lambda 详解

• 前提：

	    List all = new ArrayList();
		Wrapper queryWrapper = new QueryWrapper<>();
		all = userMapper.selectList(queryWrapper);

• 一个对象的集合转化成另一个对象的集合（ 可以是自己新建的DTO，里面取的字段可以是通过构造方法）

		List cartDTOList = all.stream().map(e -> new UserVo(e.getId(), e.getPassword(), e.getUsername())).collect(toList());

• 过滤集合

        Stream alltream = all.stream().filter(a -> a.getUsername().indexOf("z") > 0);
        List collect = alltream.collect(toList());//stream转换成List
        System.out.println(collect.size());
        collect.forEach(
                e -> System.out.println("过滤集合中的字符串:" + JSON.toJSONString(e))
        );

• 数组转字符串，字符串转数组，集合转字符串，字符串转集合

        int[] arr = {1, 2, 3, 4};
        //1、数组转逗号分隔的字符串
        String strArr = Arrays.stream(arr).boxed().map(i -> i.toString()).collect(Collectors.joining(","));
        System.out.println("int类型数组转字符串:" + strArr);
        List list = new ArrayList<>();
        list.add("1");
        list.add("2");
        list.add("3");
        list.add("4");

        //2、集合转逗号分隔的字符串
        String strList = list.stream().collect(Collectors.joining(","));
        System.out.println("integer类型的集合转字符串" + strList);
        List listInt = new ArrayList<>();
        listInt.add(1);
        listInt.add(2);
        listInt.add(3);
        listInt.add(4);
        listInt.add(1);

        //集合转逗号分隔的字符串-去重
        String str3 = listInt.stream().map(r -> r.toString()).distinct().collect(Collectors.joining(","));
        System.out.println("integer类型的集合转字符串去重：" + str3);

        //3、逗号分隔的字符串转成集合
        String ids = "1,2,3,4,5,6";
        List idss = Arrays.asList(ids.split(",")).stream().map(s -> Long.parseLong(s.trim())).collect(toList());
        idss.forEach(
                e -> System.out.println("逗号分隔的字符串转long类型的集合：" + e.toString())
        );

        //4、逗号分隔的字符串转数组
        String strToArrayList = "1,2,3,4,5,6";
        List listIds = Arrays.asList(ids.split(",")).stream().map(s -> Long.parseLong(s.trim())).collect(Collectors.toList());
        System.out.println(Arrays.toString(listIds.toArray()));

• 数组集合互转

        String ids = "1,2,3,4,5,6";
        List idss = Arrays.asList(ids.split(",")).stream().map(s -> Long.parseLong(s.trim())).collect(toList());
        /**
         * @Description:传统方式数组和list互转如下：
         * @Author: wumingdu
         * @Date: 2020/11/17 22:01
         */
        //集合转数组
        Object[] objects = (Object[]) idss.toArray();
        for (int i = 0; i < objects.length; i++) {
            //  System.out.println(objects[i].toString());
        }
        //数组转集合
        List longs = Arrays.asList(objects);
        // System.out.println(longs.toString());

        /**
         * 使用lambda 的方式转换如下：
         */
        //数组转换成list
        String[] arrays = {"a", "b", "c"};
        List listStrings = Stream.of(arrays).collect(toList());
        System.out.println(listStrings);

        //List转换为数组
        List list = new ArrayList<>();
        list.add("a");
        list.add("b");
        list.add("c");
        String[] strings = list.stream().toArray(String[]::new);
        System.out.println(strings);

• map转list

        Map map = new HashMap<>();
        map.put("1", "AAAA");
        map.put("2", "BBBB");
        map.put("3", "CCCC");
        map.put("4", "DDDD");
        map.put("5", "EEEE");
        List list = map.entrySet().stream().sorted(Comparator.comparing(e -> e.getKey())).map(e -> new User(Long.parseLong(e.getKey()), e.getValue())).collect(Collectors.toList());
        
        //将map里面的每对entrySet当做一个对象放进list中
        List