Question

So I’m trying, for all intents and purposes, to realize for myself a java version of C++’s STL algorithm inner_product for vector parameters. So far, my code(which is probably fundamentally wrong) looks like this:

public static<T,K> double inner_product(Vector<T> v1, Vector<K> v2)
{
    double isum = 0;
    for(int i=0;i<v1.size()&&i<v2.size();i++)
    {
        isum+=v1.elementAt(i)*v2.elementAt(i);
    }

    return isum;
}

The problem is that the operator * is undefined for the types T, K. However my knowledge so far doesn’t cover predefining operators, though as far as I know it’s not possible in Java as well. Any help would be appreciated in the way of realizing the function to take generics. Thanks in advance.

Answer 1

There is no nice way to do this, for two reasons:

1. Type parameters (T, K) must refer to object types, not primitives.
2. Java does not have operator overloading.

The closest you can get is something like this (modulo syntax errors, my generics are rusty):

public static<T extends Number,K extends Number> double inner_product(Vector<T> v1, Vector<K> v2)
{
    double isum = 0;
    for(int i=0;i<v1.size()&&i<v2.size();i++)
    {
        isum+=v1.elementAt(i).doubleValue()*v2.elementAt(i).doubleValue();
    }

    return isum;
}

This will work for the object wrappers around primitive types, i.e. Double but not double, etc.

Also, like your version, this returns double no matter what type was passed in. And because of type erasure, this is pretty hard to fix.