Java Generics problems

Question

I have this class Entry...

public class Entry <K, V> {
    private final K mKey;
    private final V mValue;

    public Entry() {  
        mKey = null;
        mValue = null;   
    }
}

What happens if I use an int as the mKey? As far as I know ints can't be null!

Answer 1

A variable of type Integer can be null. An int cannot be null. The latter is the primitive type, the former is a wrapper reference type for dealing with primitives as an Object. If you're using this:

Entry<Integer, String> myEntry;

Then you are necessarily using the wrapper type. Primitives can't be used as type parameters in Java so you can't have Entry<int, String> for example (it won't compile).

Answer 2

您不能将基元用作类型参数。

Answer 3

Generic type parameters need to be objects, they can't be primitives. So you can use the Integer wrapper class around mKey / mValue and set it to null, but trying to use the int primitive will always give you a compilation error.