Program: How implement bounded types (implements an interface) with generics?

package com.java2novice.generics;

public class MyBoundedInterface {

	public static void main(String a[]){
		
		//Creating object of implementation class X called Y and 
		//passing it to BoundExmp as a type parameter.
		BoundExmp<Y> bey = new BoundExmp<Y>(new Y());
		bey.doRunTest();
		//Creating object of implementation class X called Z and 
		//passing it to BoundExmp as a type parameter.
		BoundExmp<Z> bez = new BoundExmp<Z>(new Z());
		bez.doRunTest();
		//If you uncomment below code it will throw compiler error
		//becasue we restricted to only of type X  implementation classes.
		//BoundExmp<String> bes = new BoundExmp<String>(new String());
		//bea.doRunTest();
	}
}

class BoundExmp<T extends X>{
	
	private T objRef;
	
	public BoundExmp(T obj){
		this.objRef = obj;
	}
	
	public void doRunTest(){
		this.objRef.printClass();
	}
}

interface X{
	public void printClass();
}

class Y implements X{
	public void printClass(){
		System.out.println("I am in class Y");
	}
}

class Z implements X{
	public void printClass(){
		System.out.println("I am in class Z");
	}
}

I am in class Y
I am in class Z