Java assign implemented class to generic interface

Question

I am trying to achieve assigning a class that implements a generic typed interface. It's doable in C# with out keyword per https://msdn.microsoft.com/en-us/library/dd997386.aspx:

interface ICovariant<out R>
{
    R GetSomething();
}
class SampleImplementation<R> : ICovariant<R>
{
    public R GetSomething()
    {
        // Some code. 
        return default(R);
    }
}

// The interface is covariant.
ICovariant<Button> ibutton = new SampleImplementation<Button>();
ICovariant<Object> iobj = ibutton;

// The class is invariant.
SampleImplementation<Button> button = new SampleImplementation<Button>();

But when I tried to achieved the same in Java, it failed:

interface IShape<T>
{
    T GetArea();
}

class Rectangle implements IShape<Number>
{
    public Number GetArea() {
        Double d = new Double(1.03);
        return d;
    }
}

class Test {
    void Try() {
        Rectangle r = new Rectangle();
        IShape<Object> s0 = r;//assignment failed
    }
}

The error is -> Type mismatch: cannot convert from Rectangle to IShape from Java 8's compiler. How can I achieve assigning a class that implements IShape to a reference of interface IShape< T>?

Number is not the same as Object. if you adjust your interface's generic type to T extends Object instead of T, then i think it should work — stefaan dutry, Mar 09 '15 at 19:08
Either `IShape extends Number> s0 = r;` OR `IShape> s0 = r;` would compile — Hil Liao, Mar 10 '15 at 21:14

score 0 · Answer 1 · answered Mar 09 '15 at 19:10

0

Try replacing this line:

class Rectangle implements IShape<Number>

With this:

class Rectangle<T extends Number> implements IShape<T>

answered Mar 09 '15 at 19:10

Aaron

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Java assign implemented class to generic interface

1 Answers1