我的未加权GPA计算器问题

Question

我试图编写一个未加权的GPA计算器，但我遇到了一些问题。

import java.util.Scanner;

public class GPA
{
  public static void main(String[] args)
  {
    Scanner keyboard = new Scanner(System.in);
    System.out.println("How many classes are you taking?");
    int classes = keyboard.nextInt();
    keyboard.nextLine();
    int class1 = 0;
    int gpa = 0;

    for (int x = 0;  x < classes; x++)
    {
        class1++;
        System.out.println("What is your grade for class number " + class1);
        String classGrade = keyboard.nextLine();
        if (classGrade == "A" || classGrade == "a")
        {
         gpa += 4;
        }
        if (classGrade == "b" || classGrade == "B")
        {
          gpa += 3;
        }
        if (classGrade == "C" || classGrade == "c")
        {
          gpa+= 2;
        }
        if (classGrade == "D" || classGrade == "d")
        {
          gpa += 1;
        }
        if (classGrade == "F" || classGrade == "f")
        {
          gpa += 0;
        }
     }
    double calculatedGpa = gpa / classes;
    System.out.println("Your gpa is " + calculatedGpa);
    }
}

输出正常到某一点。它询问我课程的数量，并询问每个班级的成绩。但是，最后，它每次都将GPA输出为0.0。我还是Java的新手;请问我的代码有什么问题？

Answer 1

比较String时出现问题。

您应该使用equals代替==：

    if ("A".equals(classGrade) || "a".equals(classGrade))
    {
        gpa += 4;
    }
    ...

如果NullPointerException为空，我会在classGrade之前放置常量以避免classGrade。

请参阅How do I compare strings in Java?。