How do I add multiple classes to a Generic ArrayList?

Question

I'm doing an assignment where I can only add specific classes to a generic ArrayList, but the ArrayList does not seem to be adding the classes as intended.

public class ComputerOrder<T extends Product> extends GenericOrder<T> {
private List<T> products;//the list of items

    public void addProduct(T t) {
        if (t.getClass().isInstance(ComputerPart.class)) {
            products.add(t);    
        }
        if (t.getClass().isInstance(Service.class)) {
            products.add(t);   
        }
        if (t.getClass().isInstance(Peripheral.class)) {
            products.add(t);   
        }
        else System.out.println("Not the right type of object.");
   }

Main args test:

public static void main(String[] args) {

    ComputerPart c;
    c = new ComputerPart(12);

    ComputerOrder<Product> g3 = new ComputerOrder<>();
    g3.addProduct(c);
    g3.print();

}

The expected result would be that ArrayList g3 would be able to add c, since it's an instance of a ComputerPart object, but instead the ArrayList is empty. Could someone explain what I am doing wrong with my code?

Note: that "else" statement is only in there for testing purposes, and seems to indicate that the if statements are not working correctly, since it keeps getting triggered when I test.

The println will be triggered too often since you haven't used else if for the Service and Peripheral checks. But that won't solve the main problem. Try adding printlns in each conditional clause to help figure out what's going on. — Paul Hicks
– Paul Hicks, Commented May 25, 2019 at 21:01
Even with the isInstance fixed, this is a terrible way to work with generics or object-oriented programming in general. — user2357112
– user2357112, Commented May 25, 2019 at 21:05

rzwitserloot · Accepted Answer · 2019-05-25 21:08:34Z

1

Your primary problem is that you've messed up your isinstance check. That method works in reverse; what you're looking for is:

ComputerPart.class.isInstance(t), not t.getClass().isInstance(ComputerPart.class). But, you can write this a lot more simply as: t instanceof ComputerPart.

Secondarily you've messed up the sysout. Presumably you meant for each 'if' in your code to be an 'else if' instead, except for the first one of course.

answered May 25, 2019 at 21:08

rzwitserloot

107k6 gold badges74 silver badges104 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Brent R Over a year ago

Thanks very much, rzwitserloot, the t instanceof ... works. I appreciate the help!

Collectives™ on Stack Overflow

How do I add multiple classes to a Generic ArrayList?

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related