CODEWITHSUNDEEP
javagenericsbinary-tree
Rob L
Rob L

Reputation: 3303

How can I paramaterize this Java generic?

I am building a generic binary tree class in Java. I have not worked with generics too much before, so I do not know if I am doing this properly. Here is some code followed by my question:

public class Node<T> {

     T data;
     Node<T> left, right;
     //assume setters and getters
}


public class BinaryTree<T extends Comparable <T>> {

       private Node<T> root;

       //assume typical setter/getter and insert, delete, etc methods

       public void inOrderTraversal() {

             //create instance of the inorder class here and call its traverse() method.
       }
}
//define a family of algorithms for preorder, inorder and postorder traversals
public interface BinaryTreeTraversal {

    public void traverse();

}
  //of course there will be a class for each type of traversal...
  //in order, pre order, post order
  class PreOrderTraversal extends BinaryTree implements BinaryTreeTraversal {

          public void traverse() { traverse(super.getRoot() ); }

          private void traverse(Node<T>) {
                   //the three lines of code this takes
          }

    }

So the problem I am having is that I keep getting errors saying missing type T, or unknown type T. So I tried changing the class heading it to

      class PreOrderTraversal extends BinaryTree<T> implements BinaryTreeTraversal { }

and 

     class PreOrderTraversal extends BinaryTree<T extends Comparable<T>> implements BinaryTreeTraversal { }

and it did not work due to similar errors. What is a way that I can make this work? I am trying to apply a strategy pattern to the traversals. I just want to be able to have three types of traversals for any generic type. Any tips would be appreciated. Thank you.

Upvotes: 0

Views: 81

Answers (2)

Judge Mental
Judge Mental

Reputation: 5239

private class PreOrderTraversal< T extends Comparable< T > > extends BinaryTree< T > implements BinaryTreeTraversal { ... }

For generic classes, the parameter and all its constraints go right next to the name of the class being defined. Unfortunately, that syntax was not chosen for generic methods, where the generic type parameters and constraints precede the return type.

Upvotes: 1

MatthiasLaug
MatthiasLaug

Reputation: 2914

your class PreOrderTraversal implements you generic class BinaryTree so here you need to define the type of T like

java
class PreOrderTraversal extends BinaryTree<Integer> implements BinaryTreeTraversal {

    private void traverse(Node<Integer>) {
    }

}

where Integer would by the type you are storing in the binary tree, now

Upvotes: 0

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