import java.util.*;

public class TreeNode {
 
  String name = null;
  TreeNode parent = null;  
  List<TreeNode> children = new ArrayList<>();
  
  public TreeNode(String name) {
    this.name = name;
  }

  public synchronized void addChild(TreeNode child){
    if(!this.children.contains(child)) {
      this.children.add(child);
      System.out.println("Hi! Locked parent and attemping to lock child.");
      child.setParentOnly(this);
      System.out.println("Hi! Locked child as well.");
    }
  }
  
  public synchronized void addChildOnly(TreeNode child){
    if(!this.children.contains(child)){
      this.children.add(child);
    }
  }
  
  public synchronized void setParent(TreeNode parent){
    this.parent = parent;
    System.out.println("Hi! Locked child and attemping to lock parent.");
    parent.addChildOnly(this);
    System.out.println("Hi! Locked parent as well.");
  }

  public synchronized void setParentOnly(TreeNode parent){
    this.parent = parent;
  }
  
  public static void main(String args[]) {
    final TreeNode child = new TreeNode("child");
    final TreeNode parent = new TreeNode("parent");
    Thread a = new Thread(new Runnable() {
      public void run() { child.setParent(parent); }
    }); 
    a.start();
    Thread b = new Thread(new Runnable() {
      public void run() { parent.addChild(child); }
    });
    b.start();    
  }
}