Java Stream流编程入门

流式编程

在这里插入图片描述

stream流式编程分为

首先转化为stream
中间函数的链接
最后的终结函数

怎么转化为stream

单列集合

List<String> list = new ArrayList<String>();
Collections.addAll(list,"1","2","3","4","5","6","7","8","9");
list.stream().forEach(System.out::println);

双列集合

Map<String,String> map = new HashMap<String,String>();
map.put("1","2");
map.put("3","4");
map.put("5","6");
map.entrySet().stream().forEach(System.out::println);

数组

Arrays.stream(new int[0]);

多个数字

Stream.of(1,2,3,4).forEach(System.out::println);

常用中间函数

注：中间函数可连接多个

filter：过滤

List<String> list = new ArrayList<String>();
Collections.addAll(list,"1","2","3","4","5","6","7","8","9");
list.stream().filter(s -> s.equals("1")).forEach(System.out::println);

输出：1

map：将流中的每个元素传入给定的函数，得到一个新的流。

Stream.of("a","b","c","d").map(String::toUpperCase).forEach(System.out::print);

输出：ABCD

limit：取前n个

List<String> list = new ArrayList<String>();
Collections.addAll(list,"1","2","3","4","5","6","7","8","9");
list.stream().limit(3).forEach(System.out::print);

输出：123

skip：跳过前n个

List<String> list = new ArrayList<String>();
Collections.addAll(list,"1","2","3","4","5","6","7","8","9");
list.stream().skip(4).forEach(System.out::print);

输出：56789

distinct：去重，依赖hashCode(),equals()

List<String> list = new ArrayList<String>();
Collections.addAll(list,"1","2","3","4","4","3","2","1","0");
list.stream().distinct().forEach(System.out::print);

输出：12340

concat：合并两个流

List<String> l1 = new ArrayList<String>();
l1.add("1");
List<String> l2 = new ArrayList<String>();
l2.add("2");
Stream.concat(l1.stream(),l2.stream());

输出：12

sorted: 排序

List<Integer> numbers = Arrays.asList(5, 6, 4, 3, 8, 0, 1, 9, 6, 8);
//第一种   默认
numbers.stream().sorted().forEach(System.out::println);
// 第二种  自定义排序
numbers.stream()// 如果是b.compareTo(a)则降序  a.compareTo(b)则升序.sorted((a, b) -> b.compareTo(a)) .forEach(System.out::println);

输出：
0 1 3 4 5 6 6 8 8 9
9 8 8 6 6 5 4 3 1 0

终结函数

注：终结函数只能有一个

forEach：遍历
count：求数量
collect：收集

lines.stream().filter(s -> s.startsWith("张")).collect(Collectors.toList());
lines.stream().collect(Collectors.toSet());
lines.stream().collect(Collectors.toMap(s -> s.split("-")[0],s -> Integer.parseInt(s.split("-")[1])));

toArray：转化为数组

String[] strings = lines.stream().toArray(val -> new String[val]);

reduce：收纳

List<Integer> asList = Arrays.asList(1, 2, 3, 4, 5);// 示例1: 求和
Optional<Integer> sum = asList.stream().reduce(Integer::sum);
System.out.println("和: " + sum.orElse(0));// 示例2: 求乘积
Optional<Integer> product = asList.stream().reduce((x, y) -> x * y);
System.out.println("乘积: " + product.orElse(1));// 示例3: 求最大值
Optional<Integer> max = asList.stream().reduce(Integer::max);
System.out.println("最大: " + max.orElse(0));