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Is it possible to create an object using two Generics in Java?

I want to define instance variables using two generic types like belows.

class Foo<S,T>{
     private S<T> Boo ;
     .
     .
}

public class Test{
     public static void main(String[] args){
          Foo<ArrayList, String> foo = new Foo<ArrayList, String>();          

    }
}

But it won't work ... Is this the wrong grammar? I really need this gram. Thanks to

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3 answers


Perhaps you need:



class Foo<S>{
     private S Boo ;
     .
     .
}

public class Test{
     public static void main(String[] args){
          Foo<ArrayList<String>> foo = new Foo<ArrayList<String>>();          

    }
}
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This is invalid syntax

S<T>

because for anyone x<T>

, x

must be a known class or interface type.



What you want is the container type and the element type. It is possible

class Foo<C extends Collection<E>, E>{
    private C boo;

...

Foo<ArrayList<String>, String> foo = new Foo<>();
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It won't work because it ArrayList

has its own parameter and when you don't specify it it will be treated as an ArrayList.

Thus, your code will be invalidated private ArrayList<Object><String>

.

In such cases, you simply specify ArrayList<String>

and have your definition like this:

class Foo<S>{
     private S Boo ;
     .
     .
}

public class Test{
     public static void main(String[] args){
          Foo<ArrayList, String> foo = new Foo<ArrayList<String>>();          

    }
}

If you want to place constraints on T

and S

, you can do it like this:

class Foo<S extends Collection<T>, T extends Comparable<T>> {
     private S Boo;
}

But you cannot do

class Foo<S<T>, T extends Comparable<T>> {
     private S Boo;
}
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