1. Java Classes and Objects
CMSC 202
Java Classes and Objects

2. Copyright © 2008 Pearson Addison-Wesley.
Class Definitions
You already know how to use classes and the objects created from them, and how to invoke their methods
For example, you have already been using the predefined String and Scanner classes
Now you will learn how to define your own classes and their methods, and how to create your own objects from them
Aug 6, 2007
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3. Copyright © 2008 Pearson Addison-Wesley.
A Class Is a Type
A class is a special kind of programmer-defined type, and variables can be declared of a class type (recall structs from C)
A value of a class type is called an object or an instance of the class
If A is a class, then the phrases
“X is of type A“
“X is an object of the class A"
“X is an instance of the class A"
mean the same thing
A class determines the types of data that an object can contain, as well as the actions it can perform
Aug 6, 2007
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4. The Contents of a Class Definition
A class definition specifies the data items and methods that all of its objects will have
These data items and methods are sometimes called members of the object
Data items are called fields or instance variables
Instance variable declarations and method definitions can be placed in any order within the class definition
Aug 6, 2007
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5. Primitive Type Values vs. Class Type Values
A primitive type value (int, double, char) is a single piece of data
A class type value or object can have multiple pieces of data, as well as actions called methods
All objects of a class have the same methods
All objects of a class have the same pieces of data (i.e., name, type, and number)
For a given object, each piece of data can hold a different value
Aug 6, 2007
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6. Anatomy of a Java Class public class
Visibility modifier (More on this later)
Keyword class
Name of the class
public
class
Date1 {
Class body: instance variables, methods }
NO semi-colon
Aug 6, 2007

7. Anatomy of a method
Class methods are very much like functions. Methods include a visibility modifier, return type, method name, and optional parameters
Name of the method
Visibility modifier (More on this later)
return type
Optional parameters
public
double
toCelcius
(double fTemp) {
Method code: local variables and statements }

8. A Date Class The code below defines a class named Date1.
This class definition goes in a file named Date1.java
public class Date1 {
public String month; public int day;
public int year;
public void print( )
System.out.println(month + “ “ + day + “, “ + year); }
These are the (public)“data members” or “instance variables” of the class
This is a method definition and its implementation
Aug 6, 2007

9. Notes on Date1 Class
print is a method of the Date1 class. It’s definition and implementation are part of the Date1 class.
Like functions in C, class methods may be void, return a value, and (optionally) have parameters. Method parameters may be primitive types passed by value or may be objects (which need further discussion later).
Aug 6, 2007

10. Create a Date1 object named myDate
Using Date1
We can create instances of Date1, modify, and print the date. This class definition (program) goes in a file named Date1Demo.java
public class Date1Demo {
public static void main( String[ ] args ) { Date1 myDate; myDate = new Date1( ); myDate.month = “July”; myDate.day = 4; myDate.year = 2007;
myDate.print( ); }
Create a Date1 object named myDate
Give values to the data members
Invoke the print method
Aug 6, 2007

11. Notes on Date1Demo
The statement Date1 myDate; defines a variable of type Date1. But there is no Date1 object yet!
The statement myDate = new Date1( ); creates a “new” Date1 object and names it with the variable “myDate”. Now “myDate” refers to a Date1 object
For convenience, these statements can be combined
Date1 myDate = new Date1( );
In general, a statement of the form ClassVariable = new ClassName( ); creates a new object of the specified class and associates it with the class variable.
(continued)
Aug 6, 2007

12. Notes on Date1Demo
Public instance variables of an object are referenced using the “dot” operator (like members of a struct in C)
myDate.month = “July”; myDate.day = 4; myDate.year = 2007;
Instance variable can be used like any other variable of the same type.
(continued)
Aug 6, 2007

13. Notes on Date1Demo
The print method of myDate is invoked using the “dot” operator (again similar to accessing a member in a C struct).
The statement myDate.print( ); invokes the print method of myDate which refers to an object of type Date1.
In OO terminology we say that we are “sending the print message” to the object referred to by myDate.
The object myDate is referred to as the “calling object” or “host object”. It is the object in which the print method is invoked.
Aug 6, 2007

