Chapter 14 Generics and The ArrayList Class

Chapter 14 Generics and The ArrayList Class University Of Ha’il

Part 1 The ArrayList Class University Of Ha’il

Lists • list: an ordered sequence of elements, each accessible by a 0-based index • one of the most basic collections of data

The ArrayList class • Class ArrayList<E> implements the notion of a list • Think of it as an auto-resizing array • when you want to use ArrayList, remember to import java.util.*;

ArrayList vs. Array • Array String[] names = new String[5]; names[0] = "Java"; String name = names[0]; • ArrayList ArrayList<String> namesList = new ArrayList<String>(); namesList.add("Java"); String name = namesList.get(0);

The ArrayList Class • ArrayList is a class in the standard Java libraries. • Unlike arrays, which have a fixed length once they have been created, an ArrayList is an object that can grow and shrink while your program is running. • In general, an ArrayList serves the same purpose as an array, except that an ArrayList can change length while the program is running. University Of Ha’il

ArrayLists Disadvantages • An ArrayList is less efficient than an array. • It does not have the convenient square bracket notation. • The base type of an ArrayList must be a class type (or other reference type): it cannot be a primitive type (int, double, or char). University Of Ha’il

Using the ArrayList Class • In order to make use of the ArrayList class, it must first be imported from the package java.util. • An ArrayList is created and named in the same way as object of any class, except that you specify the base type as follows: ArrayList<BaseType> aList = new ArrayList<BaseType>(); • An initial capacity can be specified when creating an ArrayList as well. • e.g. The following code creates an ArrayList that stores objects of the base type String with an initial capacity of 20 items. ArrayList<String> list =new ArrayList<String>(20); University Of Ha’il

Adding elements • Elements are added dynamically to the list: ArrayList<String> list = new ArrayList<String>(); System.out.println("list = " + list); list.add("Red"); System.out.println("list = " + list); list.add("Blue"); System.out.println("list = " + list); list.add("Green"); System.out.println("list = " + list); • Output: list = [] list = [Red] list = [Red, Blue] list = [Red, Blue, Green]

Removing elements • Elements can also be removed by index: System.out.println("before remove list = " + list); list.remove(0); list.remove(1); System.out.println("after remove list = " + list); • Output: before remove list = [Tool, U2, Red, Green] after remove list = [U2, Green] • Notice that as each element is removed, the others shift downward in position to fill the hole. • Therefore, the second remove gets rid of Red, not U2.

ArrayList methods

ArrayList and for loop for (<type><name> : <collection>) { <statement(s)> ; } • This syntax can be used to examine an ArrayList: int sum = 0; for (String s : list) { sum += s.length(); } System.out.println("Total of lengths = " + sum);

The ArrayList Methods Example: ArrayList<Double> list1 = newArrayList<Double>(10); ArrayList<String> list2 = newArrayList<String>(); University Of Ha’il

The ArrayList Methods Example: ArrayList<String> list = newArrayList<String>(); int index = 2; …… list.set(index, "Here"); String S = list.get(index); University Of Ha’il

The ArrayList Methods Example: ArrayList<String> list = newArrayList<String>(); list.add("Java"); list.add("Course"); list.add(0, "Semester 201101"); University Of Ha’il

The ArrayList Methods University Of Ha’il

Why are Some Parameters of Type Base_Type and Others of type Object • When looking at the methods available in the ArrayList class, there appears to be some inconsistency. • In some cases, when a parameter is naturally an object of the base type, the parameter type is the base type. • However, in other cases, it is the type Object. • This is because the ArrayList class implements a number of interfaces, and inherits methods from various classes. • These interfaces and classes specify that certain parameters have type Object. University Of Ha’il

For-each Loop for ArrayList Objects • The ArrayList class is an example of a collection class. • Starting with version 5.0, Java has added a new kind of for loop called a for-each or enhanced for loop. • This kind of loop has been designed to cycle through all the elements in a collection (like an ArrayList). • Syntax: for (Array_Base_Type Variable : ArrayList_Object) Statement • Example: ArrayList<Integer> list = new ArrayList<Integer>(10); for (Integer element : list) element = 20 ; University Of Ha’il

Use trimToSize to Save Memory • An ArrayList automatically increases its capacity when needed. • However, the capacity may increase beyond what a program requires. • In addition, although an ArrayList grows automatically when needed, it does not shrink automatically. • If an ArrayList has a large amount of excess capacity, an invocation of the method trimToSize will shrink the capacity of the ArrayList down to the size needed. University Of Ha’il

Example (ArrayList class) import java.util.*; class ArrayListDemo { public static void main(String[] args) { ArrayList<String> list = new ArrayList<String> (20); list.add("AB") ; list.add("CD") ; list.add("GH") ; list.add("IJ") ; list.add("KL") ; list.add("MN") ; list.add("QR") ; list.add("ST") ; list.add("WX") ; list.add("YZ") ; // Print the elements of the ArrayList System.out.println("The following is the initial ArrayList:"); for(String s : list) System.out.print(s +" "); // Add elements to the ArrayList list.add(2 , "ef") ; list.add(7 , "op") ; list.add(10 , "uv") ; // Print the new ArrayList System.out.println(" "); System.out.println("The following is the new ArrayList:"); for(String s : list) System.out.print(s +" "); continued University Of Ha’il

Example (ArrayList class) // Print the element at position index1 in the ArrayList System.out.println(" "); int index1=11; String element = list.get(index1) ; System.out.println("The element at position " +" " +index1 +" " +"is" +" " +element); // Print the index of an element of the ArrayList System.out.println(" "); int index = list.indexOf("CD") ; System.out.println("The element CD is at position " +" “ +index); } } Output: The following is the initial ArrayList: AB CD GH IJ KL MN QR ST WX YZ The following is the new ArrayList: AB CD ef GH IJ KL MN op QR ST uv WX YZ The element at position 11 is WX The element BC is at position 1 University Of Ha’il

Parameterized Classes and Generics • The class ArrayList is a parameterized class. • It has a parameter, denoted by Base_Type, that can be replaced by any reference type to obtain a class for ArrayLists with the specified base type. • Starting with version 5.0, Java allows class definitions with parameters for types. • These classes that have type parameters are called parameterized class or generic definitions, or, simply, generics. University Of Ha’il

Chapter 14 Generics and The ArrayList Class