Comparable checks

hi i am trying to implement a sorted tree structure where each treenode element must be Comparable. The problem i am having is in the constructors.

I first have a constructor such that:

Code:

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public SortedTreeNode(Comparable element, ArrayList<SortedTreeNode> offspring) {
            this.offspring = offspring;
            this.element = element;
        }

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I think this is fine and will only allow a treenode to be constructed with an element that must be Comparable However i have another constructor with no parameters (Default Constructor) but i cant make element be of type Comparable when i try:

Code:

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public SortedTreeNode() {
            this.offspring = new ArrayList();
            this.element = new Comparable();
        }

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This throws error saying comparable is abstract ? How can i make the check so that when a tree node is constructed it must be of type Comparable??

Thanks.

Comparable is an interface, rather than a class. So you have to specify the behaviour of a Comparable thing if you create one. (Likewise if a class implements Comparable you have to give the specific Comparable behaviour of that class.) In the jargon Comparable is said to be "abstract": meaning that the compiler will know that element has a compareTo() method, but are supposed to actually define it - or use a class that already implements Comparable. The error you get is the compiler saying "A comparable what?!".

What did you mean the no argument constructor to do? I can see that it creates a node with no offspring. But what did you expect the payload (element) to be in that case?

Quote:

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How can i make the check so that when a tree node is constructed it must be of type Comparable?

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The most obvious way would be not to have a no argument constructor. Instead have a constructor that takes a Comparable as its only argument.

the default is supposed to just create a node with empty offspring and an empty element of type comparable.

So element could be anything comparable eg. Integer, String ...

What good is an empty Comparable object? What would it be used for?
Can you set the variable to null?

The concept of an "empty" object doesn't really make sense. The nearest, I guess, is a variable whose value is null. I agree with Norm's question: what will the element be used for?

(Comparable)null will keep the compiler happy. Although element will have the value null anyway. (If you do that you have to be careful thereafter not to call element.compareTo() since it'll give a null pointer exception.)