Generic classes and Functions Problem

[Mohit G Gupta]

abstract class Monitor1<K extends Number>
     {
     	
public abstract <V extends K> Monitor1<V> use(Monitor1<V> a); //line1
public abstract <K> Monitor1<? extends Number> use(Monitor1<? super K> k);//line2
 public abstract <K> Monitor1<? super Number> use(Monitor1<? extends K> k);//line3
} 

Line 1 compiles fines individually but gives error when line 2,3 are also used

Why?

-----------------------------------------------

abstract class Monitor1<K extends Number>
     {
     //	public abstract <V extends K> Monitor1<V> use(Monitor1<V> a);  complies & runs fine
     // public abstract <K> Monitor1<? extends Number> use(Monitor1<? super K> k);  //complies & runs fine
    // 	public abstract <K> Monitor1<? super Number> use(Monitor1<? extends K> k);       //complies & runs fine
     // public abstract <V extends K> Monitor1<V> use(Monitor1<V> a);    complies & runs fine
      //public abstract <V super K> Monitor1<V> use(Monitor<V> k);  error
     	public abstract <K> K use(Object a);  //complies & runs fine
     //	public abstract <K> Monitor1<K> use(Monitor1<K> a);error
      //public abstract <V extends Character> Monitor1<? super V> use(Monitor1<K> a);error
    //  public abstract <V super Character> Monitor1<? super V> use(Monitor1<K> a);  error 
      //public abstract K<K> use(Object a);error
     	 	
     }
     

can someone tell me why doesn't all the function
declaration in the above class fails to compile ?
---------------------------------------

[Ankit Garg]

Line 1 compiles fines individually but gives error when line 2,3 are also used. Why?

You cannot overload methods based on different generic types. Like I cannot have these methods in the same class

void method(List<Integer> list) {}
void method(List<Number> list) {}

This limitation is due to type-erasure as at runtime these two methods would be completely same...

[Arjun Srivastava]

Ankit Garg wrote:
This limitation is due to type-erasure as at runtime these two methods would be completely same

ok,at runtime typing information doesn't exist, but at compile time,compiler can able to identify the different types(type parameter),then why it doesn't compile?

[Ankit Garg]

At compile time the compiler can identify which method will be called but how will the JVM identify which method to call at runtime?? It will have two methods with the exact same signature, which method would it call??

[Arjun Srivastava]

Ankit Garg wrote:At compile time the compiler can identify which method will be called but how will the JVM identify which method to call at runtime?? It will have two methods with the exact same signature, which method would it call??

hmm..ok,got the concept,JVM(instance of JRE) which doesn't allow program to compile here in this case.

[Mohit G Gupta]

Ankit wrote:
at compile time the compiler can identify which method will be called but how will the JVM identify which method to call at runtime?? It will have two methods with the exact same signature, which method would it call??

I didn't get it.
can anyone explain