Java中映射Map的merge、compute、computeIfAbsent、computeIfPresent基本用法

下面是Java8中Map的一些新方法merge、compute、computeIfAbsent、computeIfPresent介绍。

我们在项目开发中，经常使用map，key有时存在有时不存，我们需要用containsKey方法进行判断，然后再决定如何修改value。 这样比较麻烦。能不能在一个方法调用就完成这些工作呢（如果key存在value（还可以有其他逻辑判断），就do a，如果不存在就do b）？答案是， 当然可以。

下面直接给出示例和运行结果，参看结果就明白各个方法的具体含义了。

import java.util.HashMap;
import java.util.Map;
import java.util.function.BiFunction;/*** @author yqbjtu* @data 2018年3月31日 下午6:24:16* @version 1.0**/
public class MapMain {
    /*** @param args*/public static void main(String[] args) {
    Map<String, String> myMap = new HashMap<>();String keyA = "A";String keyB = "B";String keyC = "C";String keyD = "D";String keyE = "E";String keyF = "F";String keyG = "G";String keyH = "H";myMap.put(keyA, "str01A");myMap.put(keyB, "str01B");myMap.put(keyC, "str01C");System.out.println("myMap initial content:"+ myMap);myMap.merge(keyA, "merge01", String::concat);myMap.merge(keyD, "merge01", String::concat);System.out.println("Map merge demo content:"+ myMap);BiFunction<String, String, String> biFunc = new BiFunction<String, String, String>(){
    @Overridepublic String apply(String t, String u) {
    String result = t;if (t == null) {
    result = u;}else {
    result += "," + u;}return result;}};myMap.merge(keyA, "BiFuncMerge01", biFunc);myMap.merge(keyE, "BiFuncMerge01", biFunc);System.out.println("Map customized BiFunction merge demo content:"+ myMap);String msg = "msgCompute";myMap.compute(keyB, (k, v) -> (v == null) ? msg : v.concat(msg));myMap.compute(keyF, (k, v) -> (v == null) ? msg : v.concat(msg));System.out.println("Map customized BiFunction compute demo content:"+ myMap);myMap.computeIfAbsent(keyC, k -> genValue(k));myMap.computeIfAbsent(keyG, k -> genValue(k));System.out.println("Map customized Function computeIfAbsent demo content:"+ myMap);myMap.computeIfPresent(keyC, biFunc);myMap.computeIfPresent(keyH, biFunc);System.out.println("Map customized biFunc computeIfPresent demo content:"+ myMap);}static String genValue(String str) {
      System.out.println("===");  return str + "2";  }  
}

运行结果

myMap initial content:{A=str01A, B=str01B, C=str01C}
Map merge demo content:{A=str01Amerge01, B=str01B, C=str01C, D=merge01}
Map customized BiFunction merge demo content:{A=str01Amerge01,BiFuncMerge01, B=str01B, C=str01C, D=merge01, E=BiFuncMerge01}
Map customized BiFunction compute demo content:{A=str01Amerge01,BiFuncMerge01, B=str01BmsgCompute, C=str01C, D=merge01, E=BiFuncMerge01, F=msgCompute}
===
Map customized Function computeIfAbsent demo content:{A=str01Amerge01,BiFuncMerge01, B=str01BmsgCompute, C=str01C, D=merge01, E=BiFuncMerge01, F=msgCompute, G=G2}
Map customized biFunc computeIfPresent demo content:{A=str01Amerge01,BiFuncMerge01, B=str01BmsgCompute, C=C,str01C, D=merge01, E=BiFuncMerge01, F=msgCompute, G=G2}