Download Objects and References

Catie Welsh
March 2, 2011
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Program 3 due tonight by 11:59pm
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Lab 5 due Friday by 1pm

Sample Midterm is posted on course website
◦ Solutions will be posted after class on Friday
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Midterm is Monday, March 14th
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Objects and references
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More on Classes
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



Classes
Objects
Instance variables
Methods
◦ Return types
◦ Parameters and arguments

Information hiding and encapsulation
◦ public/private
◦ accessors/mutators
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
Behave differently from variables of a
primitive type
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
When declaring a variable, a certain amount
of memory is assigned based on the declared
primitive type
int age;
double length;
char letter;
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What goes inmemor
this memory?
y
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A data value is stored in the location assigned
to a variable of a primitive type
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What goes in these variables?
Student jack;
String inputString;
memor
y
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Contain the memory address of the object
named by the variable
◦ NOT the object itself

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What is an address?
Object is stored in some other location in
memory
The address to this other location is called a
reference to the object
Class types are also called reference types
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Assume we have a class named Book
Book jacksBook = new Book(“Java”);
Book apusBook = new Book(“Java”);
vs.
Book jacksBook = new Book(“Java”);
Book apusBook = jacksBook;
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Memory
jacksBook 207
?
8
apusBook ?
?
105
207
6
8
?
1056
?
Java
?
253
?
2078
?
Java
?
137
509
Book jacksBook;
Book apusBook;
jacksBook = new Book(“Java”);
apusBook = new Book(“Java”);
jacksBook.setPage(137);
apusBook.setPage(253);
apusBook = jacksBook;
apusBook.setPage(509);
jacksBook is now on p. 509!
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Variables of a class type contain memory
addresses
◦ NOT objects themselves
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String is a class type
What happens when you have
String s1 = new String(“Hello”);
String s2 = new String(“Hello”);
boolean strEqual = (s1 == s2);
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strEqual is false! Why?
s1 and s2 store different addresses!
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
What happens when you have
String s1 = new String(“Hello”);
String s2 = new String(“Hello”);
boolean strEqual = (s1.equals(s2));
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strEqual is true! Why?
String’s .equals() method checks if all the
characters in the two Strings are the same
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public class Book
{
private String name;
private int page;
public boolean equals(Book book)
{
return (this.name.equals(book.name) &&
this.page == book.page);
}
}
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Every class has a default .equals() method if
it is not explicitly written
◦ Does not necessarily do what you want
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You decide what it means for two objects of
a specific class type to be considered equal
◦ Perhaps books are equal if the names and page
numbers are equal
◦ Perhaps only if the names are equal
◦ Put this logic inside .equals() method
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public void increaseNum(int num)
{
num++;
}
public void doStuff()
{
int x = 5;
increaseNum(x);
System.out.println(x);
}
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Prints 5. Why?
num is local to increaseNum method; does not
change x
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public void changeBook(Book book)
{
book = new Book(“Biology”);
}
public void doStuff()
{
Book jacksBook = new Book(“Java”);
changeBook(jacksBook);
System.out.println(jacksBook.getName());
}
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Prints Java. Why?
book is local to changeBook, does not change jacksBook
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public void changeBook(Book book)
{
book.setName(“Biology”);
}
public void doStuff()
{
Book jacksBook = new Book(“Java”);
changeBook(jacksBook);
System.out.println(jacksBook.getName());
}
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Prints Biology. Why?
book contains the same address as jacksBook!
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
Write a bank account class for Harry Potter
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It needs to have the ability to track
◦
◦
◦
◦
#
#
#
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of galleons
of sickles
of knuts
transaction types
 Deposit
 Withdrawal
 Inquiry
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Would you like to make a transaction? y/n
y
Would you like to make a deposit (d), withdrawal (w), or inquire (i)?
d
How many galleons, sickles, and knuts would you like to deposit?
Galleons: 5
Sickles: 6
Knuts: 4
Would you like to make a transaction? y/n
y
Would you like to make a deposit (d), withdrawal (w), or inquire (i)?
w
How many galleons, sickles, and knuts would you like to withdrawal?
Galleons: 4
Sickles: 0
Knuts: 0
Would you like to make a transaction? y/n
y
Would you like to make a deposit (d), withdrawal (w), or inquire (i)?
i
You now have:
1 galleons
6 sickles
4 knuts
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
What are instance variables?
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Why private?
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What are the instance variables for our bank
account program?
◦ What do we need to keep track of?
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public class Account
{
private int galleons = 0;
private int sickles = 0;
private int knuts = 0;
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getGalleons: int
getSickles: int
getKnuts: int
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setGalleons(int)
setSickles(int)
setKnuts(int)
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public static void main(String[] args)
{
Account potter = new Account();
.
.
.
potter.deposit(5, 7, 9);
public void deposit(int g, int s, int k)
{
galleons+=g;
sickles+=s;
knuts+=k;
}
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public static void main(String[] args)
{
Account potter = new Account();
.
.
.
potter.withdrawal(5, 7, 9);
public void withdrawal(int g, int s, int k)
{
if((g > galleons) || (s > sickles) || (k >
knuts))
{
System.out.println("You do not
have enough money");
}
else
{
galleons -= g;
sickles -= s;
knuts -= k;
}
}
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public static void main(String[] args)
{
Account potter = new Account();
.
.
.
potter.inquire();
public void inquire()
{
System.out.println("You now
have: " + galleons + " galleons
" + sickles + " sickles " + knuts
+ " knuts");
}
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Midterm Review
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Bring questions about the Sample Midterm
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