Question

I try something like this below, but the complier warns me for this type is not applicable for the argument… I know it’s not type safe, but any other way to do so?

public void addRow(List<? extends Car> list){
    list.add( list.get(list.size()-1).getClass().newInstance() );

}

Answer 1

To see the problem, let’s expand the problematic method as follows:

public <T extends Car> void addRow(List<T> list) throws InstantiationException,
        IllegalAccessException {
    T element = list.get(list.size() - 1);
    Class<T> clazz = element.getClass(); // Compilation Error Here!
    T newInstance = clazz.newInstance();
    list.add(newInstance);
}

getClass() returns Class<? extends Car>. The compiler is not smart enough to know whether or not the wildcard (?) matches T. The programmer can assert that the assignment is, in fact, safe:

public <T extends Car> void addRow(List<T> list) throws InstantiationException,
        IllegalAccessException {
    T element = list.get(list.size() - 1);
    @SuppressWarnings("unchecked")
    Class<T> clazz = (Class<T>) element.getClass();
    T newInstance = clazz.newInstance();
    list.add(newInstance);
}

Of course, now the responsibility for type safety lies with the programmer. Don’t forget that even though it might be safe today, future code changes may invalidate the assertion.