java映射的merge、compute、computeIfAbsent、computeIfPresent的使用

　　merge、compute、computeIfAbsent、computeIfPresent是java8中的语法。

　　merge：通过构建BiFunction或则是调用java中的一些函数来操作merge参数中的变量。

　　compute：通过构建BiFunction或则使用lambda表达式来操作参数中的变量，这里无论key是否存在都会执行后面的方法。

　　computeIfAbsent，computeIfPresent：前一个是当key不存在是则执性后面的方法，后一个是存在时执行后面的方法。

 

　　下面是对方法使用的源码：（源码来自：https://blog.csdn.net/russle/article/details/79772198）

 

package jihe;

import java.util.HashMap;
import java.util.Map;
import java.util.function.BiFunction;

public class MapMain {
    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>(){
            @Override
            public 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("merge"+ 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("compute"+ myMap);
        
        myMap.computeIfAbsent(keyC, k -> genValue(k));
        myMap.computeIfAbsent(keyG, k -> genValue(k));
        System.out.println("computeIfAbsent"+ myMap);
        
        myMap.computeIfPresent(keyC, biFunc);
        myMap.computeIfPresent(keyH, biFunc);
        System.out.println("computeIfPresent"+ myMap);
    }
    
    static String genValue(String str) {  
        System.out.println("===");  
        return str + "2";  
    }  

}

输出的结果为：

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