list.stream.filter常用方式
在Java8中对集合的操作可以这样操作
1.查找集合中的第一个对象
Optional1. filter 过滤
private List integers = Lists.list(30, 40, 10, 20);
Set collect = integers.stream()
.filter(i -> i > 20).collect(Collectors.toSet());
assertEquals(Sets.newTreeSet(30, 40), collect);
2. map 对每个元素执行操作,不会改变原本数量
private List integers = Lists.list(30, 40, 10, 20);
List collect = integers.stream().map(i -> i + "呵呵").collect(Collectors.toList());
assertEquals(Lists.list("30呵呵", "40呵呵", "10呵呵", "20呵呵"), collect);
3. mapToInt 转成IntSream
ArrayList list = Lists.list(2, 5, 1, 6);
IntStream intStream = list.stream().mapToInt(i -> i);
List collect = intStream.boxed()
.collect(Collectors.toList()); // boxed() 把int转成Integer??
System.out.println(collect);
// mapToDouble mapToLong 同理
4. flatMap 类似把多个集合压平成一个?
ArrayList> list = Lists.list(Lists.list(1, 2, 3), Lists.list(1, 2, 3));
Stream integerStream = list.stream().flatMap(Collection::stream);
List collect = integerStream.collect(Collectors.toList());
System.out.println(collect);//[1, 2, 3, 1, 2, 3]
// list.stream().flatMapToInt() TODO 5. distinct 去重
List integers = Lists.list(30, 40, 10, 20, 20, 10, 50, 20);
List collect = integers.stream().distinct().collect(Collectors.toList());
System.out.println(collect);//[30, 40, 10, 20, 50] 6. sorted 排序
collect = integers.stream().sorted().collect(Collectors.toList());
System.out.println(collect);// [10, 10, 20, 20, 20, 30, 40, 50]
collect = integers.stream().sorted((x, y) -> {//这里认为40 不吉利 自定义排到最后
if (y == 40) {
return -1; // 注意 排序比较时 结果 1排在前,-1排在后
}else if (x==40){
return 1;
}
return x.compareTo(y);
}).collect(Collectors.toList());
System.out.println(collect);// [10, 10, 20, 20, 20, 30, 50, 40]
7. peek 不影响主流的情况下 对当前流元素进行提取
List list = Lists.list("刘德华", "x蔡徐坤", "x吴亦凡", "朱大胖", "外星人");
List earth = new ArrayList<>();
List noFat = new ArrayList<>();
List noBadMan =
list.stream().filter(i -> !"外星人".equals(i)).peek(earth::add)
.filter(i -> !i.equals("朱大胖")).peek(noFat::add).filter(i -> !i.startsWith("x"))
.collect(Collectors.toList());
System.out.println("earth : " + earth); // earth : [刘德华, x蔡徐坤, x吴亦凡, 朱大胖]
System.out.println("noFat : " + noFat); // noFat : [刘德华, x蔡徐坤, x吴亦凡]
System.out.println("noBadMan : " + noBadMan); // noBadMan : [刘德华]
8. limit 限定流元素数量
List integers = Lists.list(30, 40, 10, 20, 20, 10, 50, 20);
List collect = integers.stream().limit(3).collect(Collectors.toList());
System.out.println(collect); // [30, 40, 10]
9. skip 跳过多少个元素
List q = Lists.list(30, 40, 10, 20, 20, 10, 50, 20);
List collect5 = q.stream().skip(2).collect(Collectors.toList());
System.out.println(collect5); // [10, 20, 20, 10, 50, 20]
补充分页：2023年6月5日10:22:49
int pageNum=1;
int pageSize=10;
结果：
List> result=Optional.ofNullable(testlist).orElseGet(()->new ArrayList<>()).stream().skip((pageNum-1)*pageSize).limit(pageSize).collect(Collectors.toList());
总数：
int total=result.size
10. foreach foreachOrdered 遍历
Stream limit = Stream.iterate(0,x->x+1).limit(100);
List list = new ArrayList();
// limit.parallel().forEachOrdered(list::add);// [0, 1, 2, 3, 4...
limit.parallel().forEach(list::add);// [62, 63, 64, 65,...
System.out.println(list);
11. reduce 合并?累加
a,b,c -> (a+b),c -> (a+b+c) -> result
List integers = Lists.list(30, 40, 10, 20, 20, 10, 50, 20);
Integer reduce = integers.stream().reduce(0, (x, y) -> x + y);
Integer integer = integers.stream().reduce((x, y) -> x + y).orElse(-1);
Integer reduce1 = integers.stream().reduce(0, (x, y) -> x + y,Integer::sum);
System.out.println(reduce); // 200
System.out.println(integer); // 200
System.out.println(reduce1); // 200
12. collect 收集
List integers = Lists.list(30, 40, 10, 20, 20, 10, 50, 20);
ArrayList 13. min 最小 count总数
List integers = Lists.list(30, 40, 10, 20, 20, 10, 50, 20);
Integer integer1 = integers.stream().min(Integer::compareTo).orElse(-1);
System.out.println(integer1); //10
long count = integers.stream().count();
System.out.println(count);//8
14. findFirst 第一个 findAny 随便一个
List integers = Lists.list(30, 40, 10, 20, 20, 10, 50, 20);
Integer integer2 = integers.stream().findFirst().orElse(-1);
System.out.println(integer2);
Integer integer3 = integers.stream().findAny().orElse(-1);
System.out.println(integer3);//30
15. Stream.iterate Stream.generate 生成一个无限流
后者可以自定义实现方法
必须有limit限制,否则无限
List collect3 = Stream.iterate(0, (x) -> ++x).limit(100).collect(Collectors.toList());
System.out.println(collect3);
List collect4 = Stream.generate(MyStreamTest::get).limit(200)
.collect(Collectors.toList());
System.out.println(collect4);
private static Integer get() {
return RandomUtils.nextInt(1, 1000);
}
16. Stream.concat 组合俩流
如果两个都排好序 那组合的也排好序
Stream limit = Stream.generate(MyStreamTest::get).limit(10).sorted();
Stream limit1 = Stream.generate(MyStreamTest::get).limit(10).sorted();
List collect = Stream.concat(limit, limit1).collect(Collectors.toList());
System.out.println(collect);