JAVA知识回顾之Java8 Merge

0x00 概述

本文主要讲述Java8 Map的merger操作。

 

0x01 用例和代码

//计算唯一的单词出现次数
var map = new HashMap<String, Integer>();
words.forEach(word -> {
    var prev = map.get(word);
    if (prev == null) {
        map.put(word, 1);
    } else {
        map.put(word, prev + 1);
    }
});

//给定输入 对应的输出结果
var words = List.of("Foo", "Bar", "Foo", "Buzz", "Foo", "Buzz", "Fizz", "Fizz");
//...
{Bar=1, Fizz=2, Foo=3, Buzz=2}

//进行重构以避免条件逻辑
words.forEach(word -> {
    map.putIfAbsent(word, 0);
    map.put(word, map.get(word) + 1);
});

putIfAbsent()是必要的，否则代码会在第一次出现未知的单词时中断，另外map.get(word) 里面map.put() 有点尴尬。

words.forEach(word -> {
    map.putIfAbsent(word, 0);
    map.computeIfPresent(word, (w, prev) -> prev + 1);
});

仅当question（word）中的键存在时才调用给定的转换。否则它什么都不做。

words.forEach(word ->
        map.compute(word, (w, prev) -> prev != null ? prev + 1 : 1)
);

compute()就像是computeIfPresent()，无论给定key是否存在都会调用它。如果键的值不存在，则prev参数为null。

实现一个简单的merge:

default V merge(K key, V value, BiFunction<V, V, V> remappingFunction) {
    V oldValue = get(key);
    V newValue = (oldValue == null) ? value :
               remappingFunction.apply(oldValue, value);
    if (newValue == null) {
        remove(key);
    } else {
        put(key, newValue);
    }
    return newValue;
}

merge() 适用于两种情况。如果给定的Key值不存在，它就变成了put(key, value)。但如果所述Key已经存值，remappingFunction可以操作合并：

• 只需返回新值覆盖旧值： (old, new) -> new
• 只需返回旧值保留旧值： (old, new) -> old
• 以某种方式合并两者，例如： (old, new) -> old + new
• 甚至删除旧值： (old, new) -> null

//1 的下word ，如果没有key就将1添加到现有值。
words.forEach(word ->
        map.merge(word, 1, (prev, one) -> prev + one)
);

使用merge()模拟一个帐户操作：

//vo
class Operation {
    private final String accNo;
    private final BigDecimal amount;
}
//针对不同账户的操作

var operations = List.of(
    new Operation("123", new BigDecimal("10")),
    new Operation("456", new BigDecimal("1200")),
    new Operation("123", new BigDecimal("-4")),
    new Operation("123", new BigDecimal("8")),
    new Operation("456", new BigDecimal("800")),
    new Operation("456", new BigDecimal("-1500")),
    new Operation("123", new BigDecimal("2")),
    new Operation("123", new BigDecimal("-6.5")),
    new Operation("456", new BigDecimal("-600"))
);

如果想要为每个账户计算余额，不使用merge：

var balances = new HashMap<String, BigDecimal>();
operations.forEach(op -> {
    var key = op.getAccNo();
    balances.putIfAbsent(key, BigDecimal.ZERO);
    balances.computeIfPresent(key, (accNo, prev) -> prev.add(op.getAmount()));
});

使用merge()：

operations.forEach(op ->
        balances.merge(op.getAccNo(), op.getAmount(), 
                (soFar, amount) -> soFar.add(amount))
);

operations.forEach(op ->
        balances.merge(op.getAccNo(), op.getAmount(), BigDecimal::add)
);

//{123=9.5, 456=-100}

完整代码：

package other;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import lombok.Data;
import org.junit.Test;

import java.util.*;

/**
 * @author liuqian
 * @date 2019/10/7 19:11.
 */
public class MapMethodsTest {

