Self Written Code on Generic Problem

[Aashu Mahajan]

Hello guys, Please me out to clearing following Generic Problems. Here is the code written by me :

class A{}
class B extends A{}
class C extends B{}
class D extends C{}

public class GenericProblems<E extends B> 
{
	public <E extends C> GenericProblems<? super E> me(GenericProblems<? super E> bol) //Line-1
	{
		return bol;
	}
	
	public static void main(String args[])
	{

//		GenericProblems<A> gpa = new GenericProblems<A>();
		GenericProblems<B> gpb = new GenericProblems<B>();
		GenericProblems<C> gpc = new GenericProblems<C>();
		GenericProblems<D> gpd = new GenericProblems<D>();
		gpd.me(gpc);
		gpc.me(gpb);
		gpb.me(gpb);
		gpb.me(gpc);  //Line-2  //Compile
	}
}

Similar code Here but with little modification from Line-1

class A{}
class B extends A{}
class C extends B{}
class D extends C{}

public class GenericProblems<E extends B> 
{
	public GenericProblems<? super E> me(GenericProblems<? super E> bol) //Line-3  //Note Here I removed '<E extends C>'
	{
		return bol;
	}
	
	public static void main(String args[])
	{

//		GenericProblems<A> gpa = new GenericProblems<A>();
		GenericProblems<B> gpb = new GenericProblems<B>();
		GenericProblems<C> gpc = new GenericProblems<C>();
		GenericProblems<D> gpd = new GenericProblems<D>();
		gpd.me(gpc);
		gpc.me(gpb);
		gpb.me(gpb);
		gpb.me(gpc);  //Line-4 //Not Compile. Why??
	}
}

I could not understand that when i removed '<E extends C>' from the line-3 the code on line-4 doesn't compile. why??
what is the use of '<E extends C>' ?
Please clarify my doubts.

[Ikpefua Jacob-Obinyan]

Aashu wrote:I could not understand that when i removed '<E extends C>' from the line-3 the code on line-4 doesn't compile. why??
what is the use of '<E extends C>' ?

In this method:

public <E extends C> GenericProblems<? super E> me(GenericProblems<? super E> bol) //Line-1

<E extends C> is a 'fresh' generic type declaration 'SPECIFIC' to the method...Hence You are saying in effect that the generic 'TYPE' 'E' passed in this method should be 'C' and 'SUB-TYPES' of 'C'.