With the advent of autoboxing in Java this is easily solved using one of the primitive wrapper classes to wrap The major caveat being that you can only store reference variables inĬollections, whereas you can store primitives and reference variables in basic arrays. In this section we look at several more collections that allow us more flexibility and structure than the indexing mechanism of a basic array. We have already used one such collection when we looked at the Array object in the Of a collection as a generic type that allows us to store and iterate over data. The head of the queue is the element that was put into it longer ago than all the other elements in the queue.In our first lesson on collections we look at the collection interfaces and classes using various diagrams and explain the terminology used when dealing with collections. The head of the queue is a smallest element, according to the ordering of the elements, or the ordering provided by a comparator.įIFO (First In First Out) queue. Objects within a Treeset should be made immutable or should be removed, changed and then re-added to the TreeSet. **Since objects are added in Comparable sort order when objects are inserted and not recalculated when the object is mutated, the sort order of the TreeSet maybe be incorrect if the object is mutated. Otherwise mutating objects within a HashSet, will result in the contains() method failing to find the object. Objects within a HashSet should be made immutable or should be removed, changed and then re-added to the HashSet. *Because an object's position within the HashSet depends on its hashCode() (and its contents), changing an object changes its position in the HashSet. Iterates through elements in ascending order Special care is required when mutating objects within a TreeSet.** Special care is required when mutating objects within a HashSet.*Įither objects need to implement the Comparable interface and be able to compare to each other or a Comparator for the objects must be provided the ordering must satisfy a 'total ordering'. Objects need to implement a hash function ( hashCode()) which is consistent with equals(). Search contains() / removal remove() by object Insertion / removal in the middle through the index Insertion / removal in the middle through an Iterator / ListIterator One step of iteration through an Iterator High priority elements get to the head first.Ī 'double-ended queue': insert and remove at both ends. Each element is associated with an index. The parent interface of the entire collections framework.Īllows you to put elements in a specific order. Class LinkedList (also implements List)Ĭomparison of Collections Comparison of Selected Interfaces.Class Stack (legacy class, use Deque, which is more powerful).Class LinkedList (also implements Deque).Here is a simplified hierarchy of the collections framework: The actual hierarchy of what extends what, and what implements what, is fairly intricate. Exceptions will be noted when introduced.Īlthough the Map interface does not extend Collection, it is usually included in discussions of collections and will be explained here. Most implementations of the Collection interface are in java.util. (For example, both ArrayList and LinkedList fulfill the general contract of the List interface, but do so differently.) These sub-interfaces decide the functionality of a collection individual classes usually differ only in implementation. There are multiple sub-interfaces of Collection that specify additional methods. However, few if any implement the interface directly.

(They can, however, hold wrappers, like .)Īll collection objects ultimately implement the interface. A drawback of collections is that they can't hold primitives. Unlike arrays, however, collections have convenience methods to, say, return a portion of the original collection. Collections are objects whose sole purpose is to store other objects, like arrays.

Collections in java