Arrays are good and fast, but they are static (once they are created they can’t grow). In this lesson we’re going to review three hierarchically related types from the Java Collections API that provide dynamic data structures:

Type extends implements Description
interface Collection  Iterable  –  General dynamic collection of elements that is iterable
interface List  Collection  Dynamic collection providing also positional access
 class ArrayList ArrayList List


Implementation of List providing fast positional access

The Collection Interface

The Collection interface is used when maximum generality is desired. The following table lists the main methods of this interface:

Basic Operations
int size()
boolean isEmpty()
boolean contains(Object element)
boolean add(E element)
boolean remove(Object element)
The method remove(e) returns true if the collection contained e
Bulk Operations
boolean containsAll(Collection<?> c)
boolean addAll(Collection<?> c)
boolean removeAll(Collection<?> c)
void clear()
Bulk operations equivalent to the basic operations, e.g.: removeAll removes all elements of this that are in c
Iterable Operations
 Iterator<E> iterator()  Implementing this interface allows an object to be the target of a for-each loop
Array Operations
 Object[] toArray()
<T> T[] toArray(T[] a)
String[] sa = c.toArray(new String[0]);

You can traverse any given collection c in two ways:

Use Iterator instead of the for-each loop when you need to:

  1. Iterate over multiple collections in parallel
  2. Remove the current element (it.remove() it’s the only safe way to do it during iteration)

The List Interface

The List interface inherits all the operations from the Collection interface and mainly adds to them positional access and search operations:

Positional Access1
E get(int index)
E set(int index, E element)
void add(int index, E element)
E remove(int index)
  • Gets element at the given index
  • Returns the element being replaced
  • Adds at the given index (shifts elements)
  • Returns the element being removed
Search Operations
int indexOf(Object o)
int lastIndexOf(Object o)
 Returns the index of the first (or last) occurrence of the specified element in this list, or -1 if the element is not found

You can think of a List as an ordered collection that allows duplicates. Two List objects are equal if they contain the same (according to equals method) elements and in the same order.

The ArrayList Class

The ArrayList class is probably the most used implementation of the List interface. It is internally backed up by an array and provides fast (constant-time) positional access.

Since it is a dynamic collection that grows automatically, you don’t have to worry about its size, except if you try to add elements at a specific index (in order to add an element at index i the ArrayList must have at least i elements or an exception will be thrown).

The Vector class is the equivalent thread-safe collection that it’s used when synchronization is needed. Because of this synchronization overhead it’s slightly slower than ArrayList.

You can create an ArrayList or a Vector object using the existing data in another Collection object to initially populate it:

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