Java 提供的两种流

10 Sept 2024 | 4 分钟阅读

在 Java 中，流主要用于提供多种编程范式，这些范式用于以高效且简洁的方式进行数据处理。

Java 包含两种主要的流类型，它们是中间流和终端流。

让我们在接下来的段落中详细了解中间流和终端流。

中间流

中间流操作可以定义为用于将一个流转换为另一个流的操作。中间流的操作通常链接在一起，用于创建转换管道。此外，中间操作是惰性的，这意味着除非调用终端操作，否则它们不会完成执行。这种惰性评估在处理大数据集时，可以实现更高的处理效率。

让我们通过一个 Java 示例程序来了解 Java 中的中间流。

示例 1

文件名: IntermediateStreamsExample.java

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
class Person {
    private String name;
    private int age;
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public int getAge() {
        return age;
    }
    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}
public class IntermediateStreamsExample {
    public static void main(String[] args) {
        // Creating a list of Person objects
        List<Person> people = Arrays.asList(
                new Person("Alice", 28),
                new Person("Bob", 35),
                new Person("Charlie", 22),
                new Person("Diana", 30),
                new Person("Eva", 25));
        // Using intermediate stream operations
        // 1. Filter people aged 30 or older
        List<Person> filteredList = people.stream()
                .filter(person -> person.getAge() >= 30)
                .collect(Collectors.toList());
        System.out.println("People aged 30 or older: " + filteredList);
        // 2. Map names to uppercase
        List<String> upperCaseNames = people.stream()
                .map(Person::getName)
                .map(String::toUpperCase)
                .collect(Collectors.toList());
        System.out.println("Names in uppercase: " + upperCaseNames);
        // 3. Sorting people by age in descending order
        List<Person> sortedList = people.stream()
                .sorted((p1, p2) -> Integer.compare(p2.getAge(), p1.getAge()))
                .collect(Collectors.toList());
        System.out.println("People sorted by age (descending): " + sortedList);
        // 4. Extract distinct ages
        List<Integer> distinctAges = people.stream()
                .map(Person::getAge)
                .distinct()
                .collect(Collectors.toList());
        System.out.println("Distinct ages: " + distinctAges);
    }
}

输出

People aged 30 or older: [Person{name='Bob', age=35}, Person{name='Diana', age=30}]
Names in uppercase: [ALICE, BOB, CHARLIE, DIANA, EVA]
People sorted by age (descending): [Person{name='Bob', age=35}, Person{name='Diana', age=30}, Person{name='Alice', age=28}, Person{name='Eva', age=25}, Person{name='Charlie', age=22}]
Distinct ages: [28, 35, 22, 30, 25]

Java 中的终端流

在 Java 中，终端流操作可以定义为用于产生结果或副作用的操作。当激活或调用单个终端操作时，流将被消耗，并且不允许再对其应用任何操作。终端操作还用于触发整个流管道的评估。

让我们通过一个 Java 示例程序来了解 Java 中的中间流。

文件名: TerminalStreamsExample.java

import java.util.Arrays;
import java.util.List;
import java.util.Optional;
class Person {
    private String name;
    private int age;
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public int getAge() {
        return age;
    }
    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}
public class TerminalStreamsExample {
    public static void main(String[] args) {
        // Creating a list of Person objects
        List<Person> people = Arrays.asList(
                new Person("Alice", 28),
                new Person("Bob", 35),
                new Person("Charlie", 22),
                new Person("Diana", 30),
                new Person("Eva", 25));
        // Using terminal stream operations
        // 1. forEach(): Print each person's details
        System.out.println("Printing each person's details:");
        people.stream()
                .forEach(System.out::println);
        // 2. collect(): Transform the stream into a List of names
        List<String> namesList = people.stream()
                .map(Person::getName)
                .collect(java.util.stream.Collectors.toList());
        System.out.println("\nList of names: " + namesList);
        // 3. reduce(): Calculate the sum of ages
        Optional<Integer> sumAges = people.stream()
                .map(Person::getAge)
                .reduce(Integer::sum);
        System.out.println("\nSum of ages: " + sumAges.orElse(0));
        // 4. count(): Count the number of people
        long count = people.stream().count();
        System.out.println("\nNumber of people: " + count);
        // 5. anyMatch(): Check if anyone is older than 35
        boolean anyMatch = people.stream()
                .anyMatch(person -> person.getAge() > 35);
        System.out.println("\nAnyone older than 35? " + anyMatch);
        // 6. allMatch(): Check if all are younger than 40
        boolean allMatch = people.stream()
                .allMatch(person -> person.getAge() < 40);
        System.out.println("All younger than 40? " + allMatch);
        // 7. noneMatch(): Check if none are older than 40
        boolean noneMatch = people.stream()
                .noneMatch(person -> person.getAge() > 40);
        System.out.println("None older than 40? " + noneMatch);
    }
}

输出

Printing each person's details:
Person{name='Alice', age=28}
Person{name='Bob', age=35}
Person{name='Charlie', age=22}
Person{name='Diana', age=30}
Person{name='Eva', age=25}
List of names: [Alice, Bob, Charlie, Diana, Eva]
Sum of ages: 140
Number of people: 5
Anyone older than 35? false
All younger than 40? true
None older than 40? true

以上是关于 Java 提供的两种流类型的所有必要信息和解释。

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