为什么java不可以在方法中这样改变对象的值

Question

如图：

public class TreeNode0 {
        public int val;
        public TreeNode0 left;
        public TreeNode0 right;
        public TreeNode0() {}
        public TreeNode0(int val) { this.val = val; }
        TreeNode0(int val, TreeNode0 left, TreeNode0 right) {
            this.val = val;
            this.left = left;
            this.right = right;
        }

}