Using Objects
Chapter 3
Spring 2006
CS 101
Aaron Bloomfield
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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
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Values versus objects
 Numbers
 Have values but they do not have behaviors
 Objects
 Have attributes and behaviors
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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
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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
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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)
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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()
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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
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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.
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Visualizing objects

Class (type) name
- width = 10
- height = 20
- ...

Attributes (properties)
+ grow (int, int) : void
+ isEmpty ( ) : void
+ setLocation ( int, int ) : void
+ resize ( int, int ) : void
+ ...

Methods (behaviors)
Rectangle
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How well do we understand using
objects?
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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.
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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
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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.
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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
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These images are not animated…
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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 + ".");
}
}
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More String methods
 trim()
 Returns the String without leading and trailing whitespace
 Whitespace is a space, tab, or return
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Reading Javadocs
 What is a Javadoc?
 Documentation of Java classes
 Where are Sun’s Javadocs?
 http://java.sun.com/j2se/1.5.0/docs/api/index.html
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