Introduction to Programming

CSCI 51Introduction to Programming March 3, 2009

Today • Review • Reserved word this • Arrays

Announcements • Program 5

ReviewScope public class Rectangle { // variables declared here are class-level // available in all methods in Rectangle class public int computeArea() { // variables declared here are method-level // only available in computeArea() } public void print() { // variables declared here are method-level // only available in print() } }

public visibility can be accessed from anywhere private visibility can only be accessed from inside the class (inside the same Java source file) ReviewVisibility Modifiers public class Rectangle { private int length; private int width; } public Rectangle () { length = 0; width = 0; } ...

ReviewVisibility Modifiers • Usually declare data members with private visibility • Declare methods that clients (other classes) are supposed to call with public visibility • service methods • Declare methods that only other methods in the class are supposed to call with private visibility • support methods

The Reference this • Reserved word • Refers to instance variables and methods of a class • Allows you to distinguish between member variables and local variables with the same name

Rectangle.java public class Rectangle { private int length; private int width; public Rectangle (int length, int width) { this.length = length; this.width = width; }

Reference Variables as Parameters If a formal parameter is a reference variable: • copies value of corresponding actual parameter • value of actual parameter is address of object where actual data is stored • both formal and actual parameter refer to same object

ReviewOverloading Methods • Overloading - the process of using the same method name for multiple methods • The signature of each overloaded method must be unique • number of parameters • type of the parameters • not the return type of the method, though • The compiler determines which version of the method is being invoked by analyzing the parameters

Review Rectangle r2 = new Rectangle (5, 10); public class Rectangle { private int length; private int width; public Rectangle () { length = 0; width = 0; } public Rectangle (int l, int w) { length = l; width = w; }

Review • A method should be relatively small • it should be able to be understood as a single entity • its name should fully describe its function • A potentially large method should be decomposed into several smaller methods as needed for clarity • A service method of an object may call one or more support methods to accomplish its goal

Thought Exercise Write a method for the Rectangle class called printBox that will print the rectangle as a box made of % example: length = 3, width = 5 %%%%% % % %%%%%

publicvoid printBox () { for (int i =1; i <= width; i++) System.out.print("%"); System.out.println( ); for (int i =1; i <= length -2; i++) { System.out.print("%"); for (int j =1; j <= width -2; j++) System.out.print(" "); System.out.println("%"); } for (int i =1; i <= width; i++) System.out.print("%"); System.out.println( );} length = 3, width = 8 %%%%%%%%% %%%%%%%%%

The toString Method • Special method in Java classes • Produces a String object based on the current object, suitable for printing • Mapped to the '+' operator • Also called when the object is a parameter in a print() or println() method • There is a default toString method, but it's better if we write our own

Rectangle.java public String toString() { String result = ""; result += "length: " + length + "\n"; result += "width: " + width; return (result); } Rectangle r = new Rectangle (2,3); System.out.println (r); length: 2 width: 3

The Modifier static • In the method heading, specifies that the method can be invoked by using the name of the class • no object has to be created in order to use the method • can't call a non-static method from a static method • can't access non-static variables from a static method • If used to declare data member, data member invoked by using the class name • no object has to be created in order to use the variables

static Variables • Shared among all objects of the class • Memory created for static variables when class is loaded • memory created for instance variables (non-static) when an object is instantiated (using new) • If one object changes the value of the static variable, it is changed for all objects of that class

public class Illustrate { private int x; public static int y; private static int count; public Illustrate() { x = 0; } public Illustrate (int a) { x = a; } public static void incrementCount() { count++; } } y 0 0 count obj1 3 x obj2 5 x Illustrate Class Illustrate obj1 = new Illustrate(3); Illustrate obj2 = new Illustrate(5); 1 1 Illustrate.incrementCount(); Illustrate.y++;

2 3 0 1 Arrays • An array is a list of values that can be represented by one variable • Members of an array must all have the same data type • Each value is stored at a specific, numbered position in the array • the number corresponding to each position is called an index or subscript • All arrays have a length • number of elements the array can hold

0 1 2 3 Declaring Arrays The array (element) data type Empty square brackets type[] name; The array (variable) name Creates a reference variable called name that can point to an array of type elements.

counter characterSet 0 1 2 3 grade Declaring ArraysExamples // array of counters (integers) int[] counter; // array of characters char[] characterSet; // array of grades (doubles) double[] grade;

0 1 2 3 Instantiating Arrays You must instantiate (create) arrays • the size of an array is typically not known before run time The assignment operator The array (variable) name The new operator name = new type[size]; The array (element) data type The number of elements

counter 0 1 2 3 4 0 1 2 3 Instantiating ArraysExamples // instantiate an array of counters counter = new int[5]; 0 <= index < size // instantiate the array of grades numStudents = 10; grade = new double[numStudents];

Declaration Instantiation 0 1 2 3 Declaration and Instantiation type[] name = new type[size];

Arrays of Objects • Can use arrays to manipulate objects • Create array of objects • Must instantiate each object in array classname[] array = new classname[size]; for(int j=0; j <array.length; j++) { array[j] = new classname(); }

