Java 中的 mapToInt()

10 Sept 2024 | 5 分钟阅读

在 Java 中，Stream API 负责存储 **mapToInt()** 方法，该方法在 Java 8 版本中引入。**mapToInt()** 方法的主要目的是用于将流中的元素转换为 **IntStream**。

让我们在接下来的段落中详细了解 mapToInt() 方法。

Java Stream API 的 mapToInt()

在 Java 中，当用户处理的是**原始 int** 值而不是常规对象时，mapToInt() 方法会得到主要利用。换句话说，mapToInt() 方法主要用于对流中的每个单独元素应用映射函数，并将它们转换为 **int** 值。

mapToInt() 方法包含两个方法签名，将在后面的段落中讨论。用户负责指定映射函数，该函数必须是 **ToIntFunction< super T >** 类型。在前面提到的语法中，**T** 用于表示原始流中元素的类型。mapToInt() 方法生成的结果将是 **IntStream**，其中存储 **int** 值。

在 Java 中使用 mapToInt() 方法的一个主要优势是它提供了潜在的性能改进。用户在使用原始数据类型时可以实现高效的内存使用。此外，与处理对象流相比，它提供了更高的性能。当用户处理大型数据集或性能敏感型应用程序时，使用 mapToInt() 方法可能会特别有益。

例如，在数学或统计相关的操作中，使用 mapToInt() 可以被证明是高效且有用的。此外，Java 为生成的 **IntStream** 提供了各种类型的方法，可以用于定制和操作 **int** 值。提供的一些方法或操作包括 sum、max、min、average 等。

让我们通过一个 Java 示例程序来详细了解 **mapToInt()** 方法。

文件名: MapToIntExample.java

// Java program to demonstrate the features and functionalities of mapToInt() method in Java. 
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
class CollegeStudent {
    private String student_name;
    private int student_age;
    private List<String> student_subjects;
    public CollegeStudent(String student_name, int student_age, String... student_subjects) {
        this.student_name = student_name;
        this.student_age = student_age;
        this.student_subjects = Arrays.asList(student_subjects);
    }
    public String get_Name() {
        return student_name;
    }
    public int get_Age() {
        return student_age;
    }
    public List<String> get_Subjects() {
        return student_subjects;
    }
    @Override
    public String toString() {
        return "CollegeStudent{" +
                "student_name='" + student_name + '\'' +
                ", student_age=" + student_age +
                ", student_subjects=" + student_subjects +
                '}';
    }
}
public class MapToIntExample {
    public static void main(String[] args) {
        // Creation of a list of CollegeStudent objects
        List<CollegeStudent> students = Arrays.asList(
                new CollegeStudent("Eshwar", 20, "Math", "Physics"),
                new CollegeStudent("Sreeram", 22, "Computer Science", "Java"),
                new CollegeStudent("Arti", 21, "English", "History"),
                new CollegeStudent("Meghana", 23, "Chemistry", "Biology"));
        // Using mapToInt to extract and process ages of students
        IntStream ageStream = students.stream().mapToInt(CollegeStudent::get_Age);
        // Displaying original list of students
        System.out.println("Original List of Students: ");
        students.forEach(System.out::println);
        // Displaying ages of students using mapToInt
        System.out.println("\nAges of Students using mapToInt: ");
        ageStream.forEach(student_age -> System.out.print(student_age + " "));
        // Using mapToInt to calculate total number of characters in all subject names
        IntStream subjectNameLengthStream = students.stream()
                .flatMap(student -> student.get_Subjects().stream()) // Flattening student_subjects
                .mapToInt(String::length); // Mapping to length of each subject student_name
        // Displaying lengths of subject names
        System.out.println("\n\nLengths of Subject Names using mapToInt: ");
        subjectNameLengthStream.forEach(length -> System.out.print(length + " "));
        // Using mapToInt and filter to get the sum of ages of students studying Java
        int sumAgesJavaStudents = students.stream()
                .filter(student -> student.get_Subjects().contains("Java")) // Filtering Java students
                .mapToInt(CollegeStudent::get_Age) // Mapping to ages
                .sum(); // Calculating sum
        System.out.println("\n\nSum of Ages of Students studying Java: " + sumAgesJavaStudents);
        // Using mapToInt and collecting to create a list of ages
        List<Integer> ageList = students.stream()
                .mapToInt(CollegeStudent::get_Age) // Mapping to ages
                .boxed() // Convert IntStream to Stream<Integer>
                .collect(Collectors.toList()); // Collecting to list
        System.out.println("\nList of Ages collected using mapToInt and collect: " + ageList);
    }
}

