好吧，我发布了一小部分代码的问题。我意识到如果他们能够看到整个代码，人们会更容易回答。我运行程序时得到nullpointerException。关于数组，我不允许使用.replace .copy .remove。我不知道为什么，但这就是我的老师想要的。
班级成绩：
public class Grades {
// Fields
private float[] listOfGrades;
private int numOfScores;
//
// Constructor
public Grades() {
float[] listOfGrades = new float[0];
}
public Grades(float[] g) {
for (int i = 0; i < g.length; i++) {
if (g[i] >= 0 && g[i] <= 100) {
listOfGrades[i] = g[i];
}
}
}
//
// AccessorMethods
public int getNumberOfScores() {
try {
int numOfScores = 0;
for (int i = 0; i < listOfGrades.length; i++) {
numOfScores++;
}
} catch (Exception e) {
}
return numOfScores;
}
public float[] getCopyOfListOfGrades() {
float[] listOfGradesCopy = new float[listOfGrades.length];
for (int i = 0; i < listOfGrades.length; i++) {
listOfGrades[i] = listOfGradesCopy[i];
}
return listOfGradesCopy = listOfGrades;
}
public float getFirstGrade() {
return listOfGrades[0];
}
public float findMax() {
float largest = listOfGrades[0];
for (int i = 0; i < listOfGrades.length; i++) {
if (listOfGrades[i] > largest) {
largest = listOfGrades[i];
}
}
return largest;
}
public float findMin() {
float largest = listOfGrades[0];
for (int i = 0; i < listOfGrades.length; i++) {
if (listOfGrades[i] < largest && listOfGrades[i] != 0) {
largest = listOfGrades[i];
}
}
return largest;
}
public float calculateAverage() {
float avg = 0;
for (int i = 0; i < listOfGrades.length; i++) {
if (listOfGrades[i] != 0) {
avg += listOfGrades[i];
}
}
return avg / 100;
}
public void addGrade(float g) {
if (g >= 0 && g <= 100) {
float[] newListOfGrades = new float[listOfGrades.length + 1];
for (int i = 0; i < newListOfGrades.length; i++) {
newListOfGrades[i] = listOfGrades[i];
if (listOfGrades.length == i) {
listOfGrades[i] = g;
}
}
} else {
System.out.println("Sorry number not in range");
}
}
public Grades deleteGrade(float g) {
float[] result = new float[listOfGrades.length - 1];
int index = 0;
for (int i = 0; i < listOfGrades.length; i++) {
if (listOfGrades[i] == g) {
result[index] = listOfGrades[i];
index++;
}
}
Grades newGrade = new Grades(result);
return newGrade;
}
}
Class GradesTest：
/* Grades Client */
public class GradesTest {
public static void main(String[] args) {
// Test the default constructor.
