工作经验|lambada处理集合的常用10种实战骚操作,我都记录下来了

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最近在项目上面经常使用lambada表达式,但是总是记不住,一直都在百度,写完之后就忘记了,感觉很费时间;这次就花点时间,把一些常用的lambada 处理集合的实例都保存了下来(去重,分组,求和,list转map等等),以后就不用到处找了,刚好也可以给同学们分享下;另外也把一些关于使用lambada时遇到的坑也给大家一起分享下,所有代码拿来即用!!!本文档持续更新…

实例演示

商品实体

@Data
@AllArgsConstructor
public class GoodInfo {
    private String mallSource;
    private String skuNo;
    private int price;
    private int monthCount;
}

排序

集合排序在项目中用的频率还蛮高,这里以按照销量排序为例

   List goodInfos = Arrays.asList();
    goodInfos.add(new GoodInfo("tb", "tb_1112312312", 199, 100000));
    goodInfos.add(new GoodInfo("tb", "tb_23534231231", 399, 10));
    goodInfos.add(new GoodInfo("jd", "jd_1110080098", 299, 100));
    goodInfos.add(new GoodInfo("jd", "jd_412313123", 99, 10000000));
    goodInfos.add(new GoodInfo("pdd", "pdd_354532431", 599, 1));
    goodInfos.add(new GoodInfo("pdd", "pdd_1423124131", 499, 10));

    // 按照销量正序 从小到大排序
    goodInfos.sort(Comparator.comparing(GoodInfo::getMonthCount));
    // 按照销量倒序 从大到小排序
    goodInfos.sort(Comparator.comparing(GoodInfo::getMonthCount).reversed());

取最大值/取最小值/求和

    List goodInfos = Arrays.asList();
    goodInfos.add(new GoodInfo("tb", "tb_1112312312", 199, 100000));
    goodInfos.add(new GoodInfo("tb", "tb_23534231231", 399, 10));
    goodInfos.add(new GoodInfo("jd", "jd_1110080098", 299, 100));
    goodInfos.add(new GoodInfo("jd", "jd_412313123", 99, 10000000));
    goodInfos.add(new GoodInfo("pdd", "pdd_354532431", 599, 1));
    goodInfos.add(new GoodInfo("pdd", "pdd_1423124131", 499, 10));

    // 获取最大销量  注意如果求最大值是在filter之后使用例如,goodInfos.stream().filter().max一定要判断filter后集合数量是否不为空,否则使用max的get方法会报错
    GoodInfo hotGoodInfo = goodInfos.stream().max(Comparator.comparing(GoodInfo::getMonthCount)).get();
    // 求最低价格商品
    GoodInfo lowPriceGoodInfo = goodInfos.stream().min(Comparator.comparing(GoodInfo::getMonthCount)).get();
    // 计算商品总价格
    int sum = goodInfos.stream().mapToInt(person -> person.getPrice()).sum();
    // 求平均价格
    double avg = goodInfos.stream().mapToInt(person -> person.getPrice()).average().getAsDouble();

遍历

   List goodInfos = Arrays.asList();
    goodInfos.add(new GoodInfo("tb", "tb_1112312312", 199, 100000));
    goodInfos.add(new GoodInfo("tb", "tb_23534231231", 399, 10));
    goodInfos.add(new GoodInfo("jd", "jd_1110080098", 299, 100));
    goodInfos.add(new GoodInfo("jd", "jd_412313123", 99, 10000000));
    goodInfos.add(new GoodInfo("pdd", "pdd_354532431", 599, 1));
    goodInfos.add(new GoodInfo("pdd", "pdd_1423124131", 499, 10));

    // 遍历输出所有商品id
    goodInfos.forEach(
        goodInfo -> {
          System.out.println(goodInfo.getSkuNo());
        });

实体集合 转 单个属性的集合

往往在我们项目中会有这样的需求:我需要提取集合中某一个属性,然后组装成集合,通常做法是先创建一个字符串集合,然后遍历原始集合,取出数据,放到字符串集合中,虽然也能实现功能,但是不免太过于繁琐,现在使用一行lambada表达式即可搞定:

    List goodInfos = Arrays.asList();
    goodInfos.add(new GoodInfo("tb", "tb_1112312312", 199, 100000));
    goodInfos.add(new GoodInfo("tb", "tb_23534231231", 399, 10));
    goodInfos.add(new GoodInfo("jd", "jd_1110080098", 299, 100));
    goodInfos.add(new GoodInfo("jd", "jd_412313123", 99, 10000000));
    goodInfos.add(new GoodInfo("pdd", "pdd_354532431", 599, 1));
    goodInfos.add(new GoodInfo("pdd", "pdd_1423124131", 499, 10));

    // 将list转为某个属性map 这里是把所有skuno全部取出来 作为集合
    List skuNos = goodInfos.stream().map(goodInfo -> goodInfo.getSkuNo()).collect(Collectors.toList());

