JDK8新特性：Stream

JDK8最大的改变：

1. lambda表达式

2. Stream

1. Steam流的入门

什么是Stream？
也叫Stream流，是jdk8开始的一套API，用于操作集合或者数组中的数据

优点：

Stream流大量结合了Lambda的语法风格来创建，提供了一种更加强大、更加简单的操作集合或者数组中的数据，代码更简洁，可读性更好

package org.example;import java.util.*;
import java.util.function.BiConsumer;
import java.util.stream.Collectors;public class Main {public static void main(String[] args) {List<String> names = new ArrayList<>();Collections.addAll(names, "张三丰", "张无忌", "王五", "李四", "张三");System.out.println(names);System.out.println("--------------------------------");// 需求： 将姓张的, 并且名字是3个字的人名放到一个新集合/*1. 集合和数组的API方法*/List<String> list = new ArrayList<>();for (String name: names) {if (name.length() == 3 && name.startsWith("张")) {list.add(name);}}System.out.println(list);System.out.println("--------------------------------");/*2. 使用Steam流*/List<String> list2 = names.stream().filter(s->s.startsWith("张")).filter(a->a.length() == 3).collect(Collectors.toList());System.out.println(list2);}
}

掌握Stream流就是掌握Stream流的创建、中间方法、终结方法

2. Stream流的创建

package org.example;import java.util.*;
import java.util.function.BiConsumer;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import java.util.stream.Stream;public class Main {public static void main(String[] args) {/*1. List 获取 stream流*/List<String> names = new ArrayList<>();Collections.addAll(names, "张三丰", "张无忌", "王五", "李四", "张三");Stream<String> stream = names.stream();stream.filter(s->s.contains("三")).forEach(System.out::println);System.out.println("--------------------------------");/*2. Set 获取stream流*/Set<String> set = new HashSet<>();set.addAll(names);System.out.println(set);System.out.println("--------------------------------");Stream<String> stream1 = set.stream();stream1.filter(s->s.contains("三")).forEach(System.out::println);System.out.println("--------------------------------");/*3. Map 获取stream流*/Map<String, Integer> map = new HashMap<>();map.put("张三丰", 1);map.put("张无忌", 2);map.put("王五", 3);map.put("李四", 4);map.put("张三", 5);Set<String> keys = map.keySet();Stream<String> stream2 = keys.stream();Collection<Integer> values = map.values();Stream<Integer> stream3 = values.stream();Set<Map.Entry<String, Integer>> entries = map.entrySet();Stream<Map.Entry<String, Integer>> kvs = entries.stream();kvs.filter(e -> e.getKey().contains("三")).forEach(e -> {System.out.println(e.getKey() + ' ' + e.getValue());});/*4. 获取数组的stream流*/String[] arr = {"1", "2", "3"};Stream<String> stream4 = Arrays.stream(arr);Stream<String> stream5 = Stream.of(arr);}
}

3. Stream流的中间方法

4. Stream流的终结方法

收集Stream流（只能收集一次）：

package org.example;import java.util.*;import java.util.stream.Collectors;
import java.util.stream.Stream;public class Main {public static void main(String[] args) {Map<String, Integer> map = new HashMap<>();map.put("张三丰", 1);map.put("张无忌", 2);map.put("王五", 3);map.put("李四", 4);map.put("张三", 5);Set<Map.Entry<String, Integer>> entries = map.entrySet();Stream<Map.Entry<String, Integer>> kvs = entries.stream();Stream<Map.Entry<String, Integer>> kvs1 = entries.stream();Map<String, Integer> map1 =  kvs.collect(Collectors.toMap(e -> e.getKey() + "t", e -> e.getValue() * 10));System.out.println(map1);/*{王五t=30, 张三丰t=10, 张三t=50, 李四t=40, 张无忌t=20}*///        Object[] arr = kvs1.toArray();Map.Entry<String, Integer>[] arr = kvs1.toArray(len -> new Map.Entry[len]);System.out.println(Arrays.toString(arr));/*[李四=4, 张三=5, 王五=3, 张三丰=1, 张无忌=2]*/}
}

最后：