CSC 142
The ArrayList class
[Reading: chapter 10]
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The ArrayList class
 Available in the package java.util.
 An ArrayList object stores a list of items. The
ArrayList automatically manages its size. The
list grows as more items are added, or
shrinks as some items are removed.
 Items stored in an ArrayList all have the same
reference type (i.e. defined by a class).
Primitive type values can’t be stored as such
in an ArrayList -> must use a wrapper type.
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Declaring an ArrayList object
 A named collection of data items of the same
reference type.
 To declare an ArrayList of Strings, write
ArrayList<String> phrases;
 To declare an ArrayList of integers, write
ArrayList<Integer> numbers;
NOT ArrayList<int> numbers;
 Always use the <> notation (= generics).
 At this point, no memory is reserved for the
ArrayList
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Initializing an ArrayList
 To initialize an ArrayList (once declared)
phrases = new ArrayList<String>();
numbers = new ArrayList<Integer>();
 Other constructors are available e.g.
// Make a copy of an existing ArrayList
copy = new ArrayList<String>(phrases);
// Or if you know the size you need
daysOfTheWeek = new ArrayList<String>(7);
// But daysOfTheWeek is NOT limited
// to 7 elements
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Manipulating ArrayList elements (1)
 To add an element to an ArrayList
ArrayList<String> a = new ArrayList<String>();
a.add(“Monday”);
a.add(“Wednesday”);
 To access an ArrayList element, use its index
(also called subscript), e.g.,
String first = a.get(0); // Monday
String second = a.get(1); // Wednesday
Indices start at 0. If an ArrayList has 10 elements, the first
element has index 0 and the last element has index 9.
• To insert in an ArrayList
a.add(1,“Tuesday”);
//Wednesday is now at index 2
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Manipulating ArrayList elements (2)
 To remove an element from an ArrayList
String s = a.remove(1); // element at index 1 is
// removed automatically, elements at indices > 1
// are shifted down
// Also:
boolean success = a.remove(s);
// removes s (= some object) if present. Returns true
// if s has been removed, and false if not.
 Other operations:
indexOf, contains, set
See java documentation
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ArrayList and loops (1)
 Loop with an index
e.g. if a is an ArrayList of Rectangle objects
for(int i = 0; i < a.size(); i ++) {
Rectangle r = a.get(i);
Number of elements
// work with r...
in a
}
 Loop with an iterator
Iterator<Rectangle> it = a.iterator();
// could also use a ListIterator
while(it.hasNext()) {
Rectangle r = it.next();
// work with r...
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}
ArrayList and loops (2)
 for each loop
for(Rectangle r : a) {
// work with r...
}
 Which one to use?
 To read the elements in no special order,


use a for each loop.
To iterate through the ArrayList in a special
way (e.g. from last to first, every other
element), use a loop with an index.
for each and iterator loops are read only.
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Memory view
ArrayList<String> a= new ArrayList<String>();
a.add(“Mercury”);
a.add(“Earth”);
a.add(1,”Venus”);
String
a
ArrayList
“Mercury”
String
0
1
2
“Venus”
String
“Earth”
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