Clarification regrading Java pass-by-value-CodePudding

I am solving a coding question where we need to remove all the sub-trees of a binary tree that only has 0 as its value . Question link https://leetcode.com/problems/binary-tree-pruning/ The solution that worked for me is this

public TreeNode pruneTree(TreeNode root) {
    if (root == null)
        return null;
    root.left = pruneTree(root.left);
    root.right = pruneTree(root.right);
    if (root.val == 0 && root.left == null && root.right == null)
        root = null;
    else
        return root;
   // pruneTree1(root);
 //   printTree(root);
    return root;
}

The solution I tried submitting earlier was this

public TreeNode pruneTree(TreeNode root) {
    pruneTree1(root);
    return root;
}

TreeNode pruneTree1 (TreeNode root) {
    if(root ==null)
        return root ;
    root.left = pruneTree1(root.left);
    root.right = pruneTree1(root.right);
    if(root.left==null && root.right==null && root.val==0) {
        System.out.println(root.val);
        root =null;
    }
    return root;
}

My question/doubt is why the second solution wasn't changing the original tree. My understanding was Java is pass by value but when we pass an Object by its variable name its a reference to original object and we can change its content.

Why was it not working in this case. Is it because I am trying to set the whole object as null and not just its value ?

I tired recreating the scenario with another example and the code behaves differently in this case . Here's what i tried

public void run1() {
        TreeNode root = new TreeNode();
        root.val = 2;
        TreeNode left = new TreeNode();
        left.val = 3;
        TreeNode right = new TreeNode();
        right.val = 4;
        TreeNode leftLeft = new TreeNode();
        leftLeft.val = 5;
        TreeNode rightRight = new TreeNode();
        rightRight.val = 6;
        root.left = left;
        root.right = right;
        left.left = leftLeft;
        right.right = rightRight;
        System.out.println(root.left.left.val);
        TreeNode root2 = makeNull(root);
        System.out.println(root.left.left);
        System.out.println(root2.left.left);

    };

 public   TreeNode  makeNull (TreeNode root){
        if(root ==null)
            return root ;
        root.left = makeNull(root.left);
        root.right = makeNull(root.right);
        if(root.val==5)
            root=null;
        //  left.left = null;
        return root;
    }

In the example i both root.left.left and root2.left.left is set as null when i print it . Why it acts like parameter is passed as reference in this case but not in the above example.

CodePudding user response：

In the second example you nowhere worked with the result of pruneTree1(). But the parameter of that method never got modified for its caller (due to pass-by-value).

Update for your added example:

root.left and root2.left are referring to the same object. As root.left.val != 5 you also don't change that.

You assign root.left and root.right to the return value of the method, but return the input parameter in the most cases. So as long as you don't return null, the same objects are still referenced.