实例化具有与其超类相同类型的泛型类型对象

Question

I am wondering if it is possible to create objects that are of generic type with the same type as the class it is contained in.

For example, consider this object

public class Foo<T> {

   private T variable;

   Foo() {

   }
}

Now consider this

public class Bar<T> {

   private Foo foo;

   Bar() {
      foo = (T) new Foo();
   }
}

I want the data type of the foo object inside the bar class to be the same data type that bar is instantiated as.

Answer 1

Yes you can do like this and it will work. But you need to have correct oveloaded constructors to handle the data types.

The output of the program is:

class java.lang.Integer
class java.lang.String

As you can see the datatype for foo variable is same as the datatype of Bar object.

And you have made one mistake in the program there should be an overloaded constructor in the Bar class.

package test;

public class test {

    public static void main(String[] args) {
        Bar<Foo> integerBar = new Bar(3);
        Foo<Integer> fooIntegerObject = (Foo) integerBar.getFoo();

        Bar<Foo> stringBar = new Bar("hello");
        Foo<String> fooStringObject = (Foo) stringBar.getFoo();

        System.out.println(fooIntegerObject.getVariable().getClass());
        System.out.println(fooStringObject.getVariable().getClass());

    }
}

class Foo<T> {

    private T variable;

    Foo(T x) {
        variable = x;
    }
    public T getVariable() {
        return variable;
    }

}

class Bar<T> {

    private T foo;

    Bar(T x) {
        foo = (T) new Foo<T>(x);
    }
    public T getFoo() {
        return foo;
    }

}

Answer 2

The constructor in Foo requires an argument; but you can do this:

public class Bar<T> {

   private Foo<T> foo;

   Bar(T x) {
      foo = new Foo<T>(x);
   }
}

Answer 3

Based on your updated question, the following will work:

class Bar<T extends Foo<T>> {  
    private Foo<T> foo;

    Bar() {
        foo = new Foo<T>();
    }
}

Note that you shouldn't use a raw type Foo .

Answer 4

I'm not sure if I understand you correctly, but do you want something like this?

        public class Bar<T> extends Foo<T> {

           private Foo<T> foo;

           Bar() {
              foo = new Foo<T>();
           }
        }