CODEWITHSUNDEEP
javagenerics
Neel
Neel

Reputation: 71

Java Generics Question

I following definitions of the class.

public class Test2<T extends Test3> {

     private T t ;


     public T foo() {
         Test3 test3 = new Test3();
         t = test3;  // I get compilation error here.**
         return t;
     }

}

class Test3 {

}

I get compilation error at line t=test3, saying "Type mismatch can not convert from Test3 to T; What is wrong?

Upvotes: 2

Views: 148

Answers (4)

Mark Elliot
Mark Elliot

Reputation: 77024

What you're doing is equivalent to this:

Integer n = new Number();

and has nothing to do with generics (note that Integer extends Number). The compiler is indicating you cannot assign a parent type to an instance of a child type, the parent type may not implement all the child type's required methods.

In this case T is the child type of Test3, or Test3 itself. So here, you're trying to assign the parent class (T) to a variable that contains the child class (Test3) and it fails, just like the example above.

Upvotes: 8

Hovercraft Full Of Eels
Hovercraft Full Of Eels

Reputation: 285405

Can a child object variable refer to a parent object?

Just test it without the generics:

public class Foo {

    public static void main(String args[]) {
        T t = new Test3();
    }
}

class T extends Test3 {
}

class Test3 {
}

edit: nevermind, I initially posted as a comment, then posted as an answer... but was too late! :)

Upvotes: 1

Dewfy
Dewfy

Reputation: 23614

Because it is downcast. T can be any class derived from Test3, if you sure (but why you need T, use always Test3) you can use follow explicit casting:

@SuppressWarnings("unchecked")
t = (T)test3;

Upvotes: 0

Ignacio Vazquez-Abrams
Ignacio Vazquez-Abrams

Reputation: 798526

T is a subclass of Test3. As such, unless it is exactly Test3 the assignment will fail since subclasses can be assigned to variables of a superclass type but the reverse is not true.

Upvotes: 2

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