1. Chapter 3 Spring 2006 CS 101 Aaron Bloomfield
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
Java treats object variables differently. Although an object variable is the symbolic name for a memory location being used by the program, the memory location for an object variable does not store a value of that object type. Instead the value in the memory location tells the program where to find a value (object) of that type.
We say that an object variable references or points to an object of the object type. Thus, a String variable references a memory location that holds a value of type String. Similarly, a BufferedReader variable references a memory location holding a BufferedReader value.

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 = 20
- ...

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?