Java generics - 桥接法

Question

When comes to bridge method, i do know that java compiler will add them if there's a need so that overriding can be done properly by the subclass (after reading SCJP by mughal and angelikalanger website). But this is a bit confusing as per below:

Before erasure:

class x <T> {  
   void set(T t){}  
}

class y <E> extends x {  
   void set(E e) {} // name clash here  
}

class z<E> extends x {  
   void set(Object y) {} // no name clash here  
}

class z1<E> extends x<T> {  
   void set(Object y) {} // name clash here  
}

after erasure:

class x {  
   void set (Object t) {}  
}

I understand the name clash for class y but why there is no name clash for class z? Also there is a name clash for class z1? Puzzling

Answer 1

class y <E> extends x {  
   void set(E e) {} // name clash here  
}

Here name clash occurs because E is not a subclass of T. So you cannot override the set method this way.see the explanation for z1 to understand better. For class y to work, you must have

class y <E> extends x<E> {  
   void set(E e) {}  
}

Next:

class z<E> extends x {  
   void set(Object y) {} // no name clash here  
}

Here there is no name clash because in the class X, set method gets interpreted as

void set(java.lang.Object) 

and in class z also the parameter of set is java.lang.Object.so no clash.

Next:

class z1<E> extends x<T> {  
   void set(Object y) {} // name clash here  
}

Again here name clash occurs because you have to have as parameter of set whatever type parameter you give to x. here, you pass to x type parameter T, but you have parameter of set method as java.lang.Object. hence name clash.

For z to work you must have:

class z1<E> extends x<Object> {  
       void set(Object y) {}  
}

Answer 2

As you say, after erasure the set method takes an Object. z extends the non-generic, after erasure x