如何将不同的类对象添加到 ArrayList

Question

I am trying to add 2 class objects in which they are an extension of another class into an ArrayList. The codes are as follows:

Student class

public class Student
{
    private String firstName;
    private String lastName;
    
    public Student(String firstName, String lastName)
    {
        this.firstName = firstName;
        this.lastName = lastName;
    }
    
    public String getName()
    {
        return firstName + " " + lastName;
    }
}

UndergradStudent class

public class UndergradStudent extends Student
{
    private int examMark;

    public UndergradStudent(String firstName, String lastName, 
        int examMark)
    {
        // parameters for Student class
        super(firstName, lastName);
        this.examMark = examMark;
    }
}

PostgradStudent class

public class PostgradStudent extends Student
{
    private int thesisMark;

    public PostgradStudent(String firstName, String lastName, 
        int thesisMark)
    {
        // parameters for Student class
        super(firstName, lastName);
        this.thesisMark = thesisMark;
    }   
}

While I am able to add them into 2 different ArrayList , is it possible to combine them into 1 ArrayList ? If so, how should I define the type for it?

The reason I had wanted to have 1 ArrayList is to minimize the amount of duplicated code lines. Eg. in my Main class below, instead of writing the print statement twice, I can write once instead. And I will also be having more methods that access either of the 2 class objects and printing out the same statements...

public class Main
{
    public static void main(String[] args)
    {   
        UndergradStudent ugs = new UndergradStudent("Day", "Parker", 65);
        PostgradStudent pgs = new PostgradStudent("Pete", "Loomis", 78);
        
        ArrayList<UndergradStudent> ugsArr = new ArrayList<UndergradStudent>();
        ArrayList<PostgradStudent> pgsArr = new ArrayList<PostgradStudent>();
        
        ugsArr.add(ugs);
        pgsArr.add(pgs);
        
        // For UndergradStudent
        for (int i=0; i< ugsArr.size(); i++)
        {
            System.out.println("ugs: " + ugsArr.get(i).getName());
        }

        for (int j=0; i< pgsArr.size(); j++)
        {
            System.out.println("pgs: " + pgsArr.get(j).getName());
        } 
    } 
}

Also I tried creating an ArrayList as ArrayList allStudents = new ArrayList(); , and I will get an error for the print statement as it cannot find getName() ..

Appreciate for any advices.

Answer 1

Yes: Use the abstract type:

List<Student> allStudents = new ArrayList<>();

One of the fundamentals of OOP is the Liskov substitution principle , which is basically that all types may safely be identified and dealt with using their abstract type.

Indeed, using the type List<Student> (as I have) instead of ArrayList<Student> is another example, and best practice.

Answer 2

You can create a list of parent class as below. Which will allow you to add both child elements to single list. As below:

List<Student> students = new ArrayList<>();

You can create reference of ArrayList as well, but it's always preferable to go with List .

Answer 3

由于UndergradStudent 和PostgradStudent 是从Student 类继承的，因此可以使用student 类来引用它们，因此您只需使用Student 对象类型的ArrayList 即可接受Student、UndergradStudent、PostgradStudent 的所有条目。

List<Student> allStudentList = new ArrayList<>();