为什么通过在泛型中将Integer类型参数设置为Integer变量而出错？

Question

I have the following program:

class MyGenClass{

    public <T> void setAge(T ageParam){
        Integer age = ageParam;
    }

}
class Program{

    public static void main(String args[]){

        MyGenClass gnClass = new MyGenClass();
        gnClass.<Integer>setAge(80);

    }

}

In fact, i am passing the Integer then why the ageParam is not assigned to age . And when i do:

class MyGenClass{

    public <T> void setAge(T ageParam){
        T age = ageParam;
    }

}

Why the generic type variable is not assigned to the Integer type variable age in fact the generic type variable ageParam is Integer . Is this compulsory that the ageParam must be assigned to the variable that is of type T ? Whats the scenario behind this?

Answer 1

There is not assured that the type T will be compatible with Integer . To make it clear, you have to use the following approach where T would be a subtype of Integer :

public <T extends Integer> void setAge(T ageParam){
    age = ageParam;
}

However, I see no point on this. Consider the following approach for the sake of variability:

class MyGenClass {
    Number age;
    public <T extends Number> void setAge(T ageParam){
        age = ageParam;
    }
}

Therefore the following is possible (the explicit type arguments can be inferred, thought):

MyGenClass gnClass = new MyGenClass();
gnClass.<Integer>setAge(80);
gnClass.<Long>setAge(80L);
gnClass.<Double>setAge(80.0);
gnClass.<Float>setAge(80.0F);

Answer 2

Look at your MyGenClass in isolation. T could be literally anything. It is not necessarily an Integer. I could call it with a String, or a HashMap or an ArrayList, or literally anything else .

MyGenClass gnClass = new MyGenClass();
gnClass.setAge("hello");
gnClass.setAge(new HashMap<String, String>());
gnClass.setAge(new ArrayList<String>());

In all of these cases, assignment to an Integer variable is invalid, hence the compiler error.

You only happen to be calling it with an Integer in your example. The compiler cannot assert that it will always be this way.

It looks as if you should not be using generics at all. Just change the signature to

public void setAge(Integer ageParam)