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
代码语言：javascript复制<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;
}
@Override
public 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
代码语言：javascript复制<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 25
List<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
代码语言：javascript复制<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 names
List<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
代码语言：javascript复制<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 set
Set<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
代码语言：javascript复制<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 ages
int 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。
代码语言：javascript复制<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
代码语言：javascript复制<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 age
List<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
代码语言：javascript复制<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 York
Optional<Person> personInNY = people.stream()
.filter(person -> "NewYork".equals(person.getCity())).findAny();
personInNY.ifPresent(System.out::println);
// Check if all people are older than 18
boolean 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。
代码语言：javascript复制<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 people
long count = people.stream().count();
System.out.println("Number of people: " count);
// Calculate average age
Double 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：
代码语言：javascript复制<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 collect
List<String> names = people.stream()
.filter(person -> person.getAge() > 20)
.map(Person::getName)
.sorted()
.collect(Collectors.toList());
names.forEach(System.out::println);
// Find the oldest person
Optional<Person> oldestPerson = people.stream()
.max(Comparator.comparing(Person::getAge));
oldestPerson.ifPresent(person -> System.out.println("Oldest Person: " person));
// Group by city
Map<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 age
IntSummaryStatistics 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>摘要： Java Stream API 是用于处理集合和数据的强大工具。它允许：
滤波：根据条件选择元素
映射：转换元素
收集：将元素收集到集合或其他数据结构中
减少：将元素组合成一个结果。
平面映射：展平嵌套结构。
排序：Order 元素。
查找和匹配：根据条件检查元素。
统计学：执行统计操作。
了解这些功能将帮助您编写更简洁、更简洁、更易读的代码。
