ArrayList Facts and Methods to Know

3 ways to initialize an arraylist:

· Create the ArrayList and add items to it later using the add method:

o ArrayList<Integer> atest = new ArrayList<Integer>();

o atest.add(1);

· Create the ArrayList with a collection of a list created using Arrays.aslist.

o yList<Integer> dtest = new ArrayList<Integer>(Arrays.asList(1,5,1,5));

· Create the ArrayList with an anonymous Arraylist subclass:

o ArrayList<Integer> dtestans = new ArrayList<Integer>( ){{ add(1); add(6); add(1);add(6);}}

Using an ArrayList

Gotcha: Remember there is an array under the ArrayList, and its indexes are maintained whenever movements are made inside the array. ArrayList will resize to the number of elements after every move.

This is a subset of methods described at: http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html

boolean	add(E e) Appends the specified element to the end of this list.
void	add(int index, E element) Inserts the specified element at the specified position in this list.
boolean	addAll(Collection<? extends E> c) Appends all of the elements in the specified collection to the end of this list, in the order that they are returned by the specified collection's Iterator.
boolean	addAll(int index, Collection<? extends E> c) Inserts all of the elements in the specified collection into this list, starting at the specified position.
void	clear() Removes all of the elements from this list.
boolean	contains(Object o) Returns true if this list contains the specified element.
E	get(int index) Returns the element at the specified position in this list.
int	indexOf(Object o) Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element.
boolean	isEmpty() Returns true if this list contains no elements.
int	lastIndexOf(Object o) Returns the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element.
E	remove(int index) Removes the element at the specified position in this list.
boolean	remove(Object o) Removes the first occurrence of the specified element from this list, if it is present.
boolean	removeAll(Collection<?> c) Removes from this list all of its elements that are contained in the specified collection.
boolean	retainAll(Collection<?> c) Retains only the elements in this list that are contained in the specified collection.
E	set(int index, E element) Replaces the element at the specified position in this list with the specified element.
int	size() Returns the number of elements in this list.
Object[]	toArray() Returns an array containing all of the elements in this list in proper sequence (from first to last element).

Read through with a For Each Loop: (Works with Primitive Arrays also) https://blog.udemy.com/for-each-loop-java/

for ( ElementType variable : ArrayListVariable) {

// do your work with variable – but you cannot change it

}

Example follows:

ArrayList <String> arr = new ArrayList<String>(Arrays.asList("a","b","c"));

for (String s : arr){

System.out.print(s);

}

Note on capacity: While capacity methods will allocate space for changes you intend to make to the array, you wont see the capacity changes reflected in the size.