Problem calling multiple instances of a class in java

i have a problem calling multiple instance of a class that i have coded (Tree, TreeNode) in the main method, the system would give the output c d j c d j even though both trees are obviously different trees. if i were to separate both postOrder() calls(each called after the tree has been pushed in to the stack)

    Stack<Tree> alphaStack = new Stack<Tree>();
    TreeNode a = new TreeNode('i');
    Tree tree = new Tree(a);
    TreeNode newleft = new TreeNode('a');
    TreeNode newright = new TreeNode('b');
    tree.setLeft(a, newleft);
    tree.setRight(a, newright);
    alphaStack.push(tree);
    Tree.postOrder(alphaStack.pop().getRoot());

    TreeNode b = new TreeNode('j');
    Tree newtree = new Tree(b);
    TreeNode left = new TreeNode('c');
    TreeNode right = new TreeNode('d');
    newtree.setLeft(b, left);
    newtree.setRight(b, right);
    alphaStack.push(newtree);

    Tree.postOrder(alphaStack.pop().getRoot());

the output would be a b i c d j.

Does this mean that my class is not being duplicated but instead being reused when i make new Trees?

Below is the code:

import java.util.Stack;

public class mast_score {

public static void main(String[] args){
    Stack<Tree> alphaStack = new Stack<Tree>();
    TreeNode a = new TreeNode('i');
    Tree tree = new Tree(a);
    TreeNode newleft = new TreeNode('a');
    TreeNode newright = new TreeNode('b');
    tree.setLeft(a, newleft);
    tree.setRight(a, newright);
    alphaStack.push(tree);

    TreeNode b = new TreeNode('j');
    Tree newtree = new Tree(b);
    TreeNode left = new TreeNode('c');
    TreeNode right = new TreeNode('d');
    newtree.setLeft(b, left);
    newtree.setRight(b, right);
    alphaStack.push(newtree);

    Tree.postOrder(alphaStack.pop().getRoot());
    Tree.postOrder(alphaStack.pop().getRoot());

} }

code for TreeNode

public class TreeNode{ Object item;

TreeNode parent;
TreeNode left;
TreeNode right;

public TreeNode (Object item) {
    this.item = item;
    parent = null;
    left = null;
    right = null;
}

public TreeNode getParent(TreeNode current) throws ItemNotFoundException
{
    if(current == null) throw new ItemNotFoundException("No parent");
    if(current.parent == null) throw new ItemNotFoundException("This
    is the root");
    else return current.parent;
}
public TreeNode getLeft(TreeNode current) throws ItemNotFoundException
{
    if(开发者_开发百科current == null) throw new ItemNotFoundException("No left or
    right child");
    if(current.left == null) throw new ItemNotFoundException("No left
    child");
    else return current.left;
}

public TreeNode getRight(TreeNode current) throws ItemNotFoundException
{
    if(current == null) throw new ItemNotFoundException("No left or
    right child");
    if(current.right == null) throw new ItemNotFoundException("No
    right child");
    else return current.right;
}

public Object getElement() throws ItemNotFoundException {
    if(this.item == null) throw new ItemNotFoundException("No such
    node");
    else return this.item;
} }

code for Tree class

import java.util.*;

public class Tree {

static TreeNode root;
int size;

public Tree() {
    root = null;
}

public Tree(TreeNode root) {
    Tree.root = root;
}

public TreeNode getRoot() {
    return this.root;
}

public int getLvl(TreeNode node) {
    return node.lvlCount;
}

public void setLeft(TreeNode node, TreeNode left) {
    node.left = left;
}

public void setRight(TreeNode node, TreeNode right) {
    node.right = right;
}
public static void postOrder(TreeNode root) {
    if (root != null) {

        postOrder(root.left);
        postOrder(root.right);
        System.out.print(root.item + " ");

    } else {
        return;
    }

}

public static int getSize(TreeNode root) {
    if (root != null) {
        return 1 + getSize(root.left) +
    getSize(root.right);
    } else {
        return 0;
    }
}

public static boolean isEmpty(Tree Tree) {
    return Tree.root == null;
} }

Your problem is here, in the Tree class:

static TreeNode root;

You should remove the word static, and replace Tree.root with this.root.

Adding the keyword static causes the variable root to be shared between all instances of Tree in your program, which is not what you want.