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。
最佳答案
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!");
}
}
}
关于java - Java 文件中的 NullPointerException 误解,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/47744678/