我在线程＆＃34; main＆＃34;中得到错误Exception。 java.lang.NullPointerException当我运行我的程序时，我不知道这意味着什么。如果有人能给我一个非常感激的提示。我已经粘贴了3个班级。感谢
它给我的整个错误是
Exception in thread "main" java.lang.NullPointerException
at CourseGrades.toString(CourseGrades.java:107)
at java.lang.String.valueOf(Unknown Source)
at java.io.PrintStream.println(Unknown Source)
at TestCourseGrades.main(TestCourseGrades.java:23)
FinalExam finalExam) {
grade[0] = labMark;
grade[1] = passFailExam;
grade[2] = essay;
grade[3] = finalExam;
}
public void setLab(GradedActivity labMark) {
grade[0] = labMark;
}
public void setPassFailExam(PassFailExam exam) {
grade[1] = exam;
}
public void setEssay(Essay essay) {
grade[2] = essay;
}
public void setFinalExam(FinalExam finalexam) {
grade[3] = finalexam;
}
public double getAverage() {
double average;
double total = 0;
for (GradedActivity currentArrayValue: grade) {
total += currentArrayValue.getScore();
}
average = total / grade.length;
return average;
}
public GradedActivity getLowest() {
GradedActivity lowest = grade[0];
for (GradedActivity currentArrayValue: grade) {
if (currentArrayValue.getScore() > lowest.getScore()) {
lowest = currentArrayValue;
}
}
return lowest;
}
public GradedActivity getHighest() { // ASK IF THERE IS A WAY TO DO IT WITHOUT ENHCANCED FOR LOOP
GradedActivity highest = grade[0]; // setting highest to the first value
for (GradedActivity currentArrayValue: grade) // going through grade array and holding the value in currentArray value
{
if (currentArrayValue.getScore() > highest.getScore()) // comparing the current array value being looped with the first value that has been set
{
highest = currentArrayValue; // if the current value is bigger than initial value than we put it as the highest
}
}
return highest; // returs the highest value
}
public String toString() {
String str; // FOR LOOP???
str = "Lab Score: " + grade[0].getScore() + ", Grade: " + grade[0].getGrade() + "\n" +
"Pass/Fail Exam Score: " + grade[1].getScore() + ", Grade: " + grade[1].getGrade() +
"Essay Score: " + grade[2].getScore() + ", Grade: " + grade[2].getGrade() + "\n" +
"Final Exam Score: " + grade[3].getScore() + ", Grade: " + grade[3].getGrade();
return str;
}
}
public class GradedActivity {
private double score;
private char letterGrade;
public GradedActivity() {
}
public GradedActivity(double lb) {
score = lb;
}
public void setScore(double s) {
score = s;
}
public void setLabMark(double lb) {
score = lb;
}
public void setLetterGrade(char lg) {
letterGrade = lg;
}
public double getScore() {
return score;
}
public double getLabMark() {
return score;
}
public char getGrade() {
if (score >= 90)
letterGrade = 'A';
else if (score >= 80)
letterGrade = 'B';
else if (score >= 70)
letterGrade = 'C';
else if (score >= 60)
letterGrade = 'D';
else
letterGrade = 'F';
return letterGrade;
}
public String toString() {
String str;
str = "Score: " + score + " Grade: " + letterGrade;
return str;
}
}
import javax.swing.JOptionPane;
public class TestCourseGrades {
public static void main(String[] args) {
String input;
double labMark;
int numQuestions;
int numMissed;
double pointsEach;
int grammar;
int spelling;
int correctLength;
int content;
CourseGrades course = new CourseGrades();
//Get lab mark
input = JOptionPane.showInputDialog("What is your lab mark?");
labMark = Double.parseDouble(input);
GradedActivity lab = new GradedActivity(labMark);
System.out.println("Lab mark is: " + lab);
course.setLab(lab);
System.out.println(course);
//Get pass/fail criteria
input = JOptionPane.showInputDialog("Pass/Fail exam: How many questions?");
numQuestions = Integer.parseInt(input);
input = JOptionPane.showInputDialog("How many questions missed?");
numMissed = Integer.parseInt(input);
input = JOptionPane.showInputDialog("minium passing mark?");
pointsEach = Double.parseDouble(input);
PassFailExam passFailExam = new PassFailExam(numQuestions, numMissed, pointsEach);
System.out.println("Pass Fail Exam:\n" + passFailExam);
course.setPassFailExam(passFailExam);
System.out.println(course);
// Get Essay marks
input = JOptionPane.showInputDialog("Essay: Grammar out of 30: ");
grammar = Integer.parseInt(input);
input = JOptionPane.showInputDialog("Essay: Spelling out of 20: ");
spelling = Integer.parseInt(input);
input = JOptionPane.showInputDialog("Essay: Correct Length out of 20: ");
correctLength = Integer.parseInt(input);
input = JOptionPane.showInputDialog("Essay: Content out of 30: ");
content = Integer.parseInt(input);
Essay essay = new Essay(grammar, spelling, correctLength, content);
System.out.println("Essay:\n" + essay);
course.setEssay(essay);
System.out.println(course);
//Get final exam
input = JOptionPane.showInputDialog("Final exam: How many questions?");
numQuestions = Integer.parseInt(input);
input = JOptionPane.showInputDialog("How many questions missed?");
numMissed = Integer.parseInt(input);
FinalExam finalExam = new FinalExam(numQuestions, numMissed);
System.out.println(finalExam);
course.setFinalExam(finalExam);
System.out.println(course);
//Create a course grade
//Set up the CourseGrades
CourseGrades courseCS = new CourseGrades(lab, passFailExam, essay, finalExam);
System.out.println("Test another course:\n" + courseCS);
System.out.println("The average of all exams is: " + courseCS.getAverage());
GradedActivity highest = courseCS.getHighest();
GradedActivity lowest = courseCS.getLowest();
System.out.println("The highest mark is: " + highest.getScore());
System.out.println("The lowest mark is: " + lowest.getScore());
}
}
`
答案 0 :(得分：0)
该例外清楚地显示了行号
at CourseGrades.toString(CourseGrades.java:107)
在CourseGrades toString（）方法中，您还没有执行任何空检查。
str = "Lab Score: " + grade[0].getScore() + ", Grade: " + grade[0].getGrade() + "\n" +
"Pass/Fail Exam Score: " + grade[1].getScore() + ", Grade: " + grade[1].getGrade() +
"Essay Score: " + grade[2].getScore() + ", Grade: " + grade[2].getGrade() + "\n" +
"Final Exam Score: " + grade[3].getScore() + ", Grade: " + grade[3].getGrade();
在TestCourseGrades中，您只设置了课程实例的LabValue而不设置其他实例。 因此，等级[1]，等级[2]，等级[3]值的值将为空。
这就是你获得空指针异常的原因
修改toString（）方法，如下所示
public String toString() {
String str=""; // FOR LOOP???
if(grade[0]!=null)
str += "Lab Score: " + grade[0].getScore() + ", Grade: " + grade[0].getGrade() + "\n";
if(grade[1]!=null)
str += "Pass/Fail Exam Score: " + grade[1].getScore() + ", Grade: " + grade[1].getGrade() ;
if(grade[2]!=null)
str += "Essay Score: " + grade[2].getScore() + ", Grade: " + grade[2].getGrade() + "\n" ;
if(grade[3]!=null)
str += "Final Exam Score: " + grade[3].getScore() + ", Grade: " + grade[3].getGrade();
return str;
}