java - 为什么在没有语法错误或运行时错误的情况下这行代码不会执行

Question

当我运行代码时，我没有得到 setDepartment 方法 所需的结果。如果用户输入了错误的选择，我会试图诱骗他们。当我执行代码并输入一个毫无疑问应该执行的值(字符串)时，它不会执行。我已经测试过，看看问题出在哪里。唯一有效的是当我尝试将正确的值硬编码到方法中时。如果我输入了错误的值并捕获了用户，并且输入了正确的值，用户仍然被困在循环中。我检查了我的循环中是否有任何逻辑错误，但我找不到问题。我完全不知所措，我迫不及待地想找到我搞砸的地方。

请忽略courseCost实例变量我还没有处理过，请忽略department后面的system.out.println(department) =在setDepartment方法中的department.toUpperCase。我在 setDepartment 方法 中使用 system.out.println(department) 来排除 department=department.toUpperCase 上可能出现的任何错误 代码行。另外，你可以忽略其他一切，其他一切都很好。感谢您抽出宝贵时间查看。

import java.util.Scanner;

public class Course {
    static Scanner keyboard = new Scanner(System.in);
    private String department;
    private int courseNumber;
    private int courseCredits;
    private double courseCost;

    public Course () {
        department = "unknown";
        courseNumber = 0;
        courseCost = 0;
        courseCredits = 0;
    }

    public Course(String department, int courseNumber, int courseCredits) {
        setDepartment(department);
        setCourseNumber(courseNumber);
        setCourseCredits(courseCredits);
        //courseCost = no value will be passed to the constructor, courseCost will be calculated as courseCredits/2 * $500, see setter below (if the course is a Lab Course add $100 to the cost)
    }

    public String getDepartment() {
        return department;
    }

    public int getCourseNumber() {
        return courseNumber;
    }

    public int getCourseCredits() {
        return courseCredits;
    }

    public double getCourseCost() {
        return courseCost;
    }

    public void setDepartment(String department) {

        boolean enteredCorrectly = false;
        //department = department.toUpperCase();

        do{
            department = department.toUpperCase();
            System.out.println(department);
            //ENGL, MATH, COMP, HIST, HUMN, SCIE, LANG, PHYS
            if( (department == "ENGL") || (department == "MATH") || (department == "COMP") || 
                (department == "HIST") || (department == "HUMN") || (department == "SCIE") || 
                (department == "LANG") || (department == "PHYS") ) {
                this.department = department;
                enteredCorrectly = true;
            } else {
                System.out.println("Please re-enter a valid department.");
                department = keyboard.nextLine();

            }
        } while(!(enteredCorrectly));

    }

    public void setCourseNumber(int courseNumber) {

        boolean enterCorrectly = false;

        do{
            if(1 <= courseNumber && courseNumber <= 399) {
                this.courseNumber = courseNumber;
                enterCorrectly = true;
            } else {
                System.out.println("Please re-enter a valid course number.");
                courseNumber = keyboard.nextInt();
                keyboard.nextLine();
            }
        } while(!(enterCorrectly));

    }

    public void setCourseCredits(int courseCredits) {

        boolean enterCorrectly = false;

        do{
            if(courseCredits == 3 || courseCredits == 4 || courseCredits == 6) {
                this.courseCredits = courseCredits;
                enterCorrectly = true;
            } else {
                System.out.println("Please re-enter a valid course credits.");
                courseCredits = keyboard.nextInt();
                keyboard.nextLine();
            }
        } while(!(enterCorrectly));

    }

    public void setCourseCost(double courseCost) {
        this.courseCost = courseCost;
    }

    public static void main(String[] args) {
        Course c = new Course ("engl", 991, 1);
    }
}

Answer 1

我认为它没有给你预期结果的原因是因为在比较 block 中你使用了运算符==，这意味着你正在比较字符串的引用:

if( (department == "ENGL") || (department == "MATH") || (department == "COMP") || 
    (department == "HIST") || (department == "HUMN") || (department == "SCIE") || 
    (department == "LANG") || (department == "PHYS") ) {
    this.department = department;
    enteredCorrectly = true;
}

在Java中，要比较字符串的值，您应该使用.equals()方法:

if( (department.equals("ENGL") || (department.equals("MATH") || (department.equals("COMP") || 
    (department.equals("HIST") || (department.equals("HUMN") || (department.equals("SCIE") || 
    (department.equals("LANG") || (department.equals("PHYS") ) {
    this.department = department;
    enteredCorrectly = true;
}