Question

I need to implement a bag data structure using the interface java.util.Collection.
I’m not asking for help on the actual implementation of the data structure. I just can’t get my program to compile. I just want to get a blank implementation of the interface (with non functional signatures of the methods) to compile before I start actually implementing methods.

class Bag<T> implements java.util.Collection<T> 
  {

  public void Collection () {

  }

  public boolean add(E e) {

  }

  public boolean addAll (Collection<? extends E> c) {

  }

  public void clear() {

  }

  public boolean contains(Object o) {

  }

  public boolean containsAll(Collection<?> c) {

  }

  public boolean equals(Object o) {

  }

  public int hashCode() {

  }

  public boolean isEmpty() {

  }

  public Interator<E> interator() {

  }

  public boolean remove(Object o) {

  }

  public boolean removeAll(Collection<?> c) {

  }

  public int size() {

  }

  public Object[] toArray() {

  }

  public <T> T[] toArray(T[] a) {

  }
}

Compiler can’t find class E in the parameters of methods like add. Am I supposed to define a class for E, or is there something I’m not understanding about what E actually is? Compiler says it can’t find class Collection (in the parameters of methods like addAll) Do I import java.util.Collection or is there something else I should know? Compiler also has no idea about class Iterator and neither do I.

I know this is all probably elementary, but I could not find anything via Google, etc yesterday and my professor’s lectures don’t follow the projects at all. I’m lost on this one. Thanks for any help!

Edit: Also, I haven’t searched as much on this but if someone could tell me anything useful about the “?”s such as public boolean addAll (Collection<? extends E> c) {}, that would be greatly appreciated.

Answer 1

Either use T or E as the type parameter, and then use it consistently throughout your class definition.

The Java documentation uses E, so you could for example change the first line to:

class Bag<E> implements java.util.Collection<E> 

You will also need to either return values or throw an exception for those methods that don’t return void.

public int size() {
    throw UnsupportedOperationException();
}