The type is a ClassName in generics

[dmjpro]

I have this code ..
Please have a look...

class GenericClass<Number>{
    Number number = null;
    GenericClass(Number number){
        this.number = number;
    }
}
 
GenericClass<String> gc = new GenericClass(100); //It gets compiled
System.out.println(gc.number); //it leads me to a ClassCastException
 

The Number is a type or ClassName?

[r035198x]

Doesn't your compiler give you a warning when you compile that? Read that warning.

[dmjpro]

I didn't see that warning. So here Number is treated as type name?
Why warning, why not error?

[r035198x]

Why should it be an error? You are allowed to hide those classes at your own peril.

[dmjpro]

@r035198x
If i add a method like this ..

void add(Number number){this.number=number;}
 

and then if i use this, then it flashes me an error ..

gc.add(200); //it shows compile error.
 

Why in case of constructor it shows a warning and in case of method it shows error?

[r035198x]

Post the code that you used and the error that you got.

P.S You'd better read the generics tutorial first before trying to use generics.

[dmjpro]

@r035198x
Here is my Code ...

class GenericClass<Number>{
    Number number = null;
    GenericClass(Number number){
        this.number = number;
    }
    void add(Number n){
        this.number = n;
    }
}
 
public class Main {
   public static void main(String[] args) {
        GenericClass<String> gc = new GenericClass(100);//it does not show me error
        gc.add(200); //it shows me error
    }
}
 

[JosAH]

@dmjpro
You do realize that the class java.lang.Number is hidden by that generic class don't you? Autoboxing goes berzerk and a general mess will be yours.

Better stay far away from that dangerous crap if you don't know what you are doing.

kind regards,

Jos

[dmjpro]

See ... What i realized..

class GenericClass<T>{
GenericClass(T t){...}
}
 
GenericClass<String> gc = new GenericClass(/*Any type of object*/);
 
GenericClass<String> gc = new GenericClass<ClassName>(/*Only ClassName type of object*/);
 

[r035198x]

The first line in your main should give you another warning as well.
You can't call add with numbers because you specified your type to be String. You can only add Strings to it. Better use the conventions (T for type and E for element) when using generics.

Don't forget to read that generics tutorial.

[JosAH]

@r035198x
IMHO it would've been much better to explicitly forbid an existing class name to be used as a generic type name; some poofters might try to come up with MyClass<String> or MyClass<Number> and be surprised that some stuff won't work as expected, especially when they want to use those (hidden) class names from within the definition of their crappy generic class.

kind regards,

Jos

[r035198x]

I thought as much as well the first time I saw this thread. They did put the warning though. Now they need to get people to read those warnings.