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【Java】Java流中的API

news 来源：原创 2024/9/29 12:24:59

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🌈个人主页：人不走空

💖系列专栏：算法专题

⏰诗词歌赋：斯是陋室，惟吾德馨

概述：

关键概念：

使用案例：

摘要：

作者其他作品：

概述：

Java Stream API 有助于处理元素序列，提供过滤、映射和减少等操作。流可用于以声明方式执行操作，类似于对数据的类似 SQL 的操作

关键概念：

流：支持顺序和并行聚合操作的元素序列

中间操作：返回另一个流且延迟的操作（例如，filter、map）

码头运营：产生结果或副作用且不懒惰的操作（例如，collect、forEach）

示例场景：
假设我们有一个 Person 对象列表，并且我们希望使用 Stream API 对该列表执行各种操作

<span style="color:var(--syntax-text-color)"><span style="color:var(--syntax-text-color)"><code>public class Person {private String name;private int age;private String city;public Person(String name, int age, String city) {this.name = name;this.age = age;this.city = city;}public String getName() {return name;}public int getAge() {return age;}public String getCity() {return city;}@Overridepublic String toString() {return "Person{name='" + name + "', age=" + age + ", city='" + city + "'}";}
}
</code></span></span>

使用案例：

滤波
映射
收集
减少
平面映射
排序
查找和匹配
统计学

滤波：过滤允许您选择与给定条件匹配的元素

<span style="color:var(--syntax-text-color)"><span style="color:var(--syntax-text-color)"><code>import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;public class Main {public static void main(String[] args) {List<Person> people = Arrays.asList(new Person("Alice", 30, "New York"),new Person("Bob", 20, "Los Angeles"),new Person("Charlie", 25, "New York"),new Person("David", 40, "Chicago"));// Filter people older than 25List<Person> filteredPeople = people.stream().filter(person -> person.getAge() > 25)                                       .collect(Collectors.toList());filteredPeople.forEach(System.out::println);}
}
</code></span></span>

映射：映射使用函数将每个元素转换为另一种形式

<span style="color:var(--syntax-text-color)"><span style="color:var(--syntax-text-color)"><code>public class Main {public static void main(String[] args) {List<Person> people = Arrays.asList(new Person("Alice", 30, "New York"),new Person("Bob", 20, "Los Angeles"),new Person("Charlie", 25, "New York"),new Person("David", 40, "Chicago"));// Get list of namesList<String> names = people.stream().map(Person::getName).collect(Collectors.toList());names.forEach(System.out::println);}
}
</code></span></span>

收集：收集将流的元素收集到集合或其他数据结构中

<span style="color:var(--syntax-text-color)"><span style="color:var(--syntax-text-color)"><code>public class Main {public static void main(String[] args) {List<Person> people = Arrays.asList(new Person("Alice", 30, "New York"),new Person("Bob", 20, "Los Angeles"),new Person("Charlie", 25, "New York"),new Person("David", 40, "Chicago"));// Collect names into a setSet<String> uniqueCities = people.stream().map(Person::getCity).collect(Collectors.toSet());uniqueCities.forEach(System.out::println);}
}
</code></span></span>

减少：Reducing 使用关联累积函数对流的元素执行 Reduction 并返回 Optional

<span style="color:var(--syntax-text-color)"><span style="color:var(--syntax-text-color)"><code>public class Main {public static void main(String[] args) {List<Person> people = Arrays.asList(new Person("Alice", 30, "New York"),new Person("Bob", 20, "Los Angeles"),new Person("Charlie", 25, "New York"),new Person("David", 40, "Chicago"));// Sum of agesint totalAge = people.stream().map(Person::getAge).reduce(0, Integer::sum);System.out.println("Total Age: " + totalAge);}
}
</code></span></span>

