AVL tree programme in java

/*
* Avl is algo that balance itself while adding new alues to tree
* by rotating branches of binary tree and make itself Binary seaarch tree
* there are four cases which has to tackle 
* rotating - left right ,left left,right right,right left 

Test Case:

AVLTree tree=new AVLTree();
		tree.insert(20);
		tree.insert(25);
		tree.insert(30);
		tree.insert(10);
		tree.insert(5);
		tree.insert(15);
		tree.insert(27);
		tree.insert(19);
		tree.insert(16);
		
		tree.display();




*/



public class AVLTree {
	private class Node{
		int data;
		int height;
		Node left;
		Node right;
		Node(int data){
			this.data=data;
			this.height=1;
		}
		
	}
	private Node root;
	public void insert(int data) {
		this.root=insert(this.root,data);
	}
	private Node insert(Node node,int item) {
		if(node==null) {
			Node add=new Node(item);
			return add;
		}
		if(node.data>item) {
			node.left=insert(node.left,item);
		}
		if(node.data<item) {
			node.right=insert(node.right,item);
		}
		node.height=Math.max(height(node.left),height(node.right))+1;
		int bf=bf(node);
		//LL case
		if(bf>1&&item<node.left.data)
			return rightRotate(node);
		//RR case
		if(bf<-1&&item>node.right.data)
			return leftRotate(node);
		//RL case
		if(bf<-1&&item<node.right.data) {
			node.right=rightRotate(node.right);
			return leftRotate(node);
		}
		//LR case
		if(bf>1&&item>node.left.data) {
			node.left=leftRotate(node.left);
			return rightRotate(node);
		}
			
		return node;
	}
	public void display() { 
		this.display(this.root);
		System.out.println(this.root.height);
	}
	private void display (Node node) {
		Strings str="";
		if(node.left!=null)
			str+=node.left.data+"=>";
		else
			str+="END=>";
		str+=node.data+"";
		if(node.right!=null)
			str+="<="+node.right.data;
		else
			str+="<=END";
		System.out.println(str);
		if(node.left!=null)
			display(node.left);
		if(node.right!=null)
			display(node.right);
	}
	private int height(Node node) {
		if(node==null) {
			return 0;
		}
		return node.height;
		
	}
	private int bf(Node node) {
		if(node==null)
			return 0;
		return height(node.left)-height(node.right);
	}
	
	private Node rightRotate(Node c) {
		Node b=c.left;
		Node T3=b.right;
		
		b.right=c;
		c.left=T3;
		c.height=Math.max(height(c.left),height(c.right))+1;
		b.height=Math.max(height(b.left),height(b.right))+1;
		return b;
		
	}
	private Node leftRotate(Node c) {
		Node b=c.right;
		Node T3=b.left;
		
		b.left=c;
		c.right=T3;
		c.height=Math.max(height(c.left),height(c.right))+1;
		b.height=Math.max(height(b.left),height(b.right))+1;
		return b;
		
	}

}