
            list2.stream().filter(v2 -> v1.getId().equals(v2.getId())).forEach(v2 -> {
                v1.setValue2(v2.getValue2());
            });

            list3.stream().filter(v2 -> v1.getId().equals(v2.getId())).forEach(v2 -> {
                v1.setValue3(v2.getValue3());
            });
            return v1;
        }).collect(Collectors.toList());
        System.out.println(collect);
    }
}

运行结果：

结果可见，“1003” 的记录没有统计，因为以 list1 为基准进行判断赋值，list1 没有 id=“1003” 的记。

解决方案：先获取所有id，再遍历。

public class MergeValue {
    public static void main(String[] args) {
        ArrayList<Total> list1 = new ArrayList<>();
        list1.add(new Total("1001","100","",""));
        list1.add(new Total("1002","200","",""));

        ArrayList<Total> list2 = new ArrayList<>();
        list2.add(new Total("1001","","200",""));

        ArrayList<Total> list3 = new ArrayList<>();
        list3.add(new Total("1001","","","300"));
        list3.add(new Total("1002","","","300"));
        list3.add(new Total("1003","","","300"));

        System.out.println(list1);
        System.out.println(list2);
        System.out.println(list3);

        // 1、获取所有集合中的 id
        HashSet<String> strings = new HashSet<>();
        list1.stream().forEach(v1 -> strings.add(v1.getId()));
        list2.stream().forEach(v1 -> strings.add(v1.getId()));
        list3.stream().forEach(v1 -> strings.add(v1.getId()));

        // 2、将获取的 id 放到新的集合中，用于基准遍历
        ArrayList<Total> result = new ArrayList<>();
        strings.stream().forEach(v1 -> result.add(new Total(v1, "", "", "")));
        System.out.println(result);

        // 3、遍历比较赋值
        List<Total> collect = result.stream().map(v1 -> {
            list1.stream().filter(v2 -> v1.getId().equals(v2.getId())).forEach(v2 -> {
                v1.setValue1(v2.getValue1());
            });

            list2.stream().filter(v2 -> v1.getId().equals(v2.getId())).forEach(v2 -> {
                v1.setValue2(v2.getValue2());
            });

            list3.stream().filter(v2 -> v1.getId().equals(v2.getId())).forEach(v2 -> {
                v1.setValue3(v2.getValue3());
            });
            return v1;
        }).collect(Collectors.toList());
        System.out.println(collect);
    }
}

运行结果：

✨ 完美！！✨