jdk8 Stream流 API

版權聲明：本文為博主原創文章，未經博主允許不得轉載： https://www.cnblogs.com/zwyu/p/9729309.html 

1、流 API 

filter()：對元素進行過濾
map()：將流的元素映射成另一個類型
distinct()：去除流中重復的元素
sorted()：對元素進行排序
forEach ：對流中的每個元素執行某個操作
peek()：與forEach()方法效果類似，不同的是，該方法會返回一個新的流，而forEach()無返回
limit()：截取流中前面幾個元素
skip()：跳過流中前面幾個元素
toArray()：將流轉換為數組
reduce()：對流中的元素歸約操作，將每個元素合起來形成一個新的值
collect()：對流的匯總操作，比如輸出成List集合
anyMatch()：匹配流中的元素，類似的操作還有allMatch()和noneMatch()方法
findFirst()：查找第一個元素，類似的還有findAny()方法
max()：求最大值
min()：求最小值
count()：求總數

2、使用案例

（1）查找、匹配、排序、歸約操作

 

import java.awt.print.Book; import java.util.*; import java.util.stream.Collectors; import java.util.stream.Stream; import static java.util.Comparator.comparing; /** * Created by zwyu on 2018/9/30. */

public class test { public static List<People> getPeople(){ Date dNow = new Date(); Calendar calendar = Calendar.getInstance(); calendar.setTime(dNow); calendar.add(Calendar.DAY_OF_MONTH, -1); Date p1 = calendar.getTime(); calendar.add(Calendar.DAY_OF_MONTH, -2); Date p2 = calendar.getTime(); calendar.add(Calendar.DAY_OF_MONTH, -3); Date p3 = calendar.getTime(); List<People> people = Arrays.asList( new People("張三", 10, p1), new People("李四", 20, p2), new People("老王", 30, p3) ); return people; } public static void test1() { //anyMatch : 任意元素符合篩選條件則返回true
        boolean hasMatch = Stream.of("Java", "C#", "PHP", "C++", "Python") .anyMatch(s -> s.equals("Java")); //allMatch : 所有元素符合篩選條件才返回true
        boolean hasAllMatch = Stream.of("C#", "C++") .allMatch(s -> s.contains("C#")); //noneMatch : 所有元素的篩選條件為false 才返回true
        boolean nonMatch = Stream.of("C#", "C++") .noneMatch(s -> s.contains("C#a")); Optional<String> element = Stream.of("Java", "C#", "PHP", "C++", "Python") .filter(s -> s.contains("C")) // .findFirst() // 獲取流中第一個元素
                .findAny();         // 獲取流中任意元素（其實也是返回第一個元素，能提高並行操作時的性能，推薦使用） // 輸出結果：element: C#, hasMatch: true, hasAllMatch: false, nonMatch: true
        System.out.println("element: " + element.get() + ", hasMatch: " + hasMatch + ", hasAllMatch: " + hasAllMatch + ", nonMatch: " + nonMatch); } public static void test2() { Stream.of(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10) .filter(n -> n > 2)     // 對元素過濾，保留大於2的元素，剩下3,4,5,6,7,8,9,10,10
                .distinct()             // 去重，類似於SQL語句中的DISTINCT，剩下3,4,5,6,7,8,9,10
                .skip(1)                // 跳過前面1個元素
                .limit(5)               // 返回開頭5個元素，類似於SQL語句中的SELECT TOP
                .sorted()               // 對結果排序，如果元素是類對象則應該使用 sorted(Comparator<? super T> comparator)方法排序
 .forEach(System.out::print); // 輸出結果：45678
 } public static void test3() { List<People> people = getPeople(); long count = people.stream().count(); // reduce：歸約操作就是將流中的元素進行合並，形成一個新的值，常見的歸約操作包括: 加減乘除、字符串拼接、求最值等功能
        Optional<Integer> totalAge = people.stream() .map(People::getAge) .reduce((n, m) -> n * m); Optional<People> maxPeople = people.stream().max(comparing(People::getAge)); // 等價於 Optional<People> maxPeople = people.stream().min(Comparator.comparing(People::getAge).reversed());
 Optional<People> minPeople = people.stream().min(comparing(People::getAge)); // 等價於Optional<People> minPeople = people.stream().max(Comparator.comparing(People::getAge).reversed()); // sorted：根據年齡將People對象進行排序, reversed()對排序結果取反: 如之前排序結果是 小到大，加reversed()方法就變為了 大到小
