1 Using Objects Chapter 3 Spring 2006 CS 101 Aaron Bloomfield

2 Variables vs. Types  The type is the recipe or template for how to create a variable Examples: int, double, char, boolean, etc.  There are only 8 primitive types There are only a few things you can do with a type:  Declare a variable int x;  Use it as a cast x = (int) 3.5; There is only one of each type  The variable is the actual instance of a type in memory It’s a spot in memory where you store a value You choose the name: width, x, thatThemThereValue, etc. You can have as may variables as you want – but only one type!  Like the difference between a recipe and a bunch of cookies

3 Values versus objects  Numbers Have values but they do not have behaviors  Objects Have attributes and behaviors

4 Classes vs. Objects  A class is a user-defined “thing” Examples: String, Scanner, Rectangle, etc.  We’ll start defining our own classes later this semester Classes are more complex than the primitive types A class is analogous to a type  It’s just more complex and user-defined There can be only one class of each name  An object is an instance of a class There is only one String class, but you can have 100 String objects An object is analogous to a variable  It just is a reference instead  A class is a “template” used for creating objects

5 Using objects  First, we create an object: Scanner stdin = new Scanner (System.in);  Most object creation lines look like this  Then we use the object stdin.nextInt(); stdin.nextDouble();  Note that we could have called the object foo, bar, or anything stdin is just what we chose to call it

6 Using Rectangle objects  Let’s create some Rectangle objects  Rectangle creation: Rectangle r = new Rectangle (10, 20);  Objects have attributes (or properties): System.out.println (r.length); System.out.println (r.width);  Objects have behaviors (or methods): r.grow (10, 20) r.isEmpty() r.setLocation (5,4)

7 Using String objects  Let’s create some String objects  String creation: String s = new String (“Hello world”);  Objects have attributes (or properties): But we can’t access them…  Objects have behaviors (or methods): s.substring(0,6) s.indexOf (“world”) s.toLowerCase()

8 The lowdown on objects  Objects are “things” that have properties (attributes) and behaviors (methods)  We first create one or more objects  We then manipulate their properties and call their methods

9 So why bother with objects?  Let’s say you want to do a lot of String manipulation  Once you create a String object, all the manipulation methods are contained therein Sun already wrote the methods for us  So we can use String objects instead of writing our own code to get the substring, indexOf, etc.

10 Visualizing objects  Class (type) name  Attributes (properties)  Methods (behaviors) + grow (int, int) : void + isEmpty ( ) : void + setLocation ( int, int ) : void + resize ( int, int ) : void +... Rectangle - width = 10 - height =

11 How well do we understand using objects?

12 Review  Variables of primitive types int, double, char, boolean, etc. Can assign a value to it Can read a value from it Can’t do much else!  Objects String, Rectangle, etc. Have many parts  Rectangle has width, length, etc. Like a complex type Have methods  String has length(), substring(), etc.

13 String methods  length(): returns the String’s length (duh!) String s = “hello world”; String t = “goodbye”; System.out.println (s.length()); System.out.println (t.length());  Prints 11 and 7  Note that calling s.length() is different than calling t.length()! Both return the length But of different Strings

14 More String methods  Consider String weddingDate = "August 21, 1976"; String month = weddingDate.substring(0, 6); System.out.println("Month is " + month + ".");  What is the output? Month is August.

15 More String methods  Consider String fruit = "banana"; String searchString = "an"; int n1 = fruit.indexOf(searchString, 0); int n2 = fruit.indexOf(searchString, n1 + 1); int n3 = fruit.indexOf(searchString, n2 + 1); System.out.println("First search: " + n1); System.out.println("Second search: " + n2); System.out.println("Third search: " + n3);  What is the output? First search: 1 Second search: 3 Third search: -1

16 These images are not animated…

17 Program WordLength.java public class WordLength { public static void main(String[] args) { Scanner stdin = new Scanner(System.in); System.out.print("Enter a word: "); String word = stdin.next(); int wordLength = word.length(); System.out.println("Word " + word + " has length " + wordLength + "."); }

18 More String methods  trim() Returns the String without leading and trailing whitespace Whitespace is a space, tab, or return

19 Reading Javadocs  What is a Javadoc? Documentation of Java classes  Where are Sun’s Javadocs?