The ArrayList Class and the enum Keyword - Learning Java Through Games - page 142

7.4 The Interface of an ArrayList
Since ArrayList is a class, the interface of objects of type ArrayList is a lot different
than the interface of arrays. For example, the interface [] to access the element of an array
is not supported. An element of an ArrayList can be accessed using the get(i) method,
where i is the index of the element. The size method returns the numbers of elements in
the ArrayList .Thisdiffersfromthevariable length , which returns the size of the array
and not the number of elements in it. In fact, the variable length is not defined for an
ArrayList because, by definition, an ArrayList can store a variable number of elements
and does not have preset capacity. To demonstrate how an ArrayList works, the following
method converts an ArrayList of employees to an array of employees.
public static Employee [ ] convert(ArrayList < Employee > emps) {
Employee [ ] result = new Employee [ emps . s ize () ] ;
for ( int i=0; i < emps . s i z e ( ) ;

i ++) {

result [ i ] = emps.get(i);
return result ;

}

The above method is for demonstration purposes only and it is not the easiest way to
convert an ArrayList to an array. Actually, Java supports the toArray method that will
convert any ArrayList into an array. The only caveat of using this method is that the result
needs to be cast to the proper array of objects. For example, the following code converts
an ArrayList of employees to an array of employees.
Employee [ ] emps = (Employee [ ] ) empArrayList . toArray () ;
Note that the get method is not identical to the [.] operator for arrays. For example,
we can write:
emps [ 0 ] = new Employee( "John" );
but we cannot use the following syntax.
emps . get (0) = new Employee( "John" );
The get method simply retrieves an element from the ArrayList and cannot be used
to modify an element of the ArrayList . An element of an ArrayList can be modified as
follows.
emps . set (0 , new Employee( "John" ));
The first parameter of the set method is the index in the ArrayList , while the sec-
ond parameter is the new value of the element. Note that the set method can only be
used to modify an existing element of an ArrayList .Itcannotbeusedtocreateanel-
ement at a position that does not exist. For example, the following code will generate
IndexOutOfBoundsException exception.
ArrayList < Employee > emps = new ArrayList < Employee > () ;
emps . set (0 , new Employee( "John" ));
The reason is that the code tries to change the element at position 0, but an element at
position 0 does not exist. The correct approach is to use the add method to insert a new
element in the ArrayList . Here is the correct version of the above code.
ArrayList < Employee > emps = new ArrayList < Employee > () ;
emps . add( new Employee( "John" ));