【Stream】简易笔记

一、常用函数

map

处理流元素对象，然后会返回处理完的对象

// bean的属性值复制到目标bean中
.map(x -> {
     SysUserAndUserServiceVO vo = new SysUserAndUserServiceVO();
     BeanUtils.copyProperties(x, vo);
     return vo;
})

// 获取Bean里面的属性值
.map(x -> x.name) 

String experimenterNames = sysUsers.stream().map(SysUser::getNickName).collect(Collectors.joining("、"));
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peek

.map(x -> {
	x.setChildren(null);
    return x;
})


// 等价于
.peek((x) -> x.setChildren(null))
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collect

流转换成集合和聚合元素

// 将流转化为List
.collect(Collectors.toList());
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forEach

对流进行遍历操作

// 对流进行遍历输出
.forEach(x -> {
	System.out.println(x)
});
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filter

// 过滤符合条件的流
.filter(x -> x.getParentId() == 0)
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Reduce

public class Test {
    public static void main(String[] args) {
        List<Integer> list = Arrays.asList(1, 3, 2, 8, 11, 4);
        // 求和方式1
        Optional<Integer> sum = list.stream().reduce((x, y) -> x + y);
        // 求和方式2
        Optional<Integer> sum2 = list.stream().reduce(Integer::sum);
        // 求和方式3
        Integer sum3 = list.stream().reduce(0, Integer::sum);
        // 求乘积
        Optional<Integer> product = list.stream().reduce((x, y) -> x * y);

        // 求最大值方式1
        Optional<Integer> max = list.stream().reduce((x, y) -> x > y ? x : y);
        // 求最大值写法2
        Integer max2 = list.stream().reduce(1, Integer::max);

        System.out.println("list求和：" + sum.get() + "," + sum2.get() + "," + sum3);
        System.out.println("list求积：" + product.get());
        System.out.println("list求和：" + max.get() + "," + max2);

        /**
         * @Result:
         * list求和：29,29,29
         * list求积：2112
         * list求和：11,11
         * 
         */
    }
}

public class MsAssessItem {
    private Integer itemNconfig;

    public Integer getItemNconfig() {
        return itemNconfig;
    }

    public void setItemNconfig(Integer itemNconfig) {
        this.itemNconfig = itemNconfig;
    }

    public static void main(String[] args) {

        List<MsAssessItem> list = new ArrayList<>();
        for (int i = 0; i < 5; i++) {
            MsAssessItem msAssessItem = new MsAssessItem();
            msAssessItem.setItemNconfig(5);
            list.add(msAssessItem);
        }
        // 0 + 5 + 5 + 5 + 5 + 5
        int sum = list.stream().map(MsAssessItem::getItemNconfig).reduce(0, Integer::sum, Integer::sum);
        System.out.println(sum);

        // 5 + 5 + 5 + 5 + 5 + 5
        System.out.println(list.stream().map(MsAssessItem::getItemNconfig).reduce(5, Integer::sum, Integer::sum));

 		List<MsAssessItem> list1 = new ArrayList<>();
        for (int i = 0; i < 5; i++) {
            MsAssessItem msAssessItem = new MsAssessItem();

            msAssessItem.setItemNconfig(i % 2 == 0 ? null : 5);
            list1.add(msAssessItem);
        }
        // 0 + null + null  + 5 + null  + 5
        int sum = list1.stream().filter(x -> x.getItemNconfig() != null).map(MsAssessItem::getItemNconfig).reduce(0, Integer::sum, Integer::sum);
        System.out.println(sum);
        // 5 + null  + null  + 5 + null  + 5
        System.out.println(list1.stream().filter(x -> x.getItemNconfig() != null).map(MsAssessItem::getItemNconfig).reduce(5, Integer::sum, Integer::sum));
    
    }
}
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二、实战

1. 满足某一条件的组装

Q：listA里面包含了所有用户集合，listB里面包含了用户所拥有的主机数量集合。a.userId =b.userId
listA：
表A：
selectUserList()
在这里插入图片描述
listB：
表B：

统计用户服务列表里面业务用户的服务数量：selectUserServiceNum()
求listC：
（有没有大佬可以用sql语句来实现结果listC的结果）

stream流

    public TableDataInfo listA() {
        // 当前业务客户listA
        List<SysUserAndUserServiceVO> list = userService.selectUserList(user).stream()
                .map(x -> {
                    SysUserAndUserServiceVO vo = new SysUserAndUserServiceVO();
                    BeanUtils.copyProperties(x, vo);
                    return vo;
                })
                .collect(Collectors.toList());
        // 统计用户服务列表里面业务用户的服务数量:listB
        List<SysUserAndUserServiceVO> listB = userService.selectUserServiceNum(sysUserAndUserServiceVO);
        // 当前业务客户list 与 服务数量的匹配
        listA.forEach(
                x -> {
                    List<SysUserAndUserServiceVO> collect = listB.stream()
                    		// 当 a.userId =b.userId 的时候
                            .filter(userServiceNum -> userServiceNum.getUserId().equals(x.getUserId()))
                            .collect(Collectors.toList());
                    if (CollectionUtils.isEmpty(collect)){
                        x.setServiceNum(0L);
                    }else {
                        x.setServiceNum(collect.get(0).getServiceNum());
                    }
                }
        );
        return getDataTable(list);
    }
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2. 树级集合的组装

数据库：暂定树的高度不超过3.
在这里插入图片描述

效果图：
在这里插入图片描述
用for循环

    public List<AiBusinessTypeVO> selectAiBusinessTypeTree(AiBusinessType aiBusinessType) {
        // 查询所有的数据
        aiBusinessType.setParentId(0L);
        aiBusinessType.setDelFlag(DelFlag.OK.getCode());
        
        List<AiBusinessTypeVO> oneList = aiBusinessTypeService.selectAiBusinessTypeListVO(aiBusinessType);
        for (AiBusinessTypeVO businessType : oneList) {
            AiBusinessType tempBusinessType = new AiBusinessType();
            List<AiBusinessTypeVO> twoList = aiBusinessTypeService.selectAiBusinessTypeListVO(tempBusinessType);
            businessType.setAiBusinessTypes(twoList);
            for (AiBusinessTypeVO type : twoList) {
                tempBusinessType = new AiBusinessType();
                List<AiBusinessTypeVO> threeList = aiBusinessTypeService.selectAiBusinessTypeListVO(tempBusinessType);
                type.setAiBusinessTypes(threeList);
            }
        }
        return getDataTable(oneList);
    }
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