具有泛型数组列表的泛型类

Question

希望我能得到一些帮助。我正在尝试使用通用 arrayList 实例化一个通用对象 Student 作为成绩列表。我无法让我的 Add 方法正常工作。我不断收到一个空指针异常——这很明显，因为它被初始化为空——只是不确定我的 add 方法有什么问题。任何帮助将不胜感激。另外，这个任务的要求非常严格——我必须这样做。谢谢。

我有两个班级：第一个，学生：

import java.util.ArrayList;

public class Student <S>{

    private String name;
    private ArrayList<S> grades;



    private double average;


    // the constructor
    public Student (String studentName, ArrayList<S> studentGrades){
        name = studentName;
        grades = studentGrades;
    }

    // get the name
    public String getName() {
        return name;
    }

    //set the name
    public void setName(String name) {
        this.name = name;
    }

    // add a grade to the array
    public void addGrade(S n){

        grades.add(n);

    }

    // return a grade from the array. This will be used for the calculation.
    public S getGrade(int n){

        return grades.get(n);


    }

    // compute the average grade for each student
    public double computeAverage(){

        Double sum = 0.00;
        for(S grade : grades){ 

            if (grade instanceof Double){
            sum += (Double)grade;
            }
            if (grade instanceof Integer){
                sum += (Integer)grade;
            }
        }
        average = sum.doubleValue()/ grades.size();
        return average;

    }

    public String toString()
    {
        return name + "'s average grade is " + average;
    }
}

第二个，测试：

public class Test {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        // create the studentList array
        ArrayList<Student> studentList = new ArrayList<Student>();
        HashMap<String, Student> studentMap = new HashMap<String, Student>();

        // the values for math.random to create the grades
        int min = 0;
        int max = 100;
        double dmin = 0.00;
        double dmax = 100.00;

        // initialize the variables

        ArrayList grades = null;

        //------------------------------------
        // Create the students (the method signature is wrong. It blows up regardless of what I try to add.)
        Student<Integer> Fred = new Student<Integer>("Fred", grades);   
        //null pointer exception. Also points to the add method in the student class.

        //-------------------------------------

        Student<Integer> Wilma = new Student<Integer>("Wilma", grades);
        Student<Double> Barney = new Student<Double>("Barney", grades);
        Student<Double> Betty = new Student<Double>("Betty", grades);   

        // add the random grades 
        for(int i = 0; i < 5; i++){
            Fred.addGrade((int) (Math.random() * (max - min) + min));
        }

        for(int i = 0; i < 5; i++){
            Wilma.addGrade((int) (Math.random() * (max - min) + min));
        }

        for(int i = 0; i < 5; i++){
            Barney.addGrade((double) (Math.random() * (dmax - dmin) + dmin));
        }

        for(int i = 0; i < 5; i++){
            Betty.addGrade((double) (Math.random() * (dmax - dmin) + dmin));
        }
        // add the students to the array list
        studentList.add(Fred);
        studentList.add(Wilma);
        studentList.add(Barney);
        studentList.add(Betty);

        studentMap.put(Fred.getName(), Fred);
        studentMap.put(Wilma.getName(), Wilma);
        studentMap.put(Barney.getName(), Barney);
        studentMap.put(Betty.getName(), Betty);


        //iterate through the list & print to the console
        Iterator<Student> itr = studentList.iterator();{
            while(itr.hasNext()){
                System.out.println(itr.next().<Student>toString());

                //Initialize the array to hold the key variables
                Set<String> mapKeys = studentMap.keySet();
                //mapKeys.add(something);

            }

        }

    }
}

Answer 1

您需要初始化 ArrayList 等级。目前您正在传递一个空值。在该行中执行以下操作：

ArrayList grades = new ArrayList();

一个好的做法是在初始化 ArrayList 时使用泛型，但由于您的成绩似乎属于不同的类型，所以我跳过它。

Answer 2

您正在初始化 Test the Grades 为 null

ArrayList grades = null;

当您尝试添加成绩时

Fred.addGrade(...);

Student 中的内部代码是这样的

grades.add(n);

但是 Grades 被初始化为 null 并创建了一个 Null 异常。

用正确的类型初始化成绩。

例子：

Student<Integer> Wilma = new Student<Integer>("Wilma", new ArrayList<Integer>());
Student<Double> Barney = new Student<Double>("Barney", new ArrayList<Double>());
Student<Double> Betty = new Student<Double>("Betty", new ArrayList<Double>());   

Answer 3

在您的测试课程中，您有 ArrayList grades = null; - 使用前需要先创建成绩ArrayList并填入数据。

Answer 4

你可以这样做：

public class Student<S>
{
    // declare array
    private final ArrayList<S> grades = new ArrayList<S>();
    private String name;

    // One arg constructor
    public Student(String name)
    {
        this.name = name;
    }

    // Two arg constructor
    public Student(String name, ArrayList<S> grades)
    {
        this(name);
        this.grades.addAll(grades);
    }

    // etc
}

Answer 5

在 Test 类中按以下方式编辑您的代码：

 Student<Integer> Fred = new Student<Integer>("Fred", new ArrayList<Integer>());   
    //null pointer exception. Also points to the add method in the student class.

    //-------------------------------------

 Student<Integer> Wilma = new Student<Integer>("Wilma", new ArrayList<Integer>());
 Student<Double> Barney = new Student<Double>("Barney", new ArrayList<Double>());
 Student<Double> Betty = new Student<Double>("Betty", new ArrayList<Double>());   

希望，这会有所帮助。