 people.stream().sorted(comparing(People::getBirthday, Comparator.nullsFirst(Date::compareTo)).reversed()).forEach(System.out::println); /** * 輸出結果： People{name='張三', age=10, birthday=Sat Sep 29 16:45:48 CST 2018} People{name='李四', age=20, birthday=Thu Sep 27 16:45:48 CST 2018} People{name='老王', age=30, birthday=Mon Sep 24 16:45:48 CST 2018} count:3, totalAge: 60, max: People{name='老王', age=30, birthday=Mon Sep 24 16:51:39 CST 2018}, min: People{name='張三', age=10, birthday=Sat Sep 29 16:51:39 CST 2018} */ System.out.println("count:" + count + ", totalAge: " + totalAge.get() + ", max: " + maxPeople.get().toString() + ", min: " + minPeople.get().toString()); } public static void test4() { // 利用reduce的歸約操作 實現集合元素的加減乘除、字符串拼接、求最值等功能
        String str = Stream.of("A", "B", "C", "D").reduce("-", String::concat); String str2 = Stream.of("A", "B", "C", "D").collect(Collectors.joining("-")); int minValue = Stream.of(1, 2, 3, 4, 5).reduce(Integer.MAX_VALUE, Integer::min); int minValue2 = Stream.of(1, 2, 3, 4, 5).reduce(0, Integer::min); int minValue3 = Stream.of(1, 2, 3, 4, 5).reduce(Integer::min).get(); int minValue4 = Stream.of(1, 2, 3, 4, 5).reduce(Integer::max).get(); int minValue5 = Stream.of(1, 2, 3, 4, 5).reduce(Integer::sum).get(); int minValue6 = Stream.of(1, 2, 3, 4, 5).reduce((n, m) -> n * m).get(); int sum = Stream.of(1, 2, 3, 4).reduce(Integer::sum).get(); int sum2 = Stream.of(1, 2, 3, 4).reduce(7, Integer::sum); /** * 輸出結果： str: -ABCD, str2: A-B-C-D, minValue: 1, minValue2: 0, minValue3: 1, minValue4: 5, minValue5: 15, minValue6: 120, sum: 10, sum2: 17 */ System.out.println("str: " + str + ", str2: " + str2 + ", minValue: " + minValue + ", minValue2: " + minValue2 + ", minValue3: " + minValue3 + ", minValue4: " + minValue4 + ", minValue5: " + minValue5 + ", minValue6: " + minValue6 + ", sum: " + sum + ", sum2: " + sum2); } public static void test5() { /** * Collectors 常用的方法有： Collectors.toList(); Collectors.toMap(); Collectors.joining(); Collectors.maxBy(); Collectors.groupingBy(); 等 */ List<People> people = getPeople(); // 求和
        long count = people.stream().collect(Collectors.counting()); // 等價於long count = people.stream().count();
 Optional<People> optional = people.stream().collect(Collectors.maxBy(comparing(People::getAge))); // 等價於Optional<People> optional = people.stream().max(comparing(People::getAge));
 Optional<People> optional2 = people.stream().collect(Collectors.minBy(comparing(People::getAge))); // 等價於Optional<People> optional2 = people.stream().min(comparing(People::getAge));
 } public static void main(String[] args) { test1(); test2(); test3(); test4(); } } class People { private String name; private int age; private Date birthday; public People(String name, int age, Date birthday) { this.name = name; this.age = age; this.birthday = birthday; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public String getName() { return name; } public void setName(String name) { this.name = name; } public Date getBirthday() { return birthday; } public void setBirthday(Date birthday) { this.birthday = birthday; } @Override public String toString() { return "People{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", birthday=" + birthday +
                '}'; } }