Question

Suppose I have a class hierarchy as follow:

class Vehicle;
class Car extends Vehicle;
class Plane extends Vehicle;

I have a function which compares the two object

public <T extends Vehicle> generateDiff(T original, T copy)

At compile time, the method above guarantees the two objects is Vehicle, but it cannot make sure the types of the two object are the same.

generateDiff(new Car(), new Car()); //OK
generateDiff(new Plane(), new Plane()); //OK
generateDiff(new Car(), new Plane()); //WRONG

Can I achive this at compile time using Generics?

P.s: currently, I’ve implemented it will throw exception if the Class of two objects are not the same. But I’m not satisfied with this.

Thanks in advance.

Answer 1

Yes, you can (kind of)!

The type T is being inferred from the arguments, but you can specify the type:

MyClass.<Car>generateDiff(new Car(), new Plane()); // generates a compile error

Without the typing the method, the type T is inferred to be the narrowest class that satisfies the bounds as used, so for parameters Car and Plane, the narrowest type that will work is Vehicle, so these two lines are equivalent:

generateDiff(new Car(), new Plane()); // type is inferred as Vehicle
MyClass.<Vehicle>generateDiff(new Car(), new Plane());

The above code assumes the generateDiff() is a static method. If it’s an instance method, you could type your class and have that type used in your method.