Use generic types to replace the object declaration
- Add one or more type parameters to its declaration.
- Replace all the uses of the type Object with the appropriate type parameter.
-
Handle the new E[] errorwith two ways :
-
Use the explicit type casting in the constructor. And use @SuppressWarnings("unchecked") to suppress warning.
// The elements array will contain only E instances from push(E).
// This is sufficient to ensure type safety, but the runtime
// type of the array won't be E[]; it will always be Object[]!
@SuppressWarnings("unchecked")
public Stack() {
elements = (E[]) new Object[DEFAULT_INITIAL_CAPACITY];
}
-
Change the type of the field elements from E[]to Object[] and explicitly cast the popped element type to E.
private Object[] elements;
public Stack() {
elements = new Object[DEFAULT_INITIAL_CAPACITY];
}
// Appropriate suppression of unchecked warning
public E pop() {
if (size==0)
throw new EmptyStackException();
// push requires elements to be of type E, so cast is correct
@SuppressWarnings("unchecked") E result = (E) elements[--size];
elements[size] = null; // Eliminate obsolete reference
return result;
}
-
/**
* @author kaibo
*
*/
// Object-based collection - a prime candidate for generics
public class Stack<E> {
private E[] elements;
private int size = 0;
private static final int DEFAULT_INITIAL_CAPACITY = 16;
public Stack() {
elements = (E[]) new Object[DEFAULT_INITIAL_CAPACITY];
}
public void push(E e) {
ensureCapacity();
elements[size++] = e;
}
public E pop() {
if (size == 0)
throw new EmptyStackException();
E result = elements[--size];
elements[size] = null; // Eliminate obsolete reference
return result;
}
// no changes in isEmpty or ensureCapacity
private void ensureCapacity() {
if (elements.length == size)
elements = Arrays.copyOf(elements, 2 * size + 1);
}
public static void main(String[] args) {
Stack<Integer> s = new Stack<Integer>();
s.push(1);
s.push(2);
s.push(3);
System.out.println(s);
}
}
Note
- You should use boxed primitive types instead of primitive type for the Generic type parameter.
-
There are some generic types that restrict the permissible values of their type parameters. For example, consider java.util.concurrent.DelayQueue, whose declaration looks like this:
class DelayQueue<E extends Delayed> implements BlockingQueue<E>
The type parameter E is known as a bounded type parameter. Note that the subtype relation is defined so that every type is a subtype of itself [JLS, 4.10], so it is legal to create a DelayQueue<Delayed>
- Generify your existing types as time permits. This will make life easier for new users of these types without breaking existing clients.
Summary
Generic types are safer and easier to use than types that require casts in client code. When you design new types, make sure that they can be used without such casts. This will often mean making the types generic. Generify your existing types as time permits. This will make life easier for new users of these types without breaking existing clients (Item 23).