stream流转map or string

        //stream对象转map， 如果存在多value则取第一个
        Map collect = proeducts.stream().collect(Collectors.toMap(Proeduct::getValue, Function.identity(), (v1, v2) -> v1));
        collect.forEach((aDouble, proeduct) -> System.out.println(aDouble.toString() + "  " + proeduct.toString()));

        //stream对象转map
        Map collect1 = proeducts.stream().collect(Collectors.toMap(Proeduct::getValue, Function.identity()));
        collect1.forEach((aDouble, proeduct) -> System.out.println(aDouble.toString() + "   " + proeduct.toString()));

Map studentMap = studentList.stream().collect(Collectors.toMap(key -> { return key.getId() + "-" + key.getName(); }, student -> student));

        //stream对象转map
        Map> stringListMap = studentList.stream().collect(Collectors.groupingBy(st -> {
            return st.getId() + "";
        }, Collectors.toList()));

        //stream对象转map
        Map> collect = studentList.stream().collect(Collectors.groupingBy(st -> {
            return st.getName() + "-" + st.getAge();
        }, LinkedHashMap::new, Collectors.toList()));

        //stream对象转map集合
        Map> map = stringList.stream().collect(Collectors.groupingBy(s -> CommonUtils.isNotEmpty(s)));
        List stringList1 = map.get(Boolean.FALSE);
        System.out.println(stringList1.size());

        //stream对象转map集合
        Map> prodMap = prodList.stream().collect(Collectors.groupingBy(Product::getCategory,
                Collectors.mapping(Product::getName, Collectors.toSet())));
        //{"啤酒":["青岛啤酒","百威啤酒"],"零食":["面包","饼干","月饼"]}

        Map stringMap = studentDTOList.stream().collect(Collectors.toMap(StudentDTO::getId, StudentDTO::getName));
        stringMap.keySet().forEach(
                key -> {
                    System.out.println(key + " : " + stringMap.get(key));
                }
        );

        //stream对象字段转字符串
        String collect2 = proeducts.stream().filter(p -> !p.getId().isEmpty()).map(proeduct -> proeduct.getId()).collect(Collectors.joining(","));
        System.out.println(collect2);

        //分组获得组内最小值
        Map> collect = proeducts.stream().collect(
                Collectors.groupingBy(
                        Proeduct::getName,
                        Collectors.minBy(Comparator.comparing(Proeduct::getValue))
                ));

        //> 分组后只获取对象中某一个属性作为一个集合
        Map> collectMap = studentList.stream().collect(
                Collectors.groupingBy(Student::getAge, //key
                        Collectors.mapping(Student::getId, //value需要的字段
                                Collectors.mapping(String::valueOf, //value转换类型
                                        Collectors.toList() //值转为集合
                                ))));

// 分组后获取对象中某一个属性进行求和
Map mapKey = studentList.stream().collect(
        Collectors.groupingBy(Student::getAge, //分组key
                Collectors.mapping(Student::getId, //求和字段
                        Collectors.mapping(String::valueOf, //求和字段转换为字符串
                                Collectors.mapping(BigDecimal::new, //字符串转为BigDecimal类型
                                        Collectors.reducing(BigDecimal.ZERO, BigDecimal::add) //求和操作
                                )))));

 

        // stream 流的使用
        String s = stringList.stream() //转为 Stream 流
                .filter(i -> !isNum(i)) //筛选字母型字符串
                .filter(i -> i.length() >= 2) //字符串长度 大于 2
                .map(i -> i.toLowerCase()) //全部转换为小写
                .distinct() //去重操作
                .sorted(Comparator.naturalOrder()) //字符串排序
                .collect(Collectors.joining(" "));

        consumer.accept("--------->>>>>>" + s);

中间结点（stream集合） 和 终止结点（非stream集合）

        appleList.stream()
                .filter(apple -> apple.getColor().equals("read") || apple.getColor().equals("green")) //过滤
                .map(apple -> apple.getWeight()) //转换
                .filter(w -> w > 300) //过滤
                .peek(w -> System.out.println("重量是：" + w)) //执行一个函数 peek 是一个中间节点
                .collect(Collectors.toList()) // 终止
                .forEach(w -> System.out.println(w)); // 执行一个函数 forEach 是一个终止节点   遍历