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

喜欢ヅ旅行 2022-01-11 02:19 216阅读 0赞

**原文：https://blog.csdn.net/qq\_36850813/article/details/84402561 **

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，我们需要是用containsKey去判断，然后再决定如何修改value。 这样比较麻烦，嫩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;
     
     
    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>(){
                @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("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";  
        }  
    }