參數化構造函數錯誤

Question

我的程序有錯誤。 它位於此處顯示的參數化構造函數中。

public class Student {
// instance variables
private String studentId;
private String firstName;
private String lastName;
private double [] grades;


/** 
 * default constructor
 * the id, first and last names are initialized to "none"
 * the array is instantiated to store 4 elements - each element is
 * initialized to -1.0
 */
public Student()
{
    studentId = "none";
    firstName = "none";
    lastName = "none";
    grades = new double [4];
    for(int i = 0; i < grades.length; i++)
    {
        grades[i] = -1.0;
    }
}

/** 
 * parameterized constructor
 * stores the parameters into the appropriate instance variables
 * @param sId the value to be stored in the instance variable studentId
 * @param sFirstName the value to be stored in the instance variable firstName
 * @param sLastName the value to be stored in the instance variable lastName
 * @param sExams the address of the array whose values will be copied into the 
 * instance variable grades
 */
public Student(String sId , String sFirstName , String sLastName , double[] sExams )
{
    studentId = sId;
    firstName = sFirstName;
    lastName = sLastName;
    sExams = new double[grades.length];
    for(int i = 0; i < grades.length; i++)
    {
        grades[i] = sExams[i];
    }




}

/**
 * setStudentId - mutator method for studentId
 * stores the parameter into the instance variable
 * @param sId the value to be stored in the instance variable studentId
 */
public void setStudentId(String sId)
{
    this.studentId = sId;
}

/**
 * setGrades - mutator method for grades
 * stores the parameter into the instance variable
 * @param sExams the address of the array whose values will be copied into the 
 * instance variable grades
 */
public void setGrades(double [] sExams)
{
    for(int i = 0; i < grades.length; i++)
    {
        grades[i] = sExams[i];
    }
}

/**
 * getStudentId - accessor method for id
 * @return a reference to the instance variable id
 */
public String getStudentId()
{
    return studentId;
}

/**
 * getFirstName - accessor method for firstName
 * @return a reference to the instance variable firstName
 */
public String getFirstName()
{
    return firstName;
}


/**
 * getLastName - accessor method for lastName
 * @return a reference to the instance variable lastName
 */
public String getLastName()
{
    return lastName;
}

/**
 * getGrades - accessor method for grades
 * @return a reference to a copy of the instance variable grades
 */
public double [] getGrades()
{
    double [] gradesCopy = new double [grades.length];
    for(int i = 0; i < grades.length; i++)
    {
        gradesCopy[i] = grades[i];
    }
    return gradesCopy;
}

/**
 * findLowestExam - find the lowest exam score in the array and returns its location 
 * in the array
 * @return the position of the lowest exam grade in the array
 */
public int findLowestExam()
{
    int lowestIndex = 0;
    for(int i = 1; i < grades.length; i++)
    {
        if(grades[i] < grades[lowestIndex])
            lowestIndex = i;
    }
    return lowestIndex;
}

/**
 * calcExamAverage - calculates the average of the exams in one of two ways 
 * if the parameter is true, the lowest exam score is dropped in 
 * calculating the average
 * if the parameter is false, no exams are dropped in the calculating
 * the average
 * @param drop - a boolean variable to specify whether or not to drop the lowest score
 * @return the average of the exams
 */

public double calcExamAverage(boolean drop)
{
    double sum = 0; 
    double average;
    if(drop == false)
    {
        for(int i = 0; i < grades.length; i++)
        {
            sum += grades[i];

        }
        average = sum / grades.length;
        return average;
    }

    else
    {
        for(int i = 0; i < grades.length; i++)
        {
            sum += grades[i];


        }
        average = (sum - grades[this.findLowestExam()]) / (grades.length - 1);
        return average;
    }

}
/**
 * toString - create and return a String with the instance variable values
 * @return a reference to a String containing the id, first and last names
 * and the exam grades
 */
public String toString()
{
    String str = "ID: " + studentId + "\n" + "Name: " + lastName + "," + firstName + "\n" + "Grades:";
    for(int i = 0; i < grades.length; i++)
    {
        str += grades[i] + " ";

    }
    return str;





}

}

對於該程序的大部分內容，我已經掌握了。 但是，我跳過了帶有數組的參數化構造函數，直到決定運行它為止。 我發現有一個錯誤，然后返回查看是否沒有在參數化構造函數中正確復制該數組。 我想知道如何解決此問題，因為我找不到解決方案。

Answer 1

問題在下一行

sExams = new double[grades.length];

您正在覆蓋參數，而不是調整成績字段的大小。 如果您將其更改為

grades = new double[sExams.length];

然后，成績字段將被正確地重新分配給sExam參數的數組大小。

Answer 2

您需要更改以下內容：

public Student(String sId , String sFirstName , String sLastName , double[] sGrades )
{
    studentId = sId;
    firstName = sFirstName;
    lastName = sLastName;
    grades = new double[sGrades.length];

    for(int i = 0; i < grades.length; i++)
    {
        grades[i] = sGrades[i];
    }
}

換句話說，您只需要使用傳入的參數來初始化grades就像您在默認構造函數中的工作方式一樣。在當前的參數化構造函數中，您嘗試設置傳入的參數。

此外，您可以這樣設置數組：

grades = sGrades.clone();
// or
System.arraycopy(sGrades, 0, grades , 0, sGrades.length());

Answer 3

這條線

sExams = new double[grades.length];

將破壞sExams的參數值。 sExams是您的輸入參數。 此外，尚未創建grades ，因此grades.length將在NullPointerException上失敗。 sExams.length可以，只要調用方為其提供一個值即可。 您想從sExams創建grades 。