Grades emptyScores = new Grades();
if (emptyScores.getNumberOfScores() != 0)
throw new Error("default constructor failed");
// Now test the parameterized Constructor
float[] a1 = { 70, 60, 80, 90, 100 };
Grades allQuizScores = new Grades(a1);
if (allQuizScores.getNumberOfScores() != 5)
throw new Error("Parameterized constructor failed");
float[] a = { 70, -34, 108, 60, 80, -8.9f, 90, 100 };
allQuizScores = new Grades(a);
// Now test getNumberOfScores
if (allQuizScores.getNumberOfScores() != 5)
throw new Error("Parameterized constructor failed with invalid grades");
if (emptyScores.getFirstGrade() != -1)
throw new Error("getFirstGrade failed to handle empty arrays");
a[0] = -100;
// Now check that allQuizScores didn't change
if (allQuizScores.getFirstGrade() == -100)
throw new Error("Constructor failed to make a copy of array");
// Check getCopyOfListOfGradeds
a = emptyScores.getCopyOfListOfGrades();
if (a.length != 0)
throw new Error("getCopyOfListOfGrades failed on empty array");
a = allQuizScores.getCopyOfListOfGrades();
a[0] = -100;
if (allQuizScores.getFirstGrade() == -100)
throw new Error("getCopyOfListOfGrades failed to make a copy of array");
// Now check that min,max, and average are correct
if (allQuizScores.findMin() != 60)
throw new Error("failed to find minimum value");
if (emptyScores.findMin() != -1)
throw new Error("findMin failed to handle case of empty array");
if (allQuizScores.findMax() != 100)
throw new Error("failed to find maximum value");
if (emptyScores.findMax() != -1)
throw new Error("findMax failed to handle case of empty array");
if (allQuizScores.calculateAverage() != 80)
throw new Error("failed to find average value");
if (emptyScores.calculateAverage() != -1)
throw new Error("findAverage failed to handle case of empty array");
// Testing addGrade
emptyScores.addGrade(10);
if (emptyScores.findMin() != 10 || emptyScores.getNumberOfScores() != 1)
throw new Error("addGrade fails on empty array");
allQuizScores.addGrade(10);
if (allQuizScores.findMin() != 10 || allQuizScores.getNumberOfScores() != 6)
throw new Error("addGrade fails on non-empty array");
allQuizScores.addGrade(-10);
if (allQuizScores.findMin() == -10)
throw new Error("addGrade fails for invalid grades");
allQuizScores.addGrade(10);
allQuizScores.addGrade(10);
Grades newQuizScores = allQuizScores.deleteGrade(allQuizScores.findMin());
if (newQuizScores.getNumberOfScores() != 7)
throw new Error("deleteGrade failed for non-empty array");
Grades emptyScores2 = emptyScores.deleteGrade(10);
if (emptyScores2.getNumberOfScores() != 0)
throw new Error("deleteGrade failed for single-item array");
emptyScores2 = emptyScores.deleteGrade(10);
if (emptyScores2.getNumberOfScores() != 0)
throw new Error("deleteGrade failed for empty array");
// Test equals
if (emptyScores.equals(allQuizScores))
throw new Error("Equals fails");
if (newQuizScores.equals(allQuizScores))
throw new Error("Equals fails");
float[] b = { 70, 60, 80, 90, 100, 10, 10 };
Grades gr = new Grades(b);
System.out.println(gr.toString());
System.out.println(newQuizScores.toString());
if (!newQuizScores.equals(gr))
throw new Error("equals fails for non-empty array");
System.out.println(emptyScores.toString());
Grades gr2 = new Grades();
if (!emptyScores2.equals(gr2))
throw new Error("equals fails for empty array");
// test toString
System.out.println("The grades without the minimum grade are:\n" + allQuizScores.toString());
System.out.println("If you are happy with the formatting, you're done!");
}
}
答案 0 :(得分：0)
带参数的构造函数没有考虑内部数组初始化为零长度。
您必须在顶部添加此行：
listofGrades = new float[g.length];
如果没有这个，将g的内容分配给listofGrades就会因NPE失败。
public Grades(float [] g){
listOfGrades = new float[g.length];
for(int i = 0; i < g.length; i++){
if(g[i] >=0 && g[i] <=100){
listOfGrades[i] = g[i];
}
}
}
如果你使用零长度数组，你需要记住，当你尝试访问它们时它们可能仍然是零长度，并做出相应的反应，例如在你的方法中：
public float getFirstGrade(){
return listOfGrades[0];
}
如果在使用默认构造函数构造对象后立即调用它，它也会抛出NullPointerException。
您的代码中存在更多奇怪内容，例如：这样：
public int getNumberOfScores(){
try{
int numOfScores = 0;
for(int i = 0; i < listOfGrades.length;i++){
numOfScores++;
}
}catch(Exception e){
}
return numOfScores;
}
你可以简单地写成：
public int getNumberOfScores(){
return listOfGrades.length;
}
这适用于零长度数组，因为在这种情况下长度为零。