Chapter 6: ArrayLists

Presentation slides for

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Java Software Solutions

for AP* Computer Science A

2nd Edition

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by John Lewis, William Loftus, and Cara Cocking

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Java Software Solutions is published by Addison-Wesley

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Presentation slides are copyright 2006 by John Lewis, William Loftus, and Cara Cocking. All rights reserved.

Instructors using the textbook may use and modify these slides for pedagogical purposes.

*AP is a registered trademark of The College Entrance Examination Board which was not involved in the production of, and does not endorse, this product.

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The ArrayList Class

• The ArrayList class is part of the java.util package (Collections API)
• Like an array, it can store a list of values and reference them with an index
• Unlike an array, an ArrayList object grows and shrinks as needed
• Items can be inserted or removed with a single method invocation
• It stores references to the Object class, which allows it to store any kind of object
• ArrayLists are not allowed to contain elements of any primitive data type.

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Specifying an ArrayList Element Type

• ArrayList is a generic type, which allows us to specify the type of data each ArrayList should hold
• For example, ArrayList<Family> holds Family objects

Example // Declare and Instantiate

ArrayList<Integer> list = new ArrayList<Integer>( );

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

//Declare and initial capacity

ArrayList<String> a = new ArrayList<String>(25);

** default initial capacity of 10

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3

Wrapper Class

ArrayList<Integer> a = new ArrayList<Integer>();

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int[] someints = { 5, 2, 5, 7, 5, 8, 5, 2 };

Auto-Boxing and Unboxing

• Auto-Boxing is automatic wrapping of primitive data types
• No primitive types in collections classes - objects only
• Numbers must be boxed before insertion
• More efficient to access primitive data types from an array than an ArrayList

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intValue( ) doubleValue( )

* Convert Integer to int

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https://docs.oracle.com/javase/7/docs/api/java/lang/Integer.html

ArrayList Efficiency

• The ArrayList class is implemented using an array
• The code of the ArrayList class automatically expands the array's capacity to accommodate additional elements
• The array is manipulated so that indexes remain continuous as elements are added or removed
• If elements are added to and removed from the end of the list, this processing is fairly efficient
• If elements are inserted and removed from the middle of the list, the elements are constantly being shifted around

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6

Common Methods

ArrayList<E> instance methods

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size( ) void clear()

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E get( int index ) vs. E set( int index, E x )

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boolean add( E x ) vs. void add( int index, E x )

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Common Methods

E remove( int index )

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1. The item at the given index is removed;
2. any items to its right "slide down" to the left (that is, their indexes are reduced by 1); and
3. the size of the ArrayList is reduced by 1.

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For Each

for ( String name : names )" expression is read as "for each String name in names".

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//Error Message

ConcurrentModificationException

"Don't change the size of this ArrayList while I'm iterating my way along it!"

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//Solution

use a regular for loop

List<E> Interface

See Interface

Practice

Ex. 115 - slide 12 ArrayList