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The ArrayList class

[Reading: chapter 10]

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, writeArrayList<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 ArrayListArrayList<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); // MondayString 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);

// work with r...

}

Loop with an iterator Iterator<Rectangle> it = a.iterator();

// could also use a ListIterator

while(it.hasNext()) {

Rectangle r = it.next();

// work with r...

}

Number of elements in a

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.

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Memory view

ArrayList<String> a= new ArrayList<String>(); a.add(“Mercury”); a.add(“Earth”); a.add(1,”Venus”);

a

0

1

2

ArrayList

String

“Mercury”

String

“Venus”

String

“Earth”