输出

Original List of Students:
CollegeStudent{student_name='Eshwar', student_age=20, student_subjects=[Math, Physics]}
CollegeStudent{student_name='Sreeram', student_age=22, student_subjects=[Computer Science, Java]}
CollegeStudent{student_name='Arti', student_age=21, student_subjects=[English, History]}
CollegeStudent{student_name='Meghana', student_age=23, student_subjects=[Chemistry, Biology]}

Ages of Students using mapToInt:
20 22 21 23

示例 2

文件名: MapToIntExample1.java

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
class OfficeEmployee {
    private String EmployeeName;
    private int EmployeeSalary;
    public OfficeEmployee(String EmployeeName, int EmployeeSalary) {
        this.EmployeeName = EmployeeName;
        this.EmployeeSalary = EmployeeSalary;
    }
    public String Get_Name() {
        return EmployeeName;
    }
    public int Get_Salary() {
        return EmployeeSalary;
    }
    @Override
    public String toString() {
        return "OfficeEmployee{" +
                "EmployeeName='" + EmployeeName + '\'' +
                ", EmployeeSalary=" + EmployeeSalary +
                '}';
    }
}
public class MapToIntExample1{
    public static void main(String[] args) {
        // Creation of a list of OfficeEmployee objects
        List<OfficeEmployee> employees = Arrays.asList(
                new OfficeEmployee("Eshwar", 50000),
                new OfficeEmployee("Sreeram", 60000),
                new OfficeEmployee("Arti", 75000),
                new OfficeEmployee("Meghana", 90000)
        );
        // Using mapToInt to calculate bonus based on EmployeeSalary
        IntStream bonusStream = employees.stream()
                .mapToInt(employee -> calculateBonus(employee.Get_Salary()));
        // Displaying original list of employees
        System.out.println("Original List of Employees: ");
        employees.forEach(System.out::println);
        // Displaying bonus amounts using mapToInt
        System.out.println("\nBonus Amounts using mapToInt: ");
        bonusStream.forEach(bonus -> System.out.print(bonus + " "));
        // Using mapToInt to categorize employees into EmployeeSalary ranges
        IntStream salaryRangeStream = employees.stream()
                .mapToInt(employee -> categorizeSalaryRange(employee.Get_Salary()));
        // Displaying EmployeeSalary ranges
        System.out.println("\n\nSalary Ranges using mapToInt: ");
        salaryRangeStream.forEach(range -> System.out.print(range + " "));
        // Using mapToInt and filter to get total EmployeeSalary of high-performing employees
        int totalHighPerformersSalary = employees.stream()
                .filter(employee -> calculatePerformanceRating(employee.Get_Salary()) > 0.8)
                .mapToInt(OfficeEmployee::Get_Salary)
                .sum();
        System.out.println("\n\nTotal Salary of High Performers: " + totalHighPerformersSalary);
        // Using mapToInt and collecting to create a list of incremented salaries
        List<Integer> incrementedSalaries = employees.stream()
                .mapToInt(employee -> employee.Get_Salary() + 10000)
                .boxed()
                .collect(Collectors.toList());
        System.out.println("\nList of Incremented Salaries using mapToInt and collect: " + incrementedSalaries);
    }
    private static int calculateBonus(int EmployeeSalary) {
        // Simulated bonus calculation based on EmployeeSalary
        return (int) (EmployeeSalary * 0.1);
    }
    private static int categorizeSalaryRange(int EmployeeSalary) {
        // Simulated EmployeeSalary range categorization
        if (EmployeeSalary < 60000) return 1;
        else if (EmployeeSalary < 80000) return 2;
        else return 3;
    }
    private static double calculatePerformanceRating(int EmployeeSalary) {
        // Simulated performance rating calculation based on EmployeeSalary
        return EmployeeSalary > 70000 ? 0.9 : 0.7;
    }
}

输出

Original List of Employees:
OfficeEmployee{EmployeeName=Eshwar, EmployeeSalary=50000}
OfficeEmployee{EmployeeName='Sreeram', EmployeeSalary=60000}
OfficeEmployee{EmployeeName='Arti', EmployeeSalary=75000}
OfficeEmployee{EmployeeName='Meghana', EmployeeSalary=90000}

Bonus Amounts using mapToInt:
5000 6000 7500 9000

Salary Ranges using mapToInt:
1 2 2 3

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Java 中的 mapToInt()

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