平面映射：FlatMapping 将嵌套结构展平到单个流中。

<span style="color:var(--syntax-text-color)"><span style="color:var(--syntax-text-color)"><code>public class Main {public static void main(String[] args) {List<List<String>> namesNested = Arrays.asList(Arrays.asList("John", "Doe"),Arrays.asList("Jane", "Smith"),Arrays.asList("Peter", "Parker"));List<String> namesFlat = namesNested.stream().flatMap(List::stream).collect(Collectors.toList());namesFlat.forEach(System.out::println);}
}
</code></span></span>

排序：排序允许您对流的元素进行排序

<span style="color:var(--syntax-text-color)"><span style="color:var(--syntax-text-color)"><code>public class Main {public static void main(String[] args) {List<Person> people = Arrays.asList(new Person("Alice", 30, "New York"),new Person("Bob", 20, "Los Angeles"),new Person("Charlie", 25, "New York"),new Person("David", 40, "Chicago"));// Sort by ageList<Person> sortedPeople = people.stream().sorted(Comparator.comparing(Person::getAge)).collect(Collectors.toList());sortedPeople.forEach(System.out::println);}
}
</code></span></span>

查找和匹配：
查找和匹配操作检查流的元素，看看它们是否与给定的谓词匹配

<span style="color:var(--syntax-text-color)"><span style="color:var(--syntax-text-color)"><code>public class Main {public static void main(String[] args) {List<Person> people = Arrays.asList(new Person("Alice", 30, "New York"),new Person("Bob", 20, "Los Angeles"),new Person("Charlie", 25, "New York"),new Person("David", 40, "Chicago"));// Find any person living in New YorkOptional<Person> personInNY = people.stream().filter(person -> "NewYork".equals(person.getCity())).findAny();personInNY.ifPresent(System.out::println);// Check if all people are older than 18boolean allAdults = people.stream().allMatch(person -> person.getAge() > 18);System.out.println("All adults: " + allAdults);}
}</code></span></span>

统计学：Stream API 还可用于执行各种统计操作，例如计数、平均等。

<span style="color:var(--syntax-text-color)"><span style="color:var(--syntax-text-color)"><code>public class Main {public static void main(String[] args) {List<Person> people = Arrays.asList(new Person("Alice", 30, "New York"),new Person("Bob", 20, "Los Angeles"),new Person("Charlie", 25, "New York"),new Person("David", 40, "Chicago"));// Count number of peoplelong count = people.stream().count();System.out.println("Number of people: " + count);// Calculate average ageDouble averageAge = people.stream().collect(Collectors.averagingInt(Person::getAge));System.out.println("Average Age: " + averageAge);}
}</code></span></span>

实际示例：
这是一个使用上述几个功能的综合示例：

<span style="color:var(--syntax-text-color)"><span style="color:var(--syntax-text-color)"><code>import java.util.*;
import java.util.stream.*;public class Main {public static void main(String[] args) {List<Person> people = Arrays.asList(new Person("Alice", 30, "New York"),new Person("Bob", 20, "Los Angeles"),new Person("Charlie", 25, "New York"),new Person("David", 40, "Chicago"));// Filter, map, sort, and collectList<String> names = people.stream().filter(person -> person.getAge() > 20).map(Person::getName).sorted().collect(Collectors.toList());names.forEach(System.out::println);// Find the oldest personOptional<Person> oldestPerson = people.stream().max(Comparator.comparing(Person::getAge));oldestPerson.ifPresent(person -> System.out.println("Oldest Person: " + person));// Group by cityMap<String, List<Person>> peopleByCity = people.stream().collect(Collectors.groupingBy(Person::getCity));peopleByCity.forEach((city, peopleInCity) -> {System.out.println("People in " + city + ": " + peopleInCity);});// Calculate total and average ageIntSummaryStatistics ageStatistics = people.stream().collect(Collectors.summarizingInt(Person::getAge));System.out.println("Total Age: " + ageStatistics.getSum());System.out.println("Average Age: " + ageStatistics.getAverage());}
}</code></span></span>