Java8 stream流式編程對List >類型數據進行簡單處理

構造list集合

List<Map<String,Object>> list = Lists.newArrayList();
IntStream.range(1,5).forEach(e->{
    Map<String,Object> map = Maps.newHashMap();
    map.put("name","張三"+(e<3?e:e-1));
    map.put("score", (int)(Math.random()*100)+1);
    list.add(map);
});
System.out.println(list);

輸出：[{score=60, name=張三1}, {score=94, name=張三2}, {score=75, name=張三2}, {score=55, name=張三3}]

根據map中的score進行過濾

//過濾分數大於60的元素
List<Map<String, Object>> filterList = list.stream().filter(
    e -> (int) e.get("score") > 60).collect(Collectors.toList());
System.out.println(filterList);

輸出：[{score=94, name=張三2}, {score=75, name=張三2}]

根據map中的score進行排序

List<Map<String, Object>> sortList = list.stream().sorted((v1, v2) -> {
    BigInteger b1 = new BigInteger(v1.get("score").toString());
    BigInteger b2 = new BigInteger(v2.get("score").toString());
    return b1.compareTo(b2);
}).collect(Collectors.toList());
System.out.println(sortList);

輸出：[{score=55, name=張三3}, {score=60, name=張三1}, {score=75, name=張三2}, {score=94, name=張三2}]

對集合中的map的value值求和

//通過Collect方式
int score = list.stream().collect(Collectors.summingInt(
    e -> Integer.parseInt(e.get("score").toString())));
//通過map方式
int sum = list.stream().mapToInt(e -> Integer.parseInt(e.get("score").toString())).sum();
System.out.println(sum);

輸出：284

根據map的name進行分組，並將name相同的score值求和

List<Map<String, Object>> collectList = list.stream().collect(
    Collectors.groupingBy(e -> e.get("name"))).values().stream().map(e -> {
    Map<String, Object> map = e.get(0);
    map.put("score", e.stream().map(
        s -> new BigInteger(s.get("score").toString())).reduce(BigInteger.ZERO, BigInteger::add));
    return map;
}).collect(Collectors.toList());
System.out.println(collectList);

輸出：[{score=55, name=張三3}, {score=60, name=張三1}, {score=169, name=張三2}]