Arrays of Objects - COMP 248 Object-Oriented Programming I PDF

COMP 248 OBJECT ORIENTED PROGRAMMING I http://www.willamette.edu/~gorr/classes/cs231/lectures/chapter9/ Chapter 6 – Part B: Arrays of Objects 1  Chapter 6 – Part B 1. Arrays are objects 2. Array...

COMP 248 OBJECT ORIENTED PROGRAMMING I http://www.willamette.edu/~gorr/classes/cs231/lectures/chapter9/ Chapter 6 – Part B: Arrays of Objects 1  Chapter 6 – Part B 1. Arrays are objects 2. Arrays of objects not covered from chap. 6: – enumerated types – methods with variable number of parameters 2  Arrays The elements of an array can be: – a primitive type or – an object reference In Java, the array itself is an object – so the name of the array is a reference – and the array itself must be created with new 3  1- Arrays are objects In Java, an array is an object – so an array can be passed as a parameter to a method Like any other object: – the reference to the array is passed (much faster) – so the formal and actual parameters become aliases of each other – so changing an array element within the method changes the original object passing the entire array ≠ passing a single element of the array 4  Example public boolean isVowel(char someChar) { } public int nbVowel(char[] someCharArray) { } char[] alphabet = {'a', 'b', 'c', 'd, …, 'z'}; if (isVowel(alphabet)) // ok ? System.out.print(“…"); if (isVowel(alphabet)) // ok? System.out.print("…"); if (nbVowel(alphabet)>3) // ok? System.out.print("…"); if (nbVowel(alphabet)>3) // ok? System.out.print("…"); 5  Example Write a method that will return the sum of an array of integers. 6 Arrays and assignment we cannot change the size of an array but we can assign an array to another… int[] a1 = {10, 20, 30}; int[] a2 = {1, 2, 3, 4, 5}; System.out.println(a1.length); System.out.println(a1); a1 = a2; System.out.println(a1.length); System.out.println(a1); Output 7  Duplicating/Coping an array to copy the content of an array into another: static int[] duplicate(int[] theOriginal) { int[] theCopy = new int[theOriginal.length]; for (int i = 0; i < theOriginal.length; i++) theCopy[i] = theOriginal[i]; return theCopy; } or use the built-in clone method (for 1-d arrays) theCopy = (int[]) theOriginal.clone();  note: clone returns an array to a generic Object, we need to cast the result to what we want (ex. int[]) 8  Example public static void main(String[] args) { int[] a1 = {10, 20, 30}; int[] a2 = {1, 2, 3, 4, 5}; with: a1 = a2; System.out.println(a1); with: a1 = duplicate(a2); // a1 = a2; // a1 = duplicate(a2); // a1 = (int[]) a2.clone(); with: a1 = (int[]) a2.clone(); System.out.println(a1); a1 = 99; System.out.println(a1); Output System.out.println(a2); } public static int[] duplicate(int[] theOriginal) { int[] theCopy = new int[theOriginal.length]; for (int i = 0; i < theOriginal.length; i++) theCopy[i] = theOriginal[i]; return theCopy; } 9 Methods That Return an Array a method may also return an array public static int[] incrementArray(int[] a, int increment) { int[] temp = new int[a.length]; int i; for (i = 0; i < a.length; i++) temp[i] = a[i] + increment; return temp; } 10 Privacy Leaks with Array Instance Variables If an accessor returns the contents of an array, special care must be taken – Just as when an accessor returns a reference to any private object public double[] getArray() { return anArray;//BAD! privacy leak } 11 Privacy Leaks with Array Instance Variables The method should return a reference to a deep copy of the private array object – public double[] getArray() { double[] temp = new double[count]; for (int i = 0; i < count; i++) temp[i] = a[i]; return temp } 12 2- Arrays of objects we can have arrays of primitive types 1 2 10 6 25 'a' 'd' 'e' 'z' 'f' we can have arrays of objects (more precisely, arrays of references to objects) 13 ex: Arrays of strings ex: 5 references to String objects String[] words = {"I", "am", "very", "hungry", "!"}; “I” “am” … … “!” 14  Arrays of objects 1. create the array of references Account[] bank = new Account; 2. create each object bank = new Account("ted", 111, 100); bank = new Account("mary", 222, 500); bank = new Account("john", 999, 5); Each object in the array must be created individually 3. manipulate each object for (int i = 0; i < bank.length; i++) { bank[i].deposit(100)); System.out.print(bank[i].getBalance()); } 15 Example Declare the class Airplane with speed, nbPassengers, Pilot. Declare an array of 100 Airplane objects 16 Example write a method that takes an array of Airplane and returns the average speed of the Airplane. If the array is empty, return 0. 17  Example public class Telephone { private int number; public Telephone(int number) { this.number = number; } public String ring() { return "dring dring"; } } Declare a 2-D array of 10x10 Telephone objects. Initialize each of the 100 telephone objects to the phone number (514) 848-2424. Call the method ring on all 100 elements of the array and display the result. 18