Develop Reference

Java extending parameterized class

I'm having trouble trying to figure out the following. Imagine that I have the generic class Node<T> for representing the nodes of a binary tree, with some methods in it.
public class Node<T> {
T info;
Node<T> left;
Node<T> right;
public Node(T info) {this.info=info;}
//and some methods
}
Now I would like to add a method for Nodes of type Integer, which would sum all the nodes that can be reached from the current one:
public int sum(){
int sum = this.info;
if(this.left!=null) sum+=left.sum();
if(this.right!=null) sum+=right.sum();
return sum;
}
I am not quite sure how to do this. I thought of creating a class that extends Node<Integer> and adding the method sum there:
public class NodeOfIntegers extends Node<Integer>{
public NodeOfIntegers (T info) {super();}
public int sum(){...}
}
but since left and right are of type Node<Integer> and not NodeOfIntegers I can't do left.sum() and right.sum().
Is there a way to do this without redefining left and right?
Thank you very much.

Use a reduce function like the Stream provides:
public static class Node<T>{
public Node(T value, Node<T> a, Node<T> b){
this.value = value;
this.a = a;
this.b = b;
}
private final Node<T> a,b;
private final T value;
private T reduce(T start,BinaryOperator<T> operator){
T reduced = operator.apply(start,value);
if(a != null)reduced = a.reduce(reduced,operator);
if(b != null)reduced = b.reduce(reduced,operator);
return reduced;
}
}
public static void main(String[] args) {
Node<Integer> integerNode = new Node<>(4,new Node<>(4,null,null),new Node<>(2,null,null));
System.out.println(integerNode.reduce(0, Integer::sum));
}

You should define your T as
class Node<T extends Number> {
}
then you can write the function of sum as
int sum() {
int sum = this.info.intValue();
}

Instead of using a NodeOfInteger class, I would define in the Node class a
public T combine(BinaryOperator<T> combiner) {
T res = this.info;
if (this.left != null) res = combine(res, this.left.combine(combiner);
if (this.right != null) res = combine(res, this.right.combine(combiner);
return res;
}
which can be used as node.combine(Integer::sum), or as node.combine(String::concat)
(Note that this can be defined outside of the Node class if needed)

Java: Implementing parallel hierarchy of trees and its nodes

In order to practice data structures I'm implementing my own library of Trees. I've begun with BST and in following I'm about to implement AVL Tree, Red-Black Tree and maybe more. AVL & RBT are BST trees as well, so some class hierarchy is rather obvious. The problem I came into is that all those trees have other types of Nodes - AvlNode has balance factor flag, RgbNode has color flag, BstNode doesn't need any additional info (despite of references to parent, children and value which all Nodes need). So I've a hierarchy of Nodes and a hierarchy of Trees. I could give some flag attribute to BstNode and use it in extending classes but it is surely not a good way to do it.
The problem is how to deal with the fact that for example Bst.findNode() will return BstNode but in Avl I need AvlNode despite the findNode() methods will be the same in both (apart of return type).
I need help with planning the hierarchies or if those parallel hierarchies (as a code smell) are in general a bad idea, I need a workaround because I have no clue how to do it in proper way.
BstTree Class:
public class BstTree<T extends Comparable> implements Iterable
{
private BstNode<T> root;
public void addValue(T value)
{
BstNode node = new BstNode(value);
addNode(node);
}
public void addNode(BstNode<T> node)
{
...
}
public boolean removeNode(T value)
{
...
}
public BstNode findNode(T value)
{
...
}
//other less significant methods
}
BstNode class:
public class BstNode<T extends Comparable>
{
private static int lastId = 0;
private int id;
private T value;
private BstNode parent = null;
private BstNode leftChild = null;
private BstNode rightChild = null;
public BstNode(T value) {
this.id = ++lastId;
this.value = value;
}
public boolean isGreaterThan(BstNode n)
{
//...
}
public boolean hasLeftChild()
{
//...
}
public boolean hasRightChild()
{
//...
}
public boolean hasParent()
{
//...
}
public boolean isLeaf()
{
//...
}
public boolean hasOnlyOneChild()
{
//...
}
public BstNode getOnlyChild(BstNode node)
{
...
}
public boolean isLeftChildren()
{
...
}
public BstNode getConsequentNode()
{
...
}
}
I can guess that the separation of responsibilities above may not be perfect, if it's wrong then I might get some of the methods from Node to Tree class but this thing is not a big problem.

I would do something like this:
public abstract class BstTree<T extends Comparable,N extends BstNode<T,N>> {
private N root;
...
public void addValue(T value)
{
N node = newNode(value);
addNode(node);
}
public abstract N newNode(T value);
public void addNode(N node)
{
// ...
}
}
public class BstNode<T extends Comparable,N extends BstNode<T,N>>
{
private T value;
private N parent = null;
private N leftChild = null;
private N rightChild = null;
public BstNode(T value) {
this.value = value;
}
public N getOnlyChild(N node)
{
// ...
}
...
}
public class AVLTree<T extends Comparable> extends BstTree<T,AVLNode<T>> {
...
#Override
public AVLNode<T> newNode(T value) {
return new AVLNode<>(value);
}
}
public class AVLNode<T extends Comparable> extends BstNode<T,AVLNode<T>> {
...
public AVLNode(T value) {
super(value);
}
#Override
public AVLNode<T> getOnlyChild(AVLNode<T> node) {
return super.getOnlyChild(node);
}
...
}

