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The java.util Package - The Collection Interface
- this is the root interface for the collection heirarchy
- it is not directly implemented by an SDK class; instead they implement the subinterfaces List or Set
- it is typically used to manipulate and pass collections around in a generic manner
- classes which implement Collection or one of it's subinterfaces must provide two constructors
- a default, no-argument constructor, which creates an empty collection, and
- a constructor which takes a Collection as an argument and creates a new collection with the same elements as the specified collection
| Query Methods | |
| contains(Object o) | returns true if the collection contains the specified element |
| isEmpty() | returns true if the collection has no elements |
| iterator() | returns an Iterator object. There is no guarantee as to the order of the returned elements unless the collection is an instance of a class that guarantees the order. |
| size() | returns the number of elements in the collection or Integer.MAX_VALUE if the collection equals or exceeds Integer.MAX_VALUE |
| toArray() | returns the collection elements as an array. If the collection class guarantees an order, the array elements are in the guaranteed order. |
| toArray(Object a[]) | returns all the elements in the collection whose type is that of the array type. If the collection does not fit in the array, a new array of the same type is returned. If the array is larger than the collection, the array element after the last collection element is set to null |
| Bulk Methods | |
| containsAll(Collection c) | returns true if the collection contains the all the elements in the specified collection |
| addAll(Collection c) | adds all the elements in the specified collection to this collection |
| clear() | removes all the elements in the collection |
| removeAll(Collection c) | removes all the this collections elements that are in the specified collection. |
| retainAll(Collection c) | retains all the elements in this collection that are contained in the specified collection |
| Modification Methods | |
| add(Object o) | adds an element to the collection. Returns false if the element is not added as the collection class guarantees no duplicates. |
| remove(Object o) | removes the specified object from the collection, if it exists. |
| equals() and hasCode() | |
| equals(Object o) | programmers may override the Object.equals() method to implement collection specific comparisons eg "value" comparison vs "reference" comparison |
| hashCode() | programmers overriding equals() must also override Object.hashCode() |
Tips
- any SDK class which implements Collection or any of it's subinterfaces will contain the two required constructors CollectionName() and CollectionName(Collection c)
