Generics and The ArrayList Class Lecture 13

1. Generics and The ArrayList Class Lecture 13 University Of Ha’il (Girls Branch)  Semester 081

2. Part 1 The ArrayList Class University Of Ha’il (Girls Branch)  Semester 081

3. 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 (Girls Branch)  Semester 081

4. 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 (Girls Branch)  Semester 081

5. 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 (Girls Branch)  Semester 081

6. The ArrayList Methods • The tools for manipulating arrays consist only of the square brackets and the instance variable length. • ArrayLists, however, come with a selection of powerful methods that can do many of the things for which code would have to be written in order to do them using arrays. University Of Ha’il (Girls Branch)  Semester 081

7. The ArrayList Methods Example: ArrayList<Double> list1 = new ArrayList<Double>(30); ArrayList<String> list2 = new ArrayList<String>(); University Of Ha’il (Girls Branch)  Semester 081

8. The ArrayList Methods Example: ArrayList<String> list = new ArrayList<String>(); list.add(“Simab”); list.add(“Shahid”); list.add(0,“Semester 081”); University Of Ha’il (Girls Branch)  Semester 081

9. The ArrayList Methods Example: ArrayList<String> list = new ArrayList<String>(); int index = 1; list.add(“Hi”); list.add(“There”); string S1 = list.set(index, “Hello”); List.add(S1); String S2 = list.get(index); University Of Ha’il (Girls Branch)  Semester 081

10. Example 01 • ArrayList<integer> list = new ArrayList<integer>(); int index = 2; list.add(1); // will add at index 0 list.add(2); // will add at index 1 index = 1; • integer i= list.set(index, 7); System.out.println(i); • Output : 2 • The set() function of ArrayList class set the index with the given value and returned the value previously stored at that index University Of Ha’il (Girls Branch)  Semester 081

11. Example 01 • ArrayList<integer> list = new ArrayList<integer>(); int index = 2; list.add(1); // will add at index 0 list.add(2); // will add at index 1 index = 1; • integer i= list.get(index); System.out.println(i); • Output : 2 • The get() function of ArrayList class returned the returned the value stored at that index University Of Ha’il (Girls Branch)  Semester 081

12. The ArrayList Methods University Of Ha’il (Girls Branch)  Semester 081

13. The ArrayList Methods University Of Ha’il (Girls Branch)  Semester 081

14. Example • ArrayList<Integer> list = new ArrayList<Integer> (20); list.add(1); list.add(2); list.add(3); int index = 1; list.remove(index);   for(Integer i : list) System.out.println(i +" "); Output: 1 3 In this code the remove function has removed the value stored at index 1. University Of Ha’il (Girls Branch)  Semester 081

15. Example • ArrayList<Integer> list = new ArrayList<Integer> (20); list.add(1); list.add(2); list.add(3); int index = 1; Integer i =list.remove(index); System.out.println(i +" "); Output: 2 University Of Ha’il (Girls Branch)  Semester 081

16. Example • ArrayList<Integer> list = new ArrayList<Integer> (20); list.add(1); list.add(2); list.add(3); list.clear(); for(Integer i : list) System.out.println(i +" "); Output: It will not print any thing because the clear() will delete all the elements of the list. University Of Ha’il (Girls Branch)  Semester 081

17. The ArrayList Methods University Of Ha’il (Girls Branch)  Semester 081

18. Example • ArrayList<Integer> list = new ArrayList<Integer> (20); list.add(1); list.add(2); list.add(3); int index ; index = list.indexOf(2); System.out.println(“Index of 2 is “ + index); Output: Index of 2 is 1 University Of Ha’il (Girls Branch)  Semester 081

19. Example • ArrayList<Integer> list = new ArrayList<Integer> (20); list.add(4); list.add(2); list.add(3); list.add(4); int index ; index = list.lastIndexOf(4); System.out.println(“Last Index of 4 is “ + index); Output: Index of 4 is 3 University Of Ha’il (Girls Branch)  Semester 081

20. The ArrayList Methods University Of Ha’il (Girls Branch)  Semester 081

21. The ArrayList Methods University Of Ha’il (Girls Branch)  Semester 081

22. The ArrayList Methods University Of Ha’il (Girls Branch)  Semester 081

23. 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 (Girls Branch)  Semester 081

24. 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 (Girls Branch)  Semester 081

25. 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 (Girls Branch)  Semester 081

26. 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 (Girls Branch)  Semester 081

27. 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 CD is at position 1 University Of Ha’il (Girls Branch)  Semester 081

28. 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 (Girls Branch)  Semester 081