Java 泛型类型非静态方法返回 Object 而不是泛型类型 T

Question

public class Module<T> {

    @SuppressWarnings("unchecked")
    public <T> T module(Class<T> clazz) {
        return clazz.cast(this);
    }
        
}

This is the base class of my module and I want to initialize it to TestModule by calling the module method above.

TestingModule testingModule = new Module().module(TestingModule.class);

However, the return type of

new Module().module(TestingModule.class)

is Object instead of TestingModule. Is there any idea that I can directly initialize it without using any casting or changing the method to static ?

Answer 1

There are a few problems. The one most directly linked to the type mismatch error is here:

TestingModule testingModule = new Module().module(TestingModule.class);

new Module() is using Module without any type argument. When you use a raw type like this, you lose all generic information in the expression. You can fix it simply by adding a type argument:

TestingModule testingModule = new Module<String>().module(TestingModule.class);

In the above code, I've added <String> to the constructor call, which resolves the problem. But String is as strange a type argument as it can get in this case, which leads to the following side note.

The declaration public <T> T module(Class<T> clazz) adds a type variable T that hides the class-level type variable of the same name. If this method is not made generic by accident, please use a different name for this variable, and use it . Otherwise, this method doesn't need to be generic.