1. Chapter 3 Spring 2005 CS 101 Aaron Bloomfield
Using Objects
Chapter 3
Spring 2005
CS 101
Aaron Bloomfield

2. About the assignment statement
Assign the value 5 to the variable x
int x;
x = 5;
5 = x; NOT VALID!
This is not a mathematical equals
It’s a Java assignment
The variable you want to copy the value to MUST be on the left
The value you want to copy MUST be on the right
Assignment copies the value on the right to the variable on the left

3. Getting classy Purpose of this chapter
Gain experience creating and manipulating objects from the standard Java types
Why
Prepares you for defining your own classes and creating and manipulating the objects of those classes

4. Values versus objects Numbers
Have values but they do not have behaviors
Objects
Have attributes and behaviors
System.in
References an InputStream
Attribute: keyboard
Behaviors: reading
System.out
References an OutputStream
Attribute: monitor
Behaviors: printing
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.

5. Java and variables Consider: int x = 7; double d; char c = ‘x’;
The variable name is the actual spot in memory where the value is stored 7
int x -
double d
‘x’
char c

6. What is a reference A reference is a memory address
References are like pointers in C/C++
But they are not the exact same thing!
C++ has references also (in addition to pointers)
You may her me call them pointers instead of references
All objects in Java are declared as references

7. Note that there is no “new” here
References 1
Consider:
int j = 5;
String s = “Hello world”;
Java translates that last line into:
String s = new String (“Hello world”);
(Not really, but close enough for this lecture)
Note that there is no “new” here

8. References 2 What’s happening in memory int j = 5;
String s = “Hello world”;
Primitive types are never references; only objects
int j = 5;
String s = “Hello world”;
String s
0x0d4fe1a8
Takes up 32 bits
(4 bytes) of memory
Takes up 32 bits
(4 bytes) of memory 5
int j
At memory location 0x0d4fe1a8
Takes up 12 bytes of memory
Hello world

9. Quick survey I understand those slides on references… I think so
I need some more review
Not really
Reference what?

10. An old prediction of the future…

11. Other Java object types
String
Rectangle
Color
JFrame

12. Representation Statements int peasPerPod = 8;
String message = "Don't look behind the door!“
message
+ length () : int
+ charAt ( int i ) : char
+ subString ( int m, int n ) : String
+ indexOf ( String s, int m ) : int
+ ...
String
- text = "Don't look behind the door!"
- length = 27
- ...
peasPerPod

13. Representation String s = “I love CS 101”; int l = s.length();
char c = s.charAt (3);
String t = s.subString(1,2);
int t = s.indexOf (t, 0);
message
+ length () : int
+ charAt ( int i ) : char
+ subString ( int m, int n ) : String
+ indexOf ( String s, int m ) : int
+ ...
String
- text = "Don't look behind the door!"
- length = 27
- ...

14. Shorthand representation
Statements
int peasPerPod = 8;
String message = "Don't look behind the door!“

15. Examples Consider String a = "excellence“; String b = a;
What is the representation?

16. Uninitialized versus null
Consider
String dayOfWeek;
Scanner inStream;
What is the representation?

17. Uninitialized versus null
Consider
String fontName = null;
Scanner fileStream = null;
What is the representation?
fontName
fileStream
null
fontName
fileStream OR

18. The null reference Sometimes you want a reference to point to nothing
Use the null reference:
String s = null;
The null reference is equivalent to a memory address of zero (0x )
No user program can exist there

19. The null reference Consider: String s = “Hello world”;
System.out.println (s.length());
What happens?
Java prints out 11 s
+ length () : int
+ charAt ( int i ) : char
+ subString ( int m, int n ) : String
+ indexOf ( String s, int m ) : int
+ ...
String
- text = “Hello world"
- length = 11
- ...

20. The null reference Consider: String s = null;
System.out.println (s.length());
This is called accessing (or following) a null pointer/reference
What happens?
Java: java.lang.NullPointerException
C/C++: Segmentation fault (core dumped)
Windows: …

21. What happens in Windows…

22. So what is a null reference good for?
Let’s say you had a method that returned a String when passed some parameters
Normally it returns a valid String
But what if it can’t? How to deal with that?
Return a null reference

23. Quick survey I understand null references… Pretty much
Once I read the text, I’ll be good
Not really
What’s a reference again?

24. A bit of humor…

25. References and memory Most modern computers are 32-bit computers
This means that a reference takes up 32 bits
232 = 4 Gb
This means that a 32-bit machine cannot access more than 4 Gb of memory!
Well, without doing some “tricks”, at least
Most machines come with 1 Gb memory these days
Will come with 4 Gb in a year or so
64-bit machines will have a maximum of 16 exabytes of memory
Giga, Tera, Peta, Exa
That’s 16 billion Gb!