Binary Tree Generic Problemm

Currently studying for an exam, and I stuck at Generic Binary Tree
public interface BinaryTreeNode <T>{
public void setParent(T binaryTreeNodeImpl);
public void setLeft(T left);
public void setRight(T right);
public T getRoot() ;
public T getParent();
public void setValue(T string);
public T getLeft();
}
İt is my interface.
public class BinaryTreeNodeImpl <T> implements BinaryTreeNode <T>{
public T value;
private T Root;
private T right;
private T parent;
private T left;
public void setRight(T right) {
this.right = right;
( (BinaryTreeNode<T>) right).setParent( (T) this);
}
public T getRoot() {
return (T) Root;
}
public void setValue(T value) {
this.value=value;
}
public T getValue() {
return value;
}
public T getParent() {
return parent;
}
public void setParent(T binaryTreeNodeImpl) {
parent= binaryTreeNodeImpl;
}
public T getRight() {
return right;
}
public T getLeft() {
return left;
}
public void setLeft(T left) {
this.left = left;
( (BinaryTreeNode<T>) left).setParent((T) this);
}
}
it is my Binary Tree Implement .When I try to run. Bellow code I've got Value method is undefined for String.
BinaryTreeNode<String> root = new BinaryTreeNodeImpl<String>();
root.setValue("abc");
BinaryTreeNode<String> left = new BinaryTreeNodeImpl<String>();
left.setValue("xyz");
root.setLeft(left);
System.out.println(left.getParent().getValue());
System.out.println(root.getLeft().getValue());
I did not understand where I did wrong ? Should I create abstract method for that ?

The reason that you had the error is that getParent() and getLeft() return T, which is String, when you meant to return a node here.
Your references to root, parent, left, and right in your BinaryTreeNode interface have type T, but that is the type of data that the node needs to reference, not the node itself. Change the datatype of the return values and parameters of the methods that don't have anything to do with the value from T to BinaryTreeNode<T>. This will of course affect the same declarations in your implementing class BinaryTreeNodeImpl.

Your getParent() has a return type T; I am under the impression that you want this to be BinaryTreeNode<T> since a parent of a BinaryTreeNode should be a binary tree node (preferably of the same type).
Your current design is akin to saying that the parent of a BinaryTreeNode can be a String or an Integer or a cat and you are calling the getValue() on that, which obviously is not defined, hence the error.
Make the same edits to the return types in the method declarations in your interface.

Java Generics in a Binary Tree, incompatible types error

I'm trying to implement a Binary Tree in java with Generics, i searched and i find this question: Implementing Binary Tree in Java with Generic Comparable<T> data?, but i couldn't resolve my doubts. So i have two classes,
BST_Tree<T>
and
Node<T extends Comparable<T>>
I want that my implementation can:
Take every type of Object and put it inside the field key in Node
Compare every node with the key field
This is the code:
public class Node < T extends Comparable < T >> {
private T key;
private Node left;
private Node right;
private Node p;
public void setKey(T key) {
this.key = key;
}
public T getKey() {
return key;
}
public Node getLeft() {
return left;
}
public Node getRight() {
return right;
}
public void setLeft(Node left) {
this.left = left;
}
public void setRight(Node right) {
this.right = right;
}
public void setP(Node p) {
this.p = p;
}
public boolean getBolCompMin(T key) {
return this.key.compareTo(key) < 0;
}
}
My Node class is suppose to extend Comparable in order to compare the key.
This is my tree:
public class BST_Tree < T > {
private ArrayList < Node > nodes;
private Node root;
public BST_Tree(Node root) {
this.root = root;
}
public void insertNode(T key) {
Node z = new Node();
z.setKey(key);
Node x = this.root;
Node y = new Node();
while (x != null) {
y = x;
if (z.getBolCompMin(x.getKey())) {
x = x.getLeft();
} else {
x = x.getRight();
}
}
z.setP(y);
if (z.getBolCompMin(y.getKey())) {
y.setLeft(z);
} else {
y.setRight(z);
}
}
public void InOderWalk(Node x) {
if (x != null) {
InOderWalk(x.getLeft());
System.out.println(x.getKey());
InOderWalk(x.getRight());
}
}
public Node getRoot() {
return root;
}
}
My tree tries to set the key in node z but it fails. This is the error:
incompatible types: T cannot be converted to java.lang.Comparable
Thank you in advance!

Your
public class BST_Tree<T>
should be
public class BST_Tree<T extends Comparable<T>>
And every Node variable inside your BST_Tree and Node classes should be Node<T>.
This would ensure that you can only instantiate your BST_Tree class with element types that implement Comparable.

Comparator not letting me compare T values

Intellij idea is giving me this error in class internalNode when trying to compare two T values: compare(capture,capture) cannot be applied to (T,T). I can't understand why. I've already changed the parametrized type to T and it still shows me the message but instead of capture just T.
Any help is very appreciated
public class BSpecialTree<T> {
private Node<T> root;
private int keysNumber;
private Comparator<? super T> cmp;
public BSpecialTree(Comparator<? super T> cmp, int keysNumber) {
this.cmp = cmp;
this.keysNumber = keysNumber;
}
public boolean belongs(T key) {
if(key == null) {
if(root == null)
return true;
return false;
}
return root.belongs(key);
}
private interface Node<T> {
boolean belongs(T key);
}
private class internalNode<T> implements Node<T> {
private T key;
private Node<T> left, right;
public internalNode(T key) {
this.key = key;
}
public boolean belongs(T key) {
if(cmp.compare(this.key, key) > 0) //ERROR
return left.belongs(key);
return right.belongs(key);
}
}
private class leafNode<T> implements Node<T> {
private List<T> keys;
public leafNode() {
keys = new ArrayList<T>(keysNumber);
}
public boolean belongs(T key) {
return keys.indexOf(key) != -1;
}
}

Your inner classes are introducing their own generic type <T> which hides <T> from BSpecialTree<T>, but comparator is declared to be using T from BSpecialTree, not from internalNode.
Maybe try with
private class internalNode implements Node<T> {
// ^remove `<T>` from here