ArrayList

• ArrayList


  an instance can serve the same purpose as an array, except that it can change in length while the program is running
• Why use ArrayList and not Arrays

  Using an instance of ArrayList is less efficient than using an array.
• An instance of ArrayList can store only objects; it cannot contain values of a primitive type, such as int, double, or char.
• ArrayList<Base_Type>variable = new ArrayList< Base_Type >();

  *** invoke the default constructor - which is the size of 10
• ArrayList< Base_Type > variable = new ArrayList< Base_Type >(20);

  *** initial capacity is 20, but can still change
• ArrayList<Base_Type> variable = new ArrayList<>()

  ***starting with JDK 7 it will not require for the base type to be repeated
• An object of ArrayList can be used like an array, but you must use methods instead of an array’s square-bracket notation.
• Example:

  String[] anArray = new String[20];

  ArrayList<String> aList = new ArrayList<>(20);

  anArray[index] = "Hi

  Mom!"; //assign values

  aList.set(index, "Hi Mom!");

  //The method set is used to change the value of

  //existing elements, not to set them for the first time.

  String temp = anArray[index]; //get element

  String temp = aList.get(index);

  To set an element for the first time, you use the method add.
• To set an element for the first time, you use the method add.

  This method adds elements at index positions 0, 1, 2, and so forth, in that order.
• ArrayList:

  This method adds elements at index positions 0, 1, 2, and so forth, in that order.

  An ArrayList object must always be filled in this order.
• ArrayList:

  The method add has two forms; that is, it is overloaded.
• ArrayList:

  When given one argument, add adds an element immediately after the last currently occupied position.
• ArrayList:

  Given two arguments, the method adds the element at the indicated position, assuming that the indicated position is not after an unoccupied position. 


  Þ All other elements will adjust position.
• public Base_Type get(int index)

  • Returns the element at the position specified by index.
• public Base_Type set(int index, Base_Type element)

  • Replaces the element at the position specified by index with the given element.
• public Base_Type remove(int index)

  • Removes and returns the element at the specified

  index.
• public Base_Type remove(int index)

  • Shifts elements at subsequent positions toward position index by decreasing their indices by 1.
• public boolean remove(Object element)

  • Removes the first occurrence of element in this list, and shifts elements at subsequent positions toward the removed element by decreasing their indices by 1.
• public boolean remove(Object element)

  Returns true if element was removed; otherwise returns false and does not alter the list.
• public void clear()

  • Removes all elements from this list.
• public boolean contains(Object element)

  • Returns true if element is in this list; otherwise, returns false.
• public int indexOf(Object element)

  • Returns the index of the first occurrence of element in this list.
• public int indexOf(Object element)

  Returns –1 if element is not on the list.
• public boolean isEmpty() 


  • Returns true if this list is empty; otherwise, returns false
• public int size()

  Returns the number of elements in this list.