Map的compute、computeIfAbsent、computeIfPresent的使用

一、代码例子：

package com.test.collection;

import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;
import java.util.function.BiFunction;
import java.util.function.Function;


public class TestMap {

    public static void main(String[] args) {
        Map<Integer, String> itemMap = new HashMap<>();
        itemMap.put(1, "ITEM_WAGNYIYUN");
        itemMap.put(2, "QQ_CARD");
        itemMap.put(3, "JD_SHOPPING");
        System.out.println("=======================compute==========================");

        /*
        itemMap.compute(1, new BiFunction<Integer, String, String>() {
            @Override
            public String apply(Integer key, String val) {
                return val + key;
            }
        });*/

        itemMap.compute(1, (key, val) -> val + key);
        System.out.println(itemMap);        //{1=ITEM_WAGNYIYUN1, 2=QQ_CARD, 3=JD_SHOPPING}

        //键不存在时插入新条目
        itemMap.compute(4, (key, oldVal) -> {
            if (oldVal == null) {
                return "TM_SHOPPING";
            } else {
                return oldVal + key;
            }
        });
        System.out.println(itemMap);       //{1=ITEM_WAGNYIYUN1, 2=QQ_CARD, 3=JD_SHOPPING, 4=TM_SHOPPING}

        //删除条目（当函数返回null时）
        itemMap.compute(4, (key, oldVal) -> null);
        System.out.println(itemMap);        //{1=ITEM_WAGNYIYUN1, 2=QQ_CARD, 3=JD_SHOPPING}


        System.out.println("=======================computeIfAbsent==========================");

        /**
         * computeIfAbsent的使用：
         * 仅当键不存在时才计算值
         * computeIfAbsent() 只接收键作为参数，而 compute() 接收键和当前值
         * computeIfAbsent() 不会删除条目（即使函数返回null）
         */
        /*
        * 只有当key=1不存在时，才会计算并插入值TM_SHOPPING
        itemMap.computeIfAbsent(1, new Function<Integer, String>() {
            @Override
            public String apply(Integer integer) {
                return "TM_SHOPPING";
            }
        });*/
        itemMap.computeIfAbsent(1, integer -> "TM_SHOPPING");
        System.out.println(itemMap);        //{1=ITEM_WAGNYIYUN1, 2=QQ_CARD, 3=JD_SHOPPING}

        String val = itemMap.computeIfAbsent(4, integer -> "TM_SHOPPING");
        System.out.println(val);            //TM_SHOPPING
        System.out.println(itemMap);        //{1=ITEM_WAGNYIYUN1, 2=QQ_CARD, 3=JD_SHOPPING, 4=TM_SHOPPING}

        Map<String, Set<String>> dictionary = new HashMap<>();

        // 添加同义词
        dictionary.computeIfAbsent("happy", k -> new HashSet<>()).add("joyful");
        dictionary.computeIfAbsent("happy", k -> new HashSet<>()).add("cheerful");
        System.out.println(dictionary);         //{happy=[cheerful, joyful]}


        System.out.println("=======================computeIfAbsent==========================");

        /**
         * computeIfPresent() - 仅当键存在时才计算值
         * 它只在键存在且对应的值不为 null 时，才会计算并更新值。
         *
         */
        String str = itemMap.computeIfPresent(4, new BiFunction<Integer, String, String>() {
            @Override
            public String apply(Integer key, String oldVal) {
                return oldVal + key;
            }
        });
        System.out.println(str);            //TM_SHOPPING4
        System.out.println(itemMap);        //{1=ITEM_WAGNYIYUN1, 2=QQ_CARD, 3=JD_SHOPPING, 4=TM_SHOPPING4}

        String str2 = itemMap.computeIfPresent(5, (k, v) -> v + k);
        System.out.println(str2);           //null   对不存在的键调用不会有任何效果
        System.out.println(itemMap);        //{1=ITEM_WAGNYIYUN1, 2=QQ_CARD, 3=JD_SHOPPING, 4=TM_SHOPPING4}
    }
}