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Building Java Programs

Chapter 10

Lecture 10-1: ArrayList

reading: 10.

2 Exercise

! Write a program that reads a file and displays

the words of that file as a list.

! First display all words. ! Then display them with all plurals (ending in "s") capitalized. ! Then display them in reverse order. ! Then display them with all plural words removed.

! Should we solve this problem using an array?

! Why or why not?

4 Lists

! list : a collection storing an ordered sequence of elements

! each element is accessible by a 0-based index ! a list has a size (number of elements that have been added) ! elements can be added to the front, back, or elsewhere ! in Java, a list can be represented as an ArrayList object

5 Idea of a list

! Rather than creating an array of boxes, create an object

that represents a "list" of items. (initially an empty list.)

[]

! You can add items to the list.

! The default behavior is to add to the end of the list. [hello, ABC, goodbye, okay]

! The list object keeps track of the element values that have

been added to it, their order, indexes, and its total size.

! Think of an "array list" as an automatically resizing array object. ! Internally, the list is implemented using an array and a size field.

7 Type Parameters (Generics)

ArrayList< Type > name = new ArrayList< Type >();

! When constructing an ArrayList, you must specify the

type of elements it will contain between < and >.

! This is called a type parameter or a generic class. ! Allows the same ArrayList class to store lists of different types. ArrayList names = new ArrayList (); names.add("Marty Stepp"); names.add("Stuart Reges");

8 Learning about classes

! The Java API Specification is a huge web page containing

documentation about every Java class and its methods.

! The link to the API Specs is on the course web site.

10 ArrayList vs. array 2

! doing something to each value that starts with "B"

for (int i = 0; i < names.length; i++) { if (names[i].startsWith("B")) { ... } } for (int i = 0; i < list.size(); i++) { if (list.get(i).startsWith("B")) { ... } }

! seeing whether the value "Benson" is found

for (int i = 0; i < names.length; i++) { if (names[i].equals("Benson")) { ... } } if (list.contains("Benson")) { ... }

11 Exercise, revisited

! Write a program that reads a file and displays

the words of that file as a list.

! First display all words. ! Then display them in reverse order. ! Then display them with all plurals (ending in "s") capitalized. ! Then display them with all plural words removed.

13 ArrayList as parameter

public static void name (ArrayList< Type > name ) {

! Example:

// Removes all plural words from the given list. public static void removePlural( ArrayList list ) { for (int i = 0; i < list.size(); i++) { String str = list.get(i); if (str.endsWith("s")) { list.remove(i); i--; } } }

! You can also return a list:

public static ArrayList< Type > methodName ( params )

14 ArrayList of primitives?

! The type you specify when creating an ArrayList must be

an object type; it cannot be a primitive type.

// illegal -- int cannot be a type parameter ArrayList list = new ArrayList ();

! But we can still use ArrayList with primitive types by

using special classes called wrapper classes in their place.

// creates a list of ints ArrayList list = new ArrayList ();

16 Exercise

! Write a program that reads a file full of numbers and

displays all the numbers as a list, then:

! Prints the average of the numbers. ! Prints the highest and lowest number. ! Filters out all of the even numbers (ones divisible by 2).

17 Exercise solution (partial) ArrayList numbers = new ArrayList(); Scanner input = new Scanner(new File("numbers.txt")); while (input.hasNextInt()) { int n = input.nextInt(); numbers.add(n); } System.out.println(numbers); filterEvens(numbers); System.out.println(numbers); ... // Removes all elements with even values from the given list. public static void filterEvens (ArrayList list) { for (int i = list.size() - 1; i >= 0; i--) { int n = list.get(i); if (n % 2 == 0) { list.remove(i); } } }

19 Out-of-bounds

! Legal indexes are between 0 and the list's size() - 1.

! Reading or writing any index outside this range will cause an IndexOutOfBoundsException. ArrayList names = new ArrayList(); names.add("Marty"); names.add("Kevin"); names.add("Vicki"); names.add("Larry"); System.out.println(names.get(0)); // okay System.out.println(names.get(3)); // okay System.out.println(names.get(-1)); // exception names.add(9, "Aimee"); // exception

index 0 1 2 3

value Marty Kevin Vicki Larry

20 ArrayList "mystery" ArrayList list = new ArrayList(); for (int i = 1; i <= 10; i++) { list.add(10 * i); // [10, 20, 30, 40, ..., 100] }

! What is the output of the following code?

for (int i = 0; i < list.size(); i++) { list.remove(i); } System.out.println(list);

! Answer:

[20, 40, 60, 80, 100]