COMP 110Introduction to Programming
Mr. Joshua StoughOctober 24, 2007
Today
• Review• Reserved word this
• Arrays
Announcements
• HW6
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()
}
}
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
public Rectangle (int l, int w){ length = l; width = w;}
public class Rectangle{
private int length;private int width;
Rectangle r2 = new Rectangle (5, 10);
public Rectangle (){ length = 0; width = 0;}
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
%%%%%% %%%%%%
public void 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: 2width: 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
Illustrate Classpublic 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++; }}
Illustrate obj1 = new Illustrate(3);
x 3
x 5
0y
count 0
obj1
obj2
Illustrate.incrementCount();
1
1
Illustrate obj2 = new Illustrate(5);
Illustrate.y++;
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
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Declaring Arrays
type[] name;
The array (element) data type Empty square brackets
The array (variable) name
Creates a reference variable called name that can point to an array of type elements.
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Declaring ArraysExamples
// array of characterschar[] characterSet;
// array of counters (integers)int[] counter;
// array of grades (doubles)double[] grade;
counter
characterSet
grade
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Instantiating Arrays
You must instantiate (create) arrays– the size of an array is typically not
known before run time
name = new type[size];
The array (element) data type
The new operatorThe array(variable) name
The assignment operator
The number of elements
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Instantiating ArraysExamples
// instantiate an array of counterscounter = new int[5];
// instantiate the array of gradesnumStudents = 10;grade = new double[numStudents];
counter
01
23
4
0 <= index < size
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Declaration and Instantiation
type[] name = new type[size];
Declaration Instantiation
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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();
}
Example
int[] num = new int[5];
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Array AccessExamples
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;
double score[] = new double[3];
score[0] = 98.3;score[1] = 57.8;score[2] = 93.4;
often use loops for access
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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 scoressumOfScores = 0;for (int i=0; i<scores.length; i++) { sumOfScores += scores[i];}
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int counter[] = {0, 0, 0, 0, 0};char[] characterSet = {‘a’,’b’,’c’}; // etc.
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
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• 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 arrayNOT int[] scores;
scores = {87, 98, 45};
Initializer Lists0 1 2 3
Next Time in COMP 110
• Arrays
• Reading Assignment: Ch 9