Java8 中 List 转 Map(Collectors.toMap) 使用技巧

public class streamTest {
    /**前端 js
     * let arr = [{id:1,name:"ls"},{id:2,name:"zs"}]
     * let res = arr.map(item => {
     *     let name = item.id;
     *     var tem = new Map()
     *     tem.set(item.id, item);
     *     return tem;
     * })
     *
     * [
     *   Map { 1 => { id: 1, name: 'ls' } },
     *   Map { 2 => { id: 2, name: 'zs' } }
     * ]
     * */

    /**
     * 关于 Collectors.toMap 方法
     * Collectors.toMap 有三个重载方法：
     *
     * toMap(Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper);
     * toMap(Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper,
     *         BinaryOperator<U> mergeFunction);
     * toMap(Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper,
     *         BinaryOperator<U> mergeFunction, Supplier<M> mapSupplier);
     * 参数含义分别是：
     *
     * keyMapper：Key 的映射函数
     *
     * valueMapper：Value 的映射函数
     *
     * mergeFunction：当 Key 冲突时，调用的合并方法
     *
     * mapSupplier：Map 构造器，在需要返回特定的 Map 时使用
     * */
    public static void main(String[] args) {
        ArrayList<UserInfo> arr = CollUtil.newArrayList();
        UserInfo userInfo = new UserInfo("lisi", "111");
        UserInfo userInfo2 = new UserInfo("zhangsan", "222");
        UserInfo userInfo3 = new UserInfo("zhangsan", "333");
        UserInfo userInfo4 = new UserInfo("beifeng", "444");
        arr.add(userInfo);
        arr.add(userInfo2);
        arr.add(userInfo3);
        arr.add(userInfo4);
        System.out.println(arr);
//        HashMap<String, String> a = CollUtil.newHashMap();
//        a.put("A","aaa");
//        a.put("B", "bbb");
//        a.put("A", "ccc");
//        System.out.println(a);
        //将一个数组转换成 一个map对象；key(string) -> value(string)
//        Map<String, String> map = arr.stream().collect(Collectors.toMap(UserInfo::getUsername, UserInfo::getPassword));
//        System.out.println(map);
        //{lisi=111, zhangsan=222}


        //将一个数组转换成 一个map对象；key(string) -> value(UserInfo)
//        Map<String, UserInfo> map2 = arr.stream().collect(Collectors.toMap(UserInfo::getUsername, v -> v));
//        System.out.println(map2);
        //Exception in thread "main" java.lang.IllegalStateException: Duplicate key 222
        //  at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
        //  at java.util.HashMap.merge(HashMap.java:1253)
        //  at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
        //  at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
        //  at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374)
        //  at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
        //  at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
        //  at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
        //  at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
        //  at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
        //  at com.ruoyi.common.demo.stream.streamTest.main(streamTest.java:46)

        // 如果有重复的key,第三个参数处理合并
        Map<String, UserInfo> map3 = arr.stream().collect(Collectors.toMap(UserInfo::getUsername, v -> v, (v1, v2) -> v1));
        System.out.println(map3);
        //{lisi=UserInfo(username=lisi, password=111), zhangsan=UserInfo(username=zhangsan, password=222)}

        // 如果有重复的key,第三个参数处理合并， 如果处理map对象的顺序
        Map<String, UserInfo> map4 = arr.stream().collect(Collectors.toMap(UserInfo::getUsername, v -> v, (v1, v2) -> v1, TreeMap:: new));
        System.out.println(map4);
        //{beifeng=UserInfo(username=beifeng, password=444), lisi=UserInfo(username=lisi, password=111), zhangsan=UserInfo(username=zhangsan, password=222)}



 //将传过来的数据 List 映射成功 另一个对象的信息
        List<UserInfo> t = arr.stream().filter(item -> Objects.nonNull(arr)).map(item -> new UserInfo("t", "1")).collect(Collectors.toList());

        System.out.println(t);
//eg
receiveThirdReqDtOList.stream()
                    .filter(receiveThirdReqDtO -> StrUtil.isNotBlank(receiveThirdReqDtO.getCouponAtno()))
                    .map(receiveThirdReqDtO -> shopPoolDao.getCouponDetail(null, receiveThirdReqDtO.getCouponAtno()))
                    .collect(Collectors.toList());
    }
}

常用使用技巧

 //给你下面一个字符串数组,如果用stream api来实现，找出以字符'a'开头长度最大的字符串，使用stream api该怎么实现呢？
        List<String> list = Arrays.asList("abb", "abcd", "fegc", "efe", "adfes");
        System.out.println(list);
        //过滤首字母是 a
        List<String> a = list.stream().filter(item -> item.startsWith("a")).collect(Collectors.toList());
        System.out.println(a);
        //
        int a2 = list.stream().filter(item -> item.startsWith("a")).mapToInt(item -> item.length()).max().orElse(0);
        System.out.println(a2);

比如过滤自己相关的 id集合

   //先过滤再映射自己需要的字段
        List<String> collect = arr.stream().filter(v -> v.getPassword().indexOf("2") < 0)
                .map(UserInfo::getUsername)
                .collect(Collectors.toList());
        System.out.println(collect);//[lisi, zhangsan, beifeng]