The ArrayList class in the Java library

  • Recall that:   the size of an array is fixed after it has been created

  • One of the most useful classes in the Java library is: 

        java.util.ArrayList      (Documentation: )

  • The java.util.ArrayList class implements a variable (= "adjustable") length array:

    • When you create an ArrayList object, the array will initially be empty

    • You can add (= insert) new elements into an ArrayList...

    • When the ArrayList object is full, the add( ) method will increase the size of the array and then insert the new element

      Note:   we have discussed how to increase the size of an array.
      See:

How to create an ArrayList object

  • The official way to create an ArrayList object is as follows: 

    import java.util.ArrayList;

public class myProg
{
    public static void main(String[] args)
    {
        ArrayList<ObjectType> varName = new ArrayList<ObjectType>();
        ...

  • Example: create an ArrayList of String 

    import java.util.ArrayList;

public class myProg
{
    public static void main(String[] args)
    {
        ArrayList<String> varName = new ArrayList<String>();
        ...

The short-hand form to create an ArrayList object

  • The short-hand form to create an ArrayList object is as follows: 

    import java.util.ArrayList;

public class myProg
{
    public static void main(String[] args)
    {
        ArrayList<ObjectType> varName = new ArrayList< >(); // empty
        ...

  • Example: shorter form to create an ArrayList of String 

    import java.util.ArrayList;

public class myProg
{
    public static void main(String[] args)
    {
        ArrayList<String> varName = new ArrayList<>(); // Shorter form
        ...

DEMO: demo/11-arrayList/01-intro/Demo.java

What is inside an ArrayList object

An ArrayList object contains:

  • size = number of elements this is currently stored in the ArrayList object

  • elementData that points to an array object that stores the elements

    • The array object contains a length variable which is the length of the array

    • The array object also contains an array to store the data
      This array is not completely filled (the number of slot used is size)

DEMO: demo/11-arrayList/01-intro/Demo2.java

Some commonly used (instance) methods of ArrayList    

These instance methods of ArrayList are used to (1) insert, (2) look up, (3) update and (4) remove elements in an ArrayList object: 

  Note:  E  can be any reference type (i.e.: not a primitive data type)

  add(E e):       
         Appends the element e to the end of this ArrayList.

  add(int index, E e)
         Inserts the element e at position index in this ArrayList.



  get(int index):
         Returns the element e at the position index in this list.



  set(int index, int newValue):
         Replace the element at position index in this ArrayList with newValue
         Returns the old element e



  remove(int index):
         Removes the element at the position index from this list.

  remove(E e):
         Removes the first occurrence of the element e from this list.

Some commonly used (instance) methods of ArrayList    

These instance methods of ArrayList are used to search for a specific element e in an ArrayList object:
  

  Note:  E  can be any reference type (i.e.: not a primitive data type)

  contains(E e):
         Returns true if e is found in this list
         Returns false otherwise 



  indexOf(E e):
         Returns the index of the first occurrence of element e in this list
	 Returns -1 when not found

  lastIndexOf(E e):
         Returns the index of the last  occurrence of element e in this list
	 Returns -1 when not found

Some less commonly used (instance) methods of ArrayList    

These instance methods of ArrayList are used to (1) find information and (2) reset an ArrayList object:
  

  size():
       Returns the number of elements in this list.

  isEmpty():
       Returns true if this list contains no elements.
       Otherwise return false



  clear():
       Removes all of the elements from this list.
       (I.e.: the ArrayList will be empty)

In the next couple of slides, we will study examples that show you how to use these (instance) methods