【JAVA】Lambda使用

List转Map

public Map<Long, String> getIdNameMap(List<Account> accounts) {
    return accounts.stream().collect(Collectors.toMap(Account::getId, Account::getUsername));
}

public Map<Long, Account> getIdAccountMap(List<Account> accounts) {
    return accounts.stream().collect(Collectors.toMap(Account::getId, account -> account));
}

排序

crowdList.sort((Comparator.comparing(o -> Optional.ofNullable(o.getSumData()).map(SumDataDTO::getCharge).orElse(0.0))));

crowdList = crowdList.stream().sorted((Comparator.comparing(o -> 
    Optional.ofNullable(o.getSumData()).map(SumDataDTO::getCharge).orElse(0.0)))
).collect(Collectors.toList());

public static void main(String[] args) {
        List<Long> listA = Lists.newArrayList(1L, 2L, 3L, 4L);
        List<Long> listB = Lists.newArrayList(1L, 2L, 6L, 8L);
        //取两数组的交集
        List<Long> result = listB.stream().filter(listA::contains).distinct().collect(Collectors.toList());
        System.out.println(result);
    }



按某个属性进行分组
Map<Long, List<AsAdgroupCrowdDataHis>> crowDatasMap = dataHisList.stream().collect(Collectors.groupingBy(AsAdgroupCrowdDataHis::getCrowdid));


提取对象某个属性
List<String> stringList = keywordList.stream().map(SubwaysAlgorithmDeleteKeyword::getWord).distinct().collect(Collectors.toList());

//对象数组按coverage倒序排序
apiHotWordList = apiHotWordList.stream().sorted(Comparator.comparing(ApiKeyWord::getCoverage).reversed()).collect(Collectors.toList());

public static void main(String[] args) {
        List<Long> listA = Lists.newArrayList(1L, 2L, 3L, 4L, 2L, 1L);
        //倒序
        listA.sort(Collections.reverseOrder());
        System.out.println(listA);
        //正序
        Collections.sort(listA);
        System.out.println(listA);
    }


public static void main(String[] args) {
        TestVO vo1 = new TestVO();
        vo1.setId(1L);
        vo1.setNum(2);

        TestVO vo2 = new TestVO();
        vo2.setId(1L);
        vo2.setNum(1);

        List<TestVO> list = new ArrayList<>();
        list.add(vo1);
        list.add(vo2);

        //汇总数组对象某个属性的和
        Integer num = list.stream().mapToInt(TestVO::getNum).sum();
        System.out.println(num);//3
        
        Map<Long, Integer> map = list.stream().collect(Collectors.groupingBy(TestVO::getId, Collectors.summingInt(TestVO::getNum)));
        System.out.println(map);
    }