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; } @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
<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
<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
<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
<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。
<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 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
<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。
<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：
<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。
