仅适用于 Integer 的通用 Java 类的方法

Question

I have a generic class like the Java Linked List with a method sum() which should only be accessible if the list is of type Integer. The constructor and sum() method look like this:

public class JList<T> {
    public JNode<T> sentinel;

    public JList() {
        this.sentinel = new JNode<T>();
    }

    // Sum of every entry
    public int sum() {
        JNode<Integer> n = (JNode<Integer>) this.sentinel.next;
        int sum = 0;

        while (n != null) {
            sum += n.element;
            n = n.next;
        }
        return sum;
    }

...

Before executing the method I dont want to ask whether the sentinel is instace of Integer. I was wondering if there was a way to not show sum() at all if the list is of type String

I tried changing JList to an abstract class and implementing a seperate class for Integers like this:

public abstract class JList<T> {
    public JNode<T> sentinel;

    public JList() {
        this.sentinel = new JNode<T>();
    }
    
    public abstract int sum();

and

public class JListInteger extends JList<Integer> {

    public int sum() {
        JNode<Integer> n = (JNode<Integer>) this.sentinel.next;
        int sum = 0;

        while (n != null) {
            sum += n.element;
            n = n.next;
        }
        return sum;
    }
}

But when trying to create a list

public class Test {
    public static void main(String[] args) {

        JList<String> l1 = new JList<>();
    }
}

it says "Cannot instantiate the type JList"

Answer 1

Your inheritance attempt is correct. Just don't make the base class abstract , so that you can instantiate it.

public class JList<T> {
    public JNode<T> sentinel;

    public JList() {
        this.sentinel = new JNode<T>();
    }
}

Note that the sum method should only be defined for the JListInteger sub class, since the base class doesn't necessarily support sum operation.