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javagenericstype-parameter

Using a class as a type parameter which may or may not contain another type parameter

I am working with Generics and i encountered a problem. So i have a Node class which has a type parameter<T> which extends comparable.

public class Node<T extends Comparable<T>> implements Comparable<Node<T>> {

    private T info;
    private Node next;
    private Node previous;

}

Now, i have designed another class say a Priority Queue class which also contains a comparable type parameter.

public class PriorityQueue<Item extends Comparable<Item>> 
implements Iterable<Item> {

....

}

But when i try and use a Node class as a Type in my Priority Queue class like this:

private MaxPriorityQueue<Node> maxPriorityQueue;

It gives me an error saying , Type parameter Node is not defined within bounds, should implement java.lang.comparable.

I think i am conceptually wrong at implementing this. What should be the correct way?

Is there a way i can change Priority Queue class, so that it can take any class(say A) which implements comparable interface (irrespective of the fact whether A contains type parameter or not) as a type parameter.?

Solution

  • If I understand correctly your requirements, you need to enforce your PriorityQueue to accept objects that implement the comparable interface for themselves. In that case, you just need:

    class PriorityQueue<Item extends Comparable<Item>> {

    and eventually if you want a type parameter for the payload of your Node:

    class Node<T> implements Comparable<Node<T>> {

    so that you can do this:

    private PriorityQueue<Node<Integer>> maxPriorityQueue;

    Like here: https://ideone.com/GBnHp7 (?) (look at the comments as well)