New to Java

I'm really tired so this is probably a really dumb mistake, but I'm having trouble printing my binary search tree. Here's the method that I made to print(it works fine if I use System.out.println, but I want to use JOptionPane) I'm open to any suggestions as to how to make the code better too. Please let me know if you need to see all of my code

Thanks

public void print() 
	{ 
	JOptionPane.showMessageDialog(null, recprint(root));
	} 
	
	private void recprint(TreeNode p) 
	{ // recursively print the tree 
		 if (p == null) 
	return; 
	recprint (p.left); 
	System.out.print(p.key + "\n"); 
 	recprint (p.right); 
	}