Inference variable T has incompatible bounds with specific Class<T> and instance <T> in-CodePudding

I'm using library which contains method I want to use. Simplified signature would look like this:

final class ThirdPartyClass
{
    public static <T> void thirdPartyMethod(Class<T> clazz, T instance)
    {
        // does something
    }
}

Inside thirdPartyMethod() it expects class of T as first argument and an instance of that specific class T as second argument.

Now I have this simple abstract class, which is calling that method:

abstract class Parent<T extends Number>
{
    public Parent(T instance)
    {
        ThirdPartyClass.thirdPartyMethod(instance.getClass(), instance);
    }
}

I'm getting an error:

method thirdPartyMethod in class ThirdPartyClass cannot be applied to given types;
required: java.lang.Class, T
found: java.lang.Class<capture#1 of ? extends java.lang.Number>, T
reason: inference variable T has incompatible bounds
equality constraints: capture#2 of ? extends java.lang.Number
lower bounds: T

How can I modify the Parent class so it conforms to expected arguments of thirdPartyMethod()? If possible with explanation.

CodePudding user response：

According to the return type of Object.getClass()

The actual result type is Class<? extends |X|> where |X| is the erasure of the static type of the expression on which getClass is called.

Since T extends Number, the resulting type would be Class<? extends Number>, i.e. means an unknown type extending Number. Which doesn't match T extends Number (i.e. a particular type extending Number which would be provided at runtime).

How can I modify the Parent class so it conforms to expected arguments of thirdPartyMethod()?

1. Introduce a second parameter of type Class<T> in your constructor.

abstract class Parent<T extends Number> {
    public Parent(T instance, Class<T> tClass) {
        ThirdPartyClass.thirdPartyMethod(tClass, instance);
    }
}

2. Perform casting (as have been mentioned in the comments):

ThirdPartyClass.thirdPartyMethod((Class<T>) instance.getClass(), instance);

By the way, instead of calling methods from a constructor (especially ones that are not developed and tested by you/your colleagues) you might consider introducing a factory method that would produce your domain object based on the result of a method call.