    @Test
    public void mapMergeTest() throws JsonProcessingException {
        ObjectMapper objectMapper = new ObjectMapper();
        List<StudentScore> studentScoreList = buildATestList();
        // 按照学生分组，求得每个学生的总分
        // 常规做法
        Map<String, Integer> studentScoreMap = new HashMap<>();
        studentScoreList.forEach(studentScore -> {
            if (studentScoreMap.containsKey(studentScore.getStuName())) {
                studentScoreMap.put(studentScore.getStuName(), studentScoreMap.get(studentScore.getStuName()) + studentScore.getScore());
            } else {
                studentScoreMap.put(studentScore.getStuName(), studentScore.getScore());
            }
        });
        // {"李四":228,"张三":215,"王五":235}
        System.out.println(objectMapper.writeValueAsString(studentScoreMap));

        // merge() 方法
        Map<String, Integer> studentScoreMap2 = new HashMap<>();
        studentScoreList.forEach(studentScore -> studentScoreMap2.merge(
                studentScore.getStuName(),
                studentScore.getScore(),
                Integer::sum));
        // {"李四":228,"张三":215,"王五":235}
        System.out.println(objectMapper.writeValueAsString(studentScoreMap2));
    }

    @Test
    public void mapComputeTest() {
        String k = "key";
        Map<String, Integer> map = new HashMap<String, Integer>() {{
            put(k, 1);
        }};
        // 2
        System.out.println(map.compute(k, (key, oldVal) -> oldVal + 1));
    }

    @Test
    public void mapCountComputeTest() {
        List<String> words = new ArrayList<String>() {{
            add("A");
            add("B");
            add("C");
            add("A");
            add("C");
            add("E");
            add("E");
            add("E");
            add("E");
            add("A");
            add("E");
        }};
        // the word appear times
        Map<String, Integer> wordCountMap = new HashMap<>();
        words.forEach(word -> wordCountMap.compute(word, (key, oldCount) -> {
            if (Objects.isNull(oldCount)) {
                return 1;
            }
            return oldCount + 1;
        }));
        System.out.println(wordCountMap);
    }

    private List<StudentScore> buildATestList() {
        List<StudentScore> studentScoreList = new ArrayList<>();
        StudentScore studentScore1 = new StudentScore() {{
            setStuName("张三");
            setSubject("语文");
            setScore(70);
        }};
        StudentScore studentScore2 = new StudentScore() {{
            setStuName("张三");
            setSubject("数学");
            setScore(80);
        }};
        StudentScore studentScore3 = new StudentScore() {{
            setStuName("张三");
            setSubject("英语");
            setScore(65);
        }};
        StudentScore studentScore4 = new StudentScore() {{
            setStuName("李四");
            setSubject("语文");
            setScore(68);
        }};
        StudentScore studentScore5 = new StudentScore() {{
            setStuName("李四");
            setSubject("数学");
            setScore(70);
        }};
        StudentScore studentScore6 = new StudentScore() {{
            setStuName("李四");
            setSubject("英语");
            setScore(90);
        }};
        StudentScore studentScore7 = new StudentScore() {{
            setStuName("王五");
            setSubject("语文");
            setScore(80);
        }};
        StudentScore studentScore8 = new StudentScore() {{
            setStuName("王五");
            setSubject("数学");
            setScore(85);
        }};
        StudentScore studentScore9 = new StudentScore() {{
            setStuName("王五");
            setSubject("英语");
            setScore(70);
        }};

        studentScoreList.add(studentScore1);
        studentScoreList.add(studentScore2);
        studentScoreList.add(studentScore3);
        studentScoreList.add(studentScore4);
        studentScoreList.add(studentScore5);
        studentScoreList.add(studentScore6);
        studentScoreList.add(studentScore7);
        studentScoreList.add(studentScore8);
        studentScoreList.add(studentScore9);

        return studentScoreList;
    }

    @Data
    private class StudentScore {
        private String stuName;
        private String subject;
        private Integer score;
    }
}