26. References 4 What happens to this? Consider:
String s1 = “first string”;
String s2 = “second string”;
s2 = s1;
System.out.println (s2);
String s1 = “first string”;
String s2 = “second string”;
s2 = s1;
System.out.println (s2);
What happens
to this?
String s1
length = 12
“first string”
“second string”
length = 13
String s2

27. Java’s garbage collection
If an object in memory does not have a reference pointing to it, Java will automagically delete the object
This is really cool!
In C/C++, you had to do this by yourself

28. Assignment Consider String word1 = "luminous";
String word2 = "graceful";
Word1 = word2;
Initial representation
Garbage
collection
time!
word1
"graceful"
word2
"luminous"

29. Using objects Consider Scanner stdin = new Scanner(System.in);
System.out.print("Enter your account name: ");
String response = stdin.next();
Suppose the user interaction is
Enter your account name: artiste
"artiste"
reponse
Scanner:
stdin

30. String representation
Consider
String alphabet = "abcdefghijklmnopqrstuvwxyz";
Standard shorthand representation
Truer representation
Another useful String member method is charAt(). This method expects a single index as its parameter and returns the value of the character at that position in its String. While such behavior seems relatively natural given the method name and its parameter, what is not natural is that in Java, as in several other programming languages, the first character of a String is located at index 0.

31. String representation
Consider
String alphabet = "abcdefghijklmnopqrstuvwxyz";
char c1 = alphabet.charAt(9);
char c2 = alphabet.charAt(15);
char c3 = alphabet.charAt(2);
What are the values of c1, c2, and c3? Why?
Another useful String member method is charAt(). This method expects a single index as its parameter and returns the value of the character at that position in its String. While such behavior seems relatively natural given the method name and its parameter, what is not natural is that in Java, as in several other programming languages, the first character of a String is located at index 0.

32. 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 + "."); }

33. 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.

34. 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

35. Quick survey I understand how one selects methods from an object.
Absolutely!
Kind of
Not really
What’s an object again?

36. Today’s dose of demotivators

37. End of lecture on 2 February 2005

38. 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 two 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
Like the difference between a recipe and a bunch of cookies

39. Quick survey I understand the difference between variables and types
Very well
With some review, I’ll be good
Not really
Not at all

40. Classes vs. Objects A class is a user-defined “thing”
Examples: String, Scanner, Rectangle, etc.
We’ll start defining our own classes next chapter
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
A object is analogous to a variable
It just is a reference instead
A class is a “template” used for creating objects

41. More on classes vs. objects

42. Quick survey I understand the difference between classes and objects
Very well
With some review, I’ll be good
Not really
Not at all

43. More String methods Consider int v1 = -12; double v2 = 3.14;
char v3 = 'a';
String s1 = String.valueOf(v1);
String s2 = String.valueOf(v2);
String s3 = String.valueOf(v3);
int v1 = -12;
double v2 = 3.14;
char v3 = 'a';
String s1 = String.valueOf(v1);
String s2 = String.valueOf(v2);
String s3 = String.valueOf(v3); v1
-12 v2
3.14 v3
‘a’
"-12" s1
"3.14" s2
"a" s3

44. Final variables Consider
final String POEM_TITLE = “Appearance of Brown";
final String WARNING = “Weather ball is black";
What is the representation?
In Chapter we used modifier final to define primitive type constants. We can also use this modifier to define object constants. However, when an object constant is defined—be it String or any other object type—we are specifying only that the reference cannot be changed. That is, the constant must always refer to the same memory location.
Although an object constant must always refer to the same memory location, it does not mean necessarily that the value stored at that location cannot be modified through its member methods.

45. Final variables The reference cannot be
modified once it is established
In general, these attributes can be
modified through member methods
Value
object
type
constant
In Chapter we used modifier final to define primitive type constants. We can also use this modifier to define object constants. However, when an object constant is defined—be it String or any other object type—we are specifying only that the reference cannot be changed. That is, the constant must always refer to the same memory location.
Although an object constant must always refer to the same memory location, it does not mean necessarily that the value stored at that location cannot be modified through its member methods.

