Java Generics

[learn2java]

:(:(:(:(:(:(:(:(:(


Hi ,

Can some one explain in java generics , if beta extends alpha , a variable of type alpha may refer to an object of type beta , a variable of type list <Alpha> may refer an object of type Arraylist<Alpha>, but a variable of typelist<Alpha> may refer an object of type Arraylist<Beta>, Can some one tell exactly what happens in it or what is this case ??

[pbrockway2]

a variable of typelist<Alpha> may refer an object of type Arraylist<Beta>

I get "Type mismatch: cannot convert from ArrayList<Test.Beta> to List<Test.Alpha>" when I try this.

import java.util.ArrayList;
import java.util.List;

public class Test {
    class Alpha {}
    class Beta extends Alpha {}
	
    public static void main(String[] args){
        List<Alpha> test = new ArrayList<Beta>();
    }
}

Did you mean a variable of type List<Alpha> may not refer an object of type ArrayList<Beta>? (If not post a SSCCE showing what you did mean).

-----

The compiler message above is explained on the Subtyping page of Oracle's Generics Tutorial. Post if you don't understand the point they make there (that a specific sort of butterfly cage is not good as a general purpose animal cage because it might well be too flimsy.)

[learn2java]

hi pbrockway2 ,

In your last statement you are exactly right , I mean the same but I can not understand the concept of it , pls can you tell in detail and explain about it ?

[anbu.gn]

Hi All
I'm new to RAD IDE. can any one explain me how to import the project in to the IDE.

[sunde887]

Please don't bump an old thread. Make a new thread with your question.