实体集合 转 map 返回

    List goodInfos = Arrays.asList();
    goodInfos.add(new GoodInfo("tb", "tb_1112312312", 199, 100000));
    goodInfos.add(new GoodInfo("tb", "tb_23534231231", 399, 10));
    goodInfos.add(new GoodInfo("jd", "jd_1110080098", 299, 100));
    goodInfos.add(new GoodInfo("jd", "jd_412313123", 99, 10000000));
    goodInfos.add(new GoodInfo("pdd", "pdd_354532431", 599, 1));
    goodInfos.add(new GoodInfo("pdd", "pdd_1423124131", 499, 10));

    // 将list转为map,key 为商品id
    Map map = goodInfos.stream().collect(Collectors.toMap(GoodInfo::getSkuNo, goodInfo -> goodInfo));

实体集合按照某个属性分组

    List goodInfos = Arrays.asList();
    goodInfos.add(new GoodInfo("tb", "tb_1112312312", 199, 100000));
    goodInfos.add(new GoodInfo("tb", "tb_23534231231", 399, 10));
    goodInfos.add(new GoodInfo("jd", "jd_1110080098", 299, 100));
    goodInfos.add(new GoodInfo("jd", "jd_412313123", 99, 10000000));
    goodInfos.add(new GoodInfo("pdd", "pdd_354532431", 599, 1));
    goodInfos.add(new GoodInfo("pdd", "pdd_1423124131", 499, 10));

    // 按照商品来源分组
    Map> map = goodInfos.stream().collect(Collectors.groupingBy(GoodInfo::getMallSource));

过滤数据(记得接收)

    List goodInfos = Arrays.asList();
    goodInfos.add(new GoodInfo("tb", "tb_1112312312", 199, 100000));
    goodInfos.add(new GoodInfo("tb", "tb_23534231231", 399, 10));
    goodInfos.add(new GoodInfo("jd", "jd_1110080098", 299, 100));
    goodInfos.add(new GoodInfo("jd", "jd_412313123", 99, 10000000));
    goodInfos.add(new GoodInfo("pdd", "pdd_354532431", 599, 1));
    goodInfos.add(new GoodInfo("pdd", "pdd_1423124131", 499, 10));

    // 过滤商品价格大于300的
    // todo 过滤后一定要使用集合接收,否则等于没有过滤

    List collect =
        goodInfos.stream()
            .filter(goodInfo -> goodInfo.getPrice() > 300)
            .collect(Collectors.toList());
    collect.forEach(
        goodInfo -> {
          System.out.println(goodInfo.getPrice());
        });

去重(两种方法可选)

方法一 set 去重

    List goodInfos = Arrays.asList();
    goodInfos.add(new GoodInfo("tb", "tb_1112312312", 199, 100000));
    goodInfos.add(new GoodInfo("tb", "tb_23534231231", 399, 10));
    goodInfos.add(new GoodInfo("jd", "jd_1110080098", 299, 100));
    goodInfos.add(new GoodInfo("jd", "jd_1110080098", 299, 100));
    goodInfos.add(new GoodInfo("jd", "jd_412313123", 99, 10000000));
    goodInfos.add(new GoodInfo("pdd", "pdd_354532431", 599, 1));
    goodInfos.add(new GoodInfo("pdd", "pdd_1423124131", 499, 10));

    // 使用treeset 集合来实现去重,这里一定要使用集合接收,不然等于没有去重
    List goodInfos1 =
        goodInfos.stream()
            .collect(
                Collectors.collectingAndThen(
                    Collectors.toCollection(
                        () -> new TreeSet<>(Comparator.comparing(o -> o.getSkuNo()))),
                    ArrayList::new));

方法二 map 去重

public static void main(String[] args) {
    List goodInfos = Arrays.asList();
    goodInfos.add(new GoodInfo("tb", "tb_1112312312", 199, 100000));
    goodInfos.add(new GoodInfo("tb", "tb_23534231231", 399, 10));
    goodInfos.add(new GoodInfo("jd", "jd_1110080098", 299, 100));
    goodInfos.add(new GoodInfo("jd", "jd_1110080098", 299, 100));
    goodInfos.add(new GoodInfo("jd", "jd_412313123", 99, 10000000));
    goodInfos.add(new GoodInfo("pdd", "pdd_354532431", 599, 1));
    goodInfos.add(new GoodInfo("pdd", "pdd_1423124131", 499, 10));

    // 使用map去重
    List goodInfos2 =
        goodInfos.stream()
            .filter(distinctByKey(goodInfo -> goodInfo.getSkuNo()))
            .collect(Collectors.toList());
  }

  public static  Predicate distinctByKey(Function keyExtractor) {
    Map seen = new ConcurrentHashMap<>();
    return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
  }

遇到的坑

坑一

  • 报错信息:
    java.util.NoSuchElementException: No value present

  • 解决方案:
    一般出现这个错都是在filter操作后面使用了max/min等操作,然后调用了get方法,取不到数据,导致报错,所以建议检查filter操作后时候还有数据,有数据再进行后续操作:

    Optional optional = goodInfos.stream().max(userComparator);
if(optional != null && optional.isPresent()) {
    recentUserServer = optional.get().getServer();
}

坑二

  • 使用了filter为什么没起作用呢?

调用filter之后,它是有返回值的,所以你需要使用新的集合去接收

。。。

后续慢慢填坑

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