Checking Java Collection Generic Type for an Empty Collection

Question

I want to implement the following function:

public boolean checkType(Vector vec)
{
  // return true if its Vector and false otherwise         


        

Answer 1

Because of type erasure, this is not possible.

Even if the vector is not empty, the collections library gives no real protection against putting things of different types in:

    Vector v = new Vector();
    v.add("foo");
    System.out.println(v.get(0));

will run without error and nicely print the string value that shouldn't have been allowed.

Thus any attempt to determine the generic type at run-time, even by checking the type of the first element, is not reliable.

What you can do in your own generic code is to add a constructor argument and a field holding the value, so that your code does know the type at runtime.

This tactic would be difficult to apply to the entire collections library, but a start at it for Vector would be:

public class TypedVector extends Vector {

    private Class genericClass;

    public TypedVector(Class clazz) {
        super();
        this.genericClass = clazz;
    }

    // many other constructors ... but probably not all of them

    public Class getGenericClass() {
        return genericClass;
    }

    @Override
    public boolean add(E e) {
        if (!(genericClass.isAssignableFrom(e.getClass())))
                throw new IllegalArgumentException("Incorrect class");
        return super.add(e);
    }

    // many other overrides for run-time type safety
}