46. Rectangle int x = 3; y = 4; The upper-left-hand width = 5;
height = 2;
Rectangle r =
new
Rectangle(x, y, width, height);
The upper-left-hand
corner of the new Rectangle
The dimensions of
the new Rectangle

47. Final variables Consider final String LANGUAGE = "Java";
The reference cannot be
modified once it is
established
"Java"
LANGUAGE

48. Rectangle final Rectangle BLOCK = new Rectangle(6, 9, 4, 2);
BLOCK.setLocation(1, 4);
BLOCK.resize(8, 3);
Consider
final Rectangle BLOCK = new Rectangle(6, 9, 4, 2);
BLOCK.setLocation(1, 4);
BLOCK.resize(8, 3);
In Chapter we used modifier final to define primitive type constants. We can also use this modifier to define object constants. However, when an object constant is defined—be it String or any other object type—we are specifying only that the reference cannot be changed. That is, the constant must always refer to the same memory location.
Although an object constant must always refer to the same memory location, it does not mean necessarily that the value stored at that location cannot be modified through its member methods.

49. String method usage String s = "Halloween";x 10 y 4
String s = "Halloween";
String t = "Groundhog Day";
String u = "May Day";
String v = s.substring(0,6);
int x = t.indexOf ("Day", 0);
int y = u.indexOf ("Day");
s = t;
u = null;
Consider:
String s = "Halloween";
String t = "Groundhog Day";
String u = "May Day";
String v = s.substring(0,6);
int x = t.indexOf ("Day", 0);
int y = u.indexOf ("Day");
s = t;
u = null;
“Hallow" v
+ length () : int
+ subString ( int m, int n ) : String
+ indexOf ( String s, int m ) : int
+ indexOf ( String s ) : int
+ ...
String
- ...
- text = “Halloween"
- length = 9
- text = “May Day"
- length = 7
- text = “Groundhog Day"
- length = 13 s
“Halloween" t
“Groundhog Day" u
“May Day"

50. String method usage Java error: cannot assign a value to finalx 10 y 4
Consider:
String s = "Halloween";
String t = "Groundhog Day";
final String u = "May Day";
String v = s.substring(0,6);
int x = t.indexOf ("Day", 0);
int y = u.indexOf ("Day");
s = t;
u = null;
s = t;
u = null;
“Hallow" v
Java error:
cannot assign a
value to final
variable u s
“Halloween" t
“Groundhog Day" u
“May Day"

51. Rectangle method usage
+ setWidth ( int w )
+ setHeight ( int wh )
+ setX ( int x )
+ setY ( int y )
+ ...
Rectangle
Consider:
Rectangle r = new Rectangle();
final Rectangle s = new
Rectangle (1, 2, 3, 4);
r.setWidth(5);
r.setHeight(6);
s.setWidth (7);
r = new Rectangle (8,9,10,11);
s = new Rectangle (12,13,14,15);
Rectangle r = new Rectangle();
final Rectangle s = new
Rectangle (1, 2, 3, 4);
r.setWidth(5);
r.setHeight(6);
s.setWidth (7);
r = new Rectangle (8,9,10,11);
s = new Rectangle (12,13,14,15);
- width = 1
- height = 2
- x = 3
- y = 4
- width = 7
- height = 2
- x = 3
- y = 4 s r
+ setWidth ( int w )
+ setHeight ( int wh )
+ setX ( int x )
+ setY ( int y )
+ ...
Rectangle
- width = 5
- height = 6
- x = 0
- y = 0
- width = 5
- height = 0
- x = 0
- y = 0
- width = 0
- height = 0
- x = 0
- y = 0
+ setWidth ( int w )
+ setHeight ( int wh )
+ setX ( int x )
+ setY ( int y )
+ ...
Rectangle
- width = 8
- height = 9
- x = 10
- y = 11

52. Scanner review To initialize a Scanner object:
Scanner stdin = new Scanner (System.in);
Scanner stdin = Scanner.create (System.in);
This one will not work!
To read an int from the keyboard:
stdin.nextInt();
To read a double from the keyboard:
stdin.nextDouble();
To read a String from the keyboard:
stdin.next();

53. Scanner usage examples
Scanner stdin = new Scanner (System.in);
int x = stdin.nextInt();
double d = stdin.nextDouble();
String s = stdin.next();
Consider:
Scanner stdin = new Scanner (System.in);
int x = stdin.nextInt();
double d = stdin.nextDouble();
String s = stdin.next();
Scanner:
stdin x 5 d
3.5
“hello world” s

54. Quick survey I felt I understood the material in this slide set…
Very well
With some review, I’ll be good
Not really
Not at all

55. Quick survey The pace of the lecture for this slide set was… Fast
About right
A little slow
Too slow

56. Quick survey
How interesting was the material in this slide set? Be honest!
Wow! That was SOOOOOOO cool!
Somewhat interesting
Rather boring
Zzzzzzzzzzz

57. A bit of humor…