226 lines
7.0 KiB
Markdown
226 lines
7.0 KiB
Markdown
# **Stream流介绍：**
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**配合lambda表达式，简化集合和数组操作**
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## **获取Stream流对象**
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## **Stream流中间操作方法：**
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## 获取对象流演示：
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### **示例：**
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```Java
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ArrayList<String> list = new ArrayList<>();
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list.add("杏菜");
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list.add("佳树");
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list.add("小鞠");
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list.add("梦子");
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//集合Stream流对象
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list.stream().filter(s -> s.startsWith("杏")).forEach(s -> System.out.println(s));
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//数组流对象
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int[] arr = {1, 2, 3};
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Arrays.stream(arr).forEach(s-> System.out.println(s));
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HashSet<Integer> hs = new HashSet<>();
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hs.add(1);
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hs.add(4);
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hs.add(3);
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hs.add(5);
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//map集合流对象，需要间接获取 - map.entrySet().stream
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HashMap<String,Integer> hm = new HashMap<>();
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hm.put("张",13);
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hm.put("李",14);
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hm.put("王",15);
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Set<Map.Entry<String, Integer>> entries = hm.entrySet();
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entries.stream()
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.filter(entry -> !entry.getKey().startsWith("张"))
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.forEach(entry -> System.out.println(entry.getKey()+"---"+entry.getValue()));
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//零散数据获取Stream流对象
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Stream.of("1","e",3).forEach(s-> System.out.println(s));
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```
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## 中间操作方法
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示例：
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```java
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ArrayList<Integer> list = new ArrayList<>();
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Stream<Integer> s2 = list.stream();
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ArrayList<Integer> list1= new ArrayList<>();
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Stream<Integer> s1 = list1.stream();
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Collections.addAll(list1,5,6,7);
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Collections.addAll(list,1,2,3,4);
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//filter过滤方法
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list.stream().filter(s -> s!=2).forEach(s-> System.out.print(s));
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System.out.println('\n');
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//limit获取前几个元素
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list.stream().limit(2).forEach(integer -> System.out.print(integer));
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System.out.println('\n');
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//skip跳过几个元素
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System.out.println("跳过之后的元素");
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list.stream().skip(1).forEach(s-> System.out.print(s));
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System.out.println('\n');
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//concat(Stream a,Stream b)合并ab为一个流
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Stream<Integer> s3 = Stream.concat(s1, s2);
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s3.forEach(new Consumer<Integer>() {
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@Override
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public void accept(Integer s) {
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System.out.println(s);
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}
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});
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//distinct方法去除流中重复的元素依赖(hashCode 和 equals方法)
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//在此会报错，因为上面已经消费过了不能重新使用
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list.stream().distinct().forEach(s-> System.out.println(s));
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```
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**注意：若流已经被使用过，或已使用终结方法，就不允许再次使用**
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## 终结方法
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**示例：**
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```java
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//count方法，返回元素个数
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Stream<Integer> s1 = Stream.of(2, 3, 4, 5, 6, 7, 8).filter(integer -> integer % 2 == 0);
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long count = s1.count();
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System.out.println(count);
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```
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## 重点：收集操作
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为何：因为Stream流操作是不会修改数据源的，图为示例：
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### **如何收集：**
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**示例：**
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```java
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//Stream流收集操作
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Stream<Integer> s1 = Stream.of(1, 2, 3, 4, 5, 6, 7);
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List list1 = s1.limit(3).collect(Collectors.toList());//JDK17版本之后可以将Collectors去除
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System.out.println(list1);
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```
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### **转为map操作（较为复杂）**
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**重点示例**
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```java
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// 将ArrayList中年龄大于15的收集到一个新的map集合中去
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public static void main(String[] args) {
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//将Stream流收集到map所需操作
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ArrayList<String> list = new ArrayList<>();
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Collections.addAll(list,"杏菜,15","佳树,16","老马,17");
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Map<String, Integer> map = list.stream().filter(new Predicate<String>() {
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@Override
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public boolean test(String s) {
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//过滤操作，将字符串数据从逗号切割装入数组，0号索引为名字，1号为年龄
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String[] split = s.split(",");
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//将字符串转为int类型并比较年龄，返回大于等于16岁的
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int age = Integer.parseInt(split[1]);
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return age >= 16;
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}
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}).collect(Collectors.toMap(new Function<String, String>() {
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//创建map集合，第一个匿名内部类为装入键值
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@Override
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public String apply(String s) {
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return s.split(",")[0];
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}
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}, new Function<String, Integer>() {
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//第二个为装入年龄值，并将类型从String改为int
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@Override
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public Integer apply(String s) {
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return Integer.parseInt(s.split(",")[1]);
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}
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}));
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System.out.println(map);
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```
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# Stream流综合案例
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```java
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package com.icacat.day12;
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import java.util.ArrayList;
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import java.util.function.Consumer;
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import java.util.function.Predicate;
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import java.util.stream.Stream;
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public class StreamDemo6 {
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public static void main(String[] args) {
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ArrayList<String> manList = new ArrayList<>();
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manList.add("温水和彦");
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manList.add("社长");
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manList.add("桐谷和人");
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manList.add("勇太");
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ArrayList<String> womanList = new ArrayList<>();
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womanList.add("杏菜");
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womanList.add("小鞠");
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womanList.add("佳树");
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womanList.add("千早");
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Stream<String> s1 = manList.stream().filter(new Predicate<String>() {
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@Override
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public boolean test(String s) {
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return s.length() == 4;
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}
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}).limit(2);
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Stream<String> s2 = womanList.stream().skip(1).filter(s -> s.startsWith("佳"));
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Stream<String> total = Stream.concat(s1, s2);
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total.forEach(new Consumer<String>() {
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@Override
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public void accept(String name) {
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Actor ac = new Actor(name);
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System.out.println(ac);
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}
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});
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}
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}
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class Actor{
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private String name;
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public Actor() {
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}
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public Actor(String name) {
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this.name = name;
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}
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/**
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* 获取
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* @return name
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*/
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public String getName() {
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return name;
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}
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/**
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* 设置
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* @param name
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*/
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public void setName(String name) {
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this.name = name;
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}
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public String toString() {
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return "Actor{name = " + name + "}";
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}
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}
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``` |