Generics and the ArrayList Class - Java - page 704

list2.set(42, index); //Sets the element at index to
//new Integer(42). This example relies on automatic boxing.
int temp2 = list2.get(index); //The expression
//v.get(index) returns the element at position index.
// This example relies on automatic unboxing.

The index must be greater than or equal to 0 and less than the current size of the
ArrayList list .

The two methods set and get give ArrayList s approximately the same functional-
ity that square brackets give to arrays. However, you need to be aware of one impor-
tant point: The method invocation

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

is not always completely analogous to

a[index] = "Hi Mom!";

The method set can replace any existing element, but you cannot use set to put an
element at just any index, as you could with an array. The method set is used to
change the value of elements, not to set them for the first time. To set an element for
the first time, you usually use the method add . The basic form of the method add adds
elements at index position 0 , position 1 , position 2 , and so forth in that order. This
means that ArrayList s must always be filled in this order. But your code can then go
back and change any individual element, just as it can in an array.
For example, suppose list is an ArrayList with base type String , which has not
yet had any elements added to it; that is, list is empty. The following statements will
add the strings "One" , "Two" , and "Three" to positions 0 , 1 , and 2 :

add

list.add("One");
list.add("Two");
list.add("Three");

The method name add is overloaded. There is also a two-argument method named
add that allows you to add an element at any currently used index position or at the
first unused position. When inserting into an ArrayList with this version of add , ele-
ments at the specified index and higher (if any) are moved up one position to make
room for the new element. For example,

list.add(0, "Zero");

adds the string "Zero" at position 0 and moves elements originally at positions 0 , 1 , 2 ,
and so forth up one position to positions 1 , 2 , 3 , and so forth.