Question

I’m trying to make a class that implements Collection, so it has to have the method that removes an Object, so I figured I’d just reuse the method I already created to remove an object of a generic type T, but it throws a compile-time error. Why won’t this work?

Code

---

public class ArrayPP<T> implements Collection<T>
{
  public boolean remove(Object o)
  {
    if (o instanceof T)
      remove((T)o, true);
    else
      return false;
    return true;
  }
}

Error

---

ArrayPP.java:5: unexpected type
found   : T
required: class or array
    if (o instanceof T)

Why, Java?

---

I’ve solved it with

if (t.getClass().isInstance(o))
  remove((T)o, true);
else
  return false;
return true;

But… I mean if someone codes “ArrayPP<String> a = new ArrayPP<String>();“, you know that T is String, right? So how come, during runtime, it can’t be seen whether o is a String? I mean, if I were to do a.add('c');, it wouldn’t compile, because it knows that, here, the add(T item) method in ArrayPP will only accept Strings, so why… ugh….

Answer 1

For remove, you don’t need to check the type. If the type doesn’t match it won’t be there and remove will return false anyway.

If you want a dynamic check for add(T t) you need to store the type in a field.

private final Class<T> tClass;

public boolean add(T t) {
   return tClass.isInstanceof(t) && collection.add(t);
}

Have you looked at Checked Collection in the Collections utility. It may do what you want.
http://download.oracle.com/javase/6/docs/api/java/util/Collections.html under checkedCollection, checkedList, checkedSet, checkedMap