14. Methods with primitive parameters
The print method of Date1 has no parameters, but like functions in C, methods may have primitive parameters as shown by the new methods on the following slide.
Aug 6, 2007

15. New Date1 Methods // change the month (using an int), day, and year.
public void setDate( int newMonth, int newDay, int newYear ) {
month = monthString( newMonth );
day = newDay;
year = newYear; }
// change month number to string
public String monthString( int monthNumber ) {
switch ( monthNumber ) {
case 1: return "January";
case 2: return "February";
case 3: return "March";
case 4: return "April";
case 5: return "May";
case 6: return "June";
case 7: return "July";
case 8: return "August";
case 9: return "September";
case 10: return "October";
case 11: return "November";
case 12: return "December";
default: return “????”;

16. Temptation
In the preceding setDate method it’s tempting to define the method using the common terms “month”, “day” and “year” as the parameters.
public void setDate( int month, int day, int year) {
month = monthString( month ); // which month is which?
day = day; // which day is which?
year = year; // which year is which? }
The problem is that the compiler gets confused between the instance variables and the parameters.
Aug 6, 2007

17. m_month, m_day, and m_year
Defeating Temptation
There are two ways to avoid this type of name duplication.
The easiest is to adopt a naming convention for your instance variables that makes them unique. Perhaps choosing a prefix like m_ or c_ or something else that makes sense to you. So for our date class, the instance variables become
m_month, m_day, and m_year
(continued)
Aug 6, 2007

18. Defeating Temptation
The 2nd way is more subtle, but leads us to a necessary discussion about methods and objects.
2. When any class method is called, the instance variables used within the method are assumed to belong to the calling object. It’s handy (and sometimes necessary) to have a name for the calling object. In Java, we use this as the name of the calling object.
(continued)
Aug 6, 2007

19. Defeating Temptation
So, if we want to name our parameters the same as our instance variables, we could write the setDate method as
public void setDate( int month, int day, int year) {
this.month = monthString( month );
this.day = day;
this.year = year; }
Note that many examples in the text use this technique for class methods.
As you can see, a naming conflict like this can lead to confusion. If we avoid this confusion then we need not use “this.” as a prefix for our instance variables since the compiler can differentiate among instance variables, local variables, and parameters.
Aug 6, 2007

20. this again
As a final example, recall the print method from Date1
public void print( ) {
System.out.println(month + “ “ + day + “ “ + year); }
Here it’s clear that month, day, and year refer to our instance variables because. To be more explicit we could have written
System.out.println(this.month
+ “ “ + this.day + “ “ + this.year);
With no chance for confusion, using the prefix this. is unnecessary and usually omitted.
Aug 6, 2007

21. Date1 Code Example Date1 newYears = new Date( );
// Determine the output from this code snippet
Date1 newYears = new Date( );
newYears.month = “January”;
newYears.day = 1;
newYears.year = 2008;
Date1 birthday = new Date( );
Birthday.month = “July”;
birthday.day = 4;
birthday.year = 1776;
newYears.print( ); // line 1
birthday.print( ); // line 2
System.out.println(birthDay.monthString(6)); // line 3
birthday.setDate( 2, 2, 2002); // line 4
birthday.print( ); // line 5
newYears.day = 42; // line 6
newYears.print( ); // line 7
The output from line 1 is “January 1, 2008”
The output from line 2 is “July 4, 1776”
The output from line 3 is “June”
No output from line 4 – it changes birthday
The output from line 5 is “February 2, 2002”
No output from line 6 – this changes newYears’s day = 42;
The output from line 7 is “January 42, 2008”
Sept 7, 2007

22. January 42, 2008
It appears that classes and structs both allow the user to change the data anytime he chooses and can possibly make the data invalid
That’s true so far because we have defined our instance variables with public access. This is rarely the case in real applications.
Our focus today was on classes vs objects and the syntax used for accessing methods.
Next time we’ll see a more robust class that will prevent the user from making our data invalid.
Sep 7, 2007