java - Java 文件中的 NullPointerException 误解

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 5 年前。

我必须编写一个程序，将字母形式的成绩(如 A、B、C、D 或 F)转换为数字形式(分别为 4、3、2、1、0)。等级也可以有+或-(+增加等级0.3，-减少相同数字)。我写了这个程序:

public class Grade{
   private String letterGrade;
   private String letter;
   private String symbol = "n";
   private double numericGrade;
   private double grade;
   private final double PLUS = 0.3;
   private final double MENO = -0.3;

   public Grade(String letterGrade) {
      this.letterGrade = letterGrade;
   }

   public double getNumericGrade() {
      if (letterGrade.length() > 1) {
         letter = letterGrade.substring(0, 1);
         symbol = letterGrade.substring(1, 2);
      }

      if (letter.equals("A")) {
         numericGrade = 4.0;

         if (symbol.equals("+")) {
            symbol = "n";
         }
      }
      else if (letter.equals("B")) {
         numericGrade = 3.0;
      }
      else if (letter.equals("C")) {
         numericGrade = 2.0;
      }
      else if (letter.equals("D")) {
         numericGrade = 1.0;
      }
      else if (letter.equals("F")) {
         numericGrade = 0.0;
      }

      if (symbol.equals("+")) {
         grade = numericGrade + PLUS;
      }
      else if (symbol.equals("-")) {
         grade = numericGrade + MENO;
      }
      else {
         grade = numericGrade;
      }

      return grade;
   }
}

程序测试人员:

import java.util.Scanner;
public class GradeTester{
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);

        System.out.print("Please enter the grade in letter form: ");
        String letterGrade = in.next();

        if (letterGrade.equals("A") || letterGrade.equals("A+") || letterGrade.equals("A-")) {
            Grade graduation = new Grade(letterGrade);

            System.out.println("The grade in number form is: " + graduation.getNumericGrade());
        }
        else if (letterGrade.equals("B") || letterGrade.equals("B+") || letterGrade.equals("B-")) {
            Grade graduation = new Grade(letterGrade);

            System.out.println("The grade in number form is: " + graduation.getNumericGrade());
        }
        else if (letterGrade.equals("C") || letterGrade.equals("C+") || letterGrade.equals("C-")) {
            Grade graduation = new Grade(letterGrade);

            System.out.println("The grade in number form is: " + graduation.getNumericGrade());
        }
        else if (letterGrade.equals("D") || letterGrade.equals("D+") || letterGrade.equals("D-")) {
            Grade graduation = new Grade(letterGrade);

            System.out.println("The grade in number form is: " + graduation.getNumericGrade()); 
        }
        else if (letterGrade.equals("F")) {
            Grade graduation = new Grade(letterGrade);

            System.out.println("The grade in number form is: " + graduation.getNumericGrade());
        }
        else {
            System.out.println("Not permissed!");
        }
    }
}

但是当我尝试时，仅对于没有 + 或 - 的字母会出现此错误:

Exception in thread "main" java.lang.NullPointerException
        at Grade.getNumericGrade(Grade.java:20)
        at GradeTester.main(GradeTester.java:13)

我不明白问题出在哪里。

Answer 1

尝试使用 "A".equals(letter) 代替 letter.equals("A") ，这将阻止 NullPointers，因为“A”永远不会为 null : -)

您可能想对所有 equals() 执行此操作，首先是常量，然后是变量，然后再次尝试您的代码...

oh 和 Grade 中的 letter 始终为 null，因为它仅在给出符号时才设置。

只是因为我感觉很慷慨:-)这是一个工作版本

package nl.ivonet;

import java.util.Scanner;

public class Grade {
    private final String letterGrade;
    private String letter;
    private String symbol = "n";
    private double numericGrade;
    private static final double PLUS = 0.3;
    private static final double MENO = -0.3;

    public Grade(final String letterGrade) {
        this.letterGrade = letterGrade;
    }

    public double getNumericGrade() {
        if (this.letterGrade.length() > 1) {
            this.letter = this.letterGrade.substring(0, 1);
            this.symbol = this.letterGrade.substring(1, 2);
        } else {
            this.letter = this.letterGrade;
        }

        if ("A".equals(this.letter)) {
            this.numericGrade = 4.0;

            if ("+".equals(this.symbol)) {
                this.symbol = "n";
            }

        } else if ("B".equals(this.letter)) {
            this.numericGrade = 3.0;

        } else if ("C".equals(this.letter)) {
            this.numericGrade = 2.0;

        } else if ("D".equals(this.letter)) {
            this.numericGrade = 1.0;

        } else if ("F".equals(this.letter)) {
            this.numericGrade = 0.0;

        }

        final double grade;
        if ("+".equals(this.symbol)) {
            grade = this.numericGrade + PLUS;

        } else if ("-".equals(this.symbol)) {
            grade = this.numericGrade + MENO;

        } else {
            grade = this.numericGrade;

        }

        return grade;
    }


    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);

        System.out.print("Please enter the grade in letter form: ");
        String letterGrade = in.next();

        if ("A".equals(letterGrade) || "A+".equals(letterGrade) || "A-".equals(letterGrade)) {
            Grade graduation = new Grade(letterGrade);

            System.out.println("The grade in number form is: " + graduation.getNumericGrade());
        } else if ("B".equals(letterGrade) || "B+".equals(letterGrade) || "B-".equals(letterGrade)) {
            Grade graduation = new Grade(letterGrade);

            System.out.println("The grade in number form is: " + graduation.getNumericGrade());
        } else if ("C".equals(letterGrade) || "C+".equals(letterGrade) || "C-".equals(letterGrade)) {
            Grade graduation = new Grade(letterGrade);

            System.out.println("The grade in number form is: " + graduation.getNumericGrade());
        } else if ("D".equals(letterGrade) || "D+".equals(letterGrade) || "D-".equals(letterGrade)) {
            Grade graduation = new Grade(letterGrade);

            System.out.println("The grade in number form is: " + graduation.getNumericGrade());
        } else if ("F".equals(letterGrade)) {
            Grade graduation = new Grade(letterGrade);

            System.out.println("The grade in number form is: " + graduation.getNumericGrade());
        } else {
            System.out.println("Not permissed!");
        }
    }
}