0 1 2 3 Example int[] num = new int[5];

0 1 2 3 Array AccessExamples double score[] = new double[3]; score[0] = 98.3; score[1] = 57.8; score[2] = 93.4; averageScore = (score[0]+score[1]+score[2])/3; numStudents = 3; totalScore = 0; for (int i = 0; i < numStudents; i++) { totalScore += score[i]; } averageScore = totalScore/numStudents; often use loops for access

0 1 2 3 Array Length Arrays have length • an internal variable called length • number of elements in array • access the length variable using the “dot’ notation (arrayname.length) // loop through the array of test scores sumOfScores = 0; for (int i=0; i<scores.length; i++) { sumOfScores += scores[i]; }

0 1 2 3 Initializing Arrays • Array elements are variables too! • if you don’t initialize, the contents are undefined • When and how? • if you don’t yet know the size • initialize at run time, typically with a loop • if you know how many elements • perhaps use an initializer list int counter[] = {0, 0, 0, 0, 0}; char[] characterSet = {‘a’,’b’,’c’}; // etc.

0 1 2 3 Initializer Lists • Lists the initial value for the elements of an array • Items are separated by commas and the list is in braces {} • The size of the array is determined by the number of items in the list int[] scores = {87, 98, 45}; • Can only be used in the same statement as declaring the array NOT int[] scores; scores = {87, 98, 45};

0 1 2 3 Array Bounds • Arrays have finite size • If you access an element outside of the array, you’ll get an ArrayIndexOutOfBounds Exception Example: int[] grades = {99, 98, 95, 96}; System.out.println (grades[4]);

0 1 2 3 Example Specify Array Size During Program Execution (Assume that keyboard has already been declared and instantiated.) intarraySize; System.out.print ("Enter the size of the array:"); arraySize = Integer.parseInt(keyboard.readLine()); int[] list = new int[arraySize];

0 1 2 3 Example Initialize Array to Specific Value (10.00) (Assume that sale has already been declared and instantiated.) for (int ind = 0; ind < sale.length; ind++) { sale[ind] = 10.00; }

0 1 2 3 Example Read Data into Array (Assume that sale has already been declared and instantiated, and that keyboard has already been declared and instantiated.) for (int ind = 0; ind < sale.length; ind++) { sale[ind] = Double.parseDouble(keyboard.readLine()); }

0 1 2 3 Example Print Array (Assume that sale has already been declared and instantiated.) for (int ind = 0; ind < sale.length; ind++) { System.out.print(sale[ind] + " "); }

0 1 2 3 Example Find Sum and Average of Array (Assume that sale has already been declared and instantiated, and that sum and average have already been declared.) sum = 0; for(int ind = 0; ind < sale.length; ind++) { sum = sum + sale[ind]; } if(sale.length != 0) average = sum / sale.length; else average = 0.0;

0 1 2 3 Example Determining Largest Element in Array (Assume that sale has already been declared and instantiated, and that maxIndex and largestSale have already been declared.) maxIndex = 0; for (int ind = 1; ind < sale.length; ind++) { if (sale[maxIndex] < sale[ind]) { maxIndex = ind; } } largestSale = sale[maxIndex]; 5 7 4 6 1 3 0 2 25.00 19.60 12.50 98.23 8.35 14.00 39.43 35.90

0 1 2 3 Parallel Arrays Arrays are parallel if corresponding components hold related information String[] studentName; double[] studentGPA; For example, studentName and studentGPA are parallel if studentGPA[3] is the GPA of the student with studentName[3].

0 1 2 3 In-Class Exercises • Declare an array of integers called numbers Hint: type[] name; • Declare and instantiate an array of 26 characters called alphabet Hint: type[] name = new type[size]; int[] numbers; char[] alphabet = new char[26];

0 1 2 3 In-Class Exercises • Declare an array of 5 characters called grades and initialize it with the letters: A, B, C, D, F Hint: type[] name = {initialization list}; • Write a loop to print the contents of an array named zipCodes Hint: to access array elementname[index] char[] grades = {'A', 'B', 'C', 'D', 'F'}; for (int i=0; i<zipCodes.length; i++) { System.out.println (zipCodes[i]); }

0 1 2 3 In-Class Exercises • Write a loop to change all the values of the integer array numbers to index + 1 for (int i=0; i<numbers.length; i++) { numbers[i] = i+1; }

0 1 2 3 ArraysSummary • Why use them? • maintain a list of related items • How use them? • first declare a variable to reference the array • when your program knows how many elements, it can then instantiate (create), initialize, and access the array • design code to index the array only within the array bounds

0 1 2 3 ReviewArrays • Declaration int[] counts; • Instantiation counts = new int[50]; • Initialization / Access for (int i=0; i<counts.length; i++) { counts[i] = 0; } • Initializer List • declaration, instantiation, and initialization double[] grades = {98.7, 72.4, 87.5}; int[] numbers = {num, num+1, num+2, num+3}; can use variables and expressions as initial values

Next Time in CSCI 51 • More on Arrays • Reading Assignment: Ch 10

Introduction to Programming - Reserved words, Arrays, and Visibility Modifiers