[数据结构与算法]HashMap的jdk1.8的新方法使用

package com.zyp.test;

import com.google.common.collect.Maps;

import java.util.HashMap;

/**
 *put():key不存在则直接插入,返回值为null,key存在则对应的value值被新值覆盖,返回值为旧值
 *compute():key不存在则直接插入,返回值为新值,key存在时,则通过remappingFunction计算新的value,返回计算后的value值，key值不变
 *computeIfAbsent():key不存在则value进行mappingFunction运算(i -> i + "a",i为key值),返回值新值,key存在,value不变，返回值旧值
 *computeIfPresent():key不存在则不插入，返回值为null,key存在时value进行remappingFunction，返回值为新值
 *merge():key不存在时,则把第二个参数当做value插入到map中,返回第二个参数,key存在时,
 *对原有value值和第二个参数值进行remappingFunction,返回计算后的value
 */
public class Map18 {

    public static void main(String[] args) {
        HashMap<Integer, String> map = Maps.newHashMap();
        //key不存在则直接插入,返回值为null,key存在则对应的value值被新值覆盖,返回值为旧值
        String put = map.put(1, "1");
        System.out.println("put->"+put);
        String put1 = map.put(1, "2");
        System.out.println("put1->"+put1);
        //key不存在则直接插入,返回值为新值,key存在时,则通过remappingFunction计算新的value,返回计算后的value值，key值不变
        String compute = map.compute(2, (K, V) -> "2");
        System.out.println("compute->"+compute);
        String compute1 = map.compute(2, (K, V) -> V+"b");
        System.out.println("compute1->"+compute1);
        //key不存在则value进行mappingFunction运算(i -> i + "a",i为key值),返回值新值,key存在,value不变，返回值旧值
        String computeIfAbsent = map.computeIfAbsent(2, i -> i + "a");
        System.out.println("computeIfAbsent->"+computeIfAbsent);
        String computeIfAbsent2 = map.computeIfAbsent(4, i -> i + "a");
        System.out.println("computeIfAbsent2->"+computeIfAbsent2);
        System.out.println(map);
        //key不存在则不插入，返回值为null,key存在时value进行remappingFunction，返回值为新值
        String computeIfPresent = map.computeIfPresent(4, (K, V) -> V + "a");
        System.out.println("computeIfPresent->"+computeIfPresent);
        String computeIfPresent2 = map.computeIfPresent(5, (K, V) -> V + "a");
        System.out.println("computeIfPresent2->"+computeIfPresent2);
        System.out.println(map);
        //key不存在时,则把第二个参数当做value插入到map中,返回第二个参数,
        //key存在时,对原有value值和第二个参数值进行remappingFunction,返回计算后的value
        String merge = map.merge(5, "5", (old, new1) -> old + new1);
        System.out.println("merge->"+merge);
        String merge2 = map.merge(5, "6", (old, new1) -> old + new1);
        System.out.println("merge2->"+merge2);
        String merge1 = map.merge(6, "6", (old, new1) -> old + new1);
        System.out.println("merge1->"+merge1);
        System.out.println(map);
    }
}

使用举例：

package com.zyp.test;

import com.google.common.collect.Maps;

import java.util.HashMap;

/**
 * @author syl
 * @description TODO
 * @since 2022/3/8
 */
public class MapTest {
    public static void main(String[] args) {
        //统计随机字符串中每个字符的个数
        String a="fiensdkajfhquebdjsankroiwqfhusdbvjnkwoqdas";
        //方法1
        HashMap<Character, Integer> map = Maps.newHashMap();
        for (int i = 0; i <a.length() ; i++) {
            char c = a.charAt(i);
            Integer value = map.get(c);
            if(null==value){
                value=1;
            }else{
                ++value;
            }
            map.put(c, value);
        }
        System.out.println("map->"+map);
        System.out.println();
        map.clear();
        //方法2
        for (int i = 0; i < a.length(); i++) {
            map.compute(a.charAt(i),(K,V)->{
                if(V==null){
                   V=1;
                }else{
                    ++V;
                }
                return V;
            });
        }
        System.out.println("map1->"+map);
        //方法3
        map.clear();
        for (int i = 0; i < a.length(); i++) {
            map.merge(a.charAt(i), 1, (s,s1)->s+s1);
        }
        System.out.println("map2->"+map);
    }
}