java8 list某两列转成map/从集合中取出某个字段的集合

• list某两列转成map
• 从集合中取出某个字段的集合
• group by
• 遍历集合的时候给某个属性挨个重新赋值

list某两列转成map

List -> Map 需要注意的是： toMap 如果集合对象有重复的key，会报错Duplicate key ....
apple1,apple12的id都为1。 可以用 (k1,k2)->k1 来设置，如果有重复的key,则保留key1,舍弃key2

//key无重复值
List<Entity> list = new ArrayList<>();
Map<Integer, String> map = list.stream().collect(Collectors.toMap(Entity::getId, Entity::getType));

//key有重复值的情况下
Map<Integer, Apple> appleMap = appleList.stream().collect(Collectors.toMap(Apple::getId, a -> a,(k1,k2)->k1));

从集合中取出某个字段的集合

List<String> names = list.stream().map(p -> p.getName()).collect(Collectors.toList());

计算

//集合中某个属性的和(BigDecimal类型)
BigDecimal couponMoneyTotal = redPackageCoupons.stream().map(RedPackageCoupon::getCouponMoney).reduce(BigDecimal::add).get();
//集合中某个属性的最大值
BigDecimal couponMoneyMax = redPackageCoupons.stream().max(Comparator.comparing(RedPackageCoupon::getCouponMoney)).get().getCouponMoney();
//集合中某个属性的最小值
Long couponDaysMin = redPackageCoupons.stream().min(Comparator.comparing(RedPackageCoupon::getCouponDays)).get().getCouponDays();

group by

//根据某属性分组
Map<String,List<Apple>> map = appleList.stream().collect(Collectors.groupingBy(Apple::getType));

 //根据某属性分组后,计算
 Map<String, LongSummaryStatistics> collect = appleList.stream().collect(
         Collectors.groupingBy(Apple::getType,
                 Collectors.summarizingLong(Apple::getCount)));
 for (Map.Entry<String, LongSummaryStatistics> entry : collect.entrySet()) {
    
    
     LongSummaryStatistics longSummaryStatistics = entry.getValue();
     System.out.println("----------------key----------------" + entry.getKey());
     System.out.println("求和:" + longSummaryStatistics.getSum());
     System.out.println("求平均" + longSummaryStatistics.getAverage());
     System.out.println("求最大:" + longSummaryStatistics.getMax());
     System.out.println("求最小:" + longSummaryStatistics.getMin());
     System.out.println("求总数:" + longSummaryStatistics.getCount());
 }

遍历集合的时候给某个属性挨个重新赋值

Bean bean1 = new Bean(1, 2);
Bean bean2 = new Bean(3, 4);
List<Bean> list = Lists.newArrayList(bean1, bean2);
System.out.println("list1 : " + list);
list.stream().filter(bean -> {
    
    
    if (1 == bean.getNum1())
        bean.setNum1(2);
    return true;
}).collect(Collectors.toList());
System.out.println("list2 : " + list);

输出结果
list1 : [Bean(num1=1, num2=2.0), Bean(num1=3, num2=4.0)]
list2 : [Bean(num1=2, num2=2.0), Bean(num1=3, num2=4.0)]