[Java知识库]Java Stream如何写出高雅又装*的代码

git仓库直达

如何让同事看不懂你写的代码 然后觉得你非常牛逼 这里用到了stream()与Lambda 需要有点基础,没基础你炫个🔨

优雅永不过时~ 看下面文章时记得穿燕尾服 拿高脚杯

List<String> strings = Lists.newArrayList("name=kk", "sex=1", "tel=1111", "email=xx@qq.com","name=ww","name=yy");

一. 冷静分析

如上代码,你现在要做一个分组的Map将上面list的值读取出来,然后分组统计类似于Map<String,List< String >> 将Key相同的Val全部存到一个list里面

二. 直接开装

不会真有人还直接遍历吧,那别人不就看的懂了~ 为了炫技直接装一波

定义封装对象

  	@Data
    @Builder
    static class ObiectMap{
        private String key;
        private String value;
    }

2.1 初级炫

 System.out.println("map1: ");
        Map<String, List<String>> collect1 = strings.stream().map(e -> { //封装成对象
            String[] split = e.split("\\=", 2);
            return ObiectMap.builder().key(split[0]).value(split[1]).build();
        }).collect(Collectors.toMap(ObiectMap::getKey,v->Lists.newArrayList(v.getValue()), (List<String> newList, List<String> oldList) -> { // Collectors.toMap(a,b,(n1,n2)) -> a代表Map的key (这里直接用方法引用拿到key) b代表value (n1,n2)代表key相同时value的处理办法,直接合并List
            oldList.addAll(newList);
            return oldList;
        }));
        collect1.forEach((k,v)->{
            System.out.printf(k+" { ");
            String vList = v.stream().collect(Collectors.joining(",")); //加上逗号,最后一个不加
            System.out.printf(vList);
            System.out.printf(" }");
            System.out.println();
        });
        System.out.println();

结果如下

2.2 普通炫

System.out.println("map2: ");
        Map<String, List<String>> collect2 = strings.stream().map(e -> {
            String[] split = e.split("\\=", 2);
            return ObiectMap.builder().key(split[0]).value(split[1]).build();
        }).collect(Collectors.groupingBy(ObiectMap::getKey, Collectors.mapping(ObiectMap::getValue, Collectors.toList()))); //Collectors.groupingBy(a,b) a还是通过key来分组 ,b将value收集起来做list value
        collect2.forEach((k,v)->{
            System.out.printf(k+" { ");
            String vList = v.stream().collect(Collectors.joining(",")); //加上逗号,最后一个不加
            System.out.printf(vList);
            System.out.printf(" }");
            System.out.println();
        });
        System.out.println();

结果如下

2.3 高级炫

用Guava来炫

import com.google.common.collect.*

System.out.println("map3: get name List with Multimap");
        List<ObiectMap> collect3 = strings.stream().map(e -> {
            String[] split = e.split("\\=", 2);
            return ObiectMap.builder().key(split[0]).value(split[1]).build();
        }).collect(Collectors.toList());
        Multimap<String, String> multiMap = ArrayListMultimap.create();  //使用Guava的Multimap来存 value直接是collection,可以随意转换
        collect3.forEach(c -> multiMap.put(c.getKey(), c.getValue()));
        System.out.println(multiMap.get("name").stream().collect(Collectors.joining(","))); 
        System.out.println();

结果如下

2.4 再炫一波 拿到当前key与对应的数量

System.out.println("map4: get name with count");
        Map<String, Long> collect4 = strings.stream().map(e -> {
            String[] split = e.split("\\=", 2);
            return ObiectMap.builder().key(split[0]).value(split[1]).build();
        }).collect(Collectors.groupingBy(ObiectMap::getKey, Collectors.counting())); // 拿到数量
        collect4.forEach((k,v)->{
            System.out.printf(k+" { ");
            System.out.printf("%d",v);
            System.out.printf(" }");
            System.out.println();
        });
        System.out.println();

结果如下

2.5 Map Reduce炫

这个不能处理重复value,只是将单一Map聚合到一个Map

List<String> strings = Lists.newArrayList("name=kk", "sex=1", "tel=1111", "email=xx@qq.com");
        Map<String, String> reduce = strings.stream().map(e -> {
            String[] split = e.split("\\=", 2); // 正则划分为单map
            return Collections.singletonMap(split[0], split[1]);
        }).reduce(new HashMap<String, String>(), (accMap, singleMap) -> { // reduce(a,(b,c))-> a 为初始值 b为累加值 c为当前值 操作返回当前值与累加值后的结果 将多个单KV的Map组合成一个Map
            accMap.putAll(singleMap);
            return accMap;
        });

        reduce.forEach((k,v)->{
            System.out.println(k+" = "+v);
        });

好家伙直接装一波,直接起飞~ 制作不易,转载请标注