1. Barb Ericson Georgia Institute of Technology June 2005
Creating Classes
Barb Ericson
Georgia Institute of Technology
June 2005
Georgia Institute of Technology

2. Georgia Institute of Technology
Learning Goals
Computing concepts
Identifying objects and classes
Declaring a class
Declaring a constructor
Overloading constructors
Declaring fields
Creating accessors and modifiers
Declaring a main method
Javadoc comments
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3. Identifying Objects and Classes
Object-oriented programs
Consist of interacting objects
Which are defined by and created by classes
To identify the objects in a task
What are the things that are doing the work or being acted upon?
How do you classify them?
What data (fields) do they need to know to do the task?
What procedures (methods) do they need?
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4. Identifying the Objects and Classes
Say that we want to write a program to track the grades of students.
One way to start is to underline the nouns
grades, student
Grades is a list of numbers that belongs to a student
Simple data like numbers and strings are fields in a class
So this would be a field in a student class
A student has grades and a name
So it needs to be a class
With fields for grades and name
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Class Definition
Each class is defined in a file
With the same name as the class.java
Class names
Are singular (Student not Students)
Start with an uppercase letter
The rest of the word is lowercase
Uppercase the first letter of each additional word
The syntax for a class definition is:
visibility class Name {}
Inside the class definition goes:
Fields, constructors, and methods
The order of the items in a class definition doesn’t matter to Java. Usually you group the fields together and the constructors together, and the methods together. You can actually put the methods first, then the constructors, and then the fields.
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Class Declaration
To declare a Student class
Click on the New button in DrJava
Type in:
public class Student { }
Save it in Student.java
Click on File then Save
Click the Compile All button to compile it
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Student Fields
A student should have a name and some grades
What type should we use for each of these?
The Field name can be a String object
Maybe we will have at most 20 grades and they can have a decimal point in them
Since they can have a decimal point let’s use double as the type
We could name them grade1, grade2, grade3 …
Or, we can use an array
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Declaring Fields
Syntax
visiblity type name;
visibility type name = expression;
Usually use private for the visibility
So that other classes can’t access it directly
The type is any of the primitive types, a class name , or an interface name
Arrays are declared with [] after the type or after the name
type[] name; or type name[];
Names start with a lowercase letter
The first letter of each additional word is uppercased
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Default Field Values
If you don’t specify an initial value for a field
It will get one anyway when it is created
Numbers = 0
Objects = null (not referring to any object yet)
boolean = false
public class Student {
/////////////// fields ///////////////
private String name;
private double[ ] gradeArray; }
The name field will start off with a value of null since strings are objects. The gradeArray field will also start off with a value of null since arrays are objects.
Initial value will be null
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10. Testing the Student Class
Add the fields to the Student class definition and compile it
Try the following in the interactions pane
Student studentObj = new Student();
System.out.println(studentObj);
What happens?
you may not get the exact same thing
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Inherited Methods
When you executed
System.out.println(studentObj);
The class Student was checked for a toString method
Since it didn’t have one the parent class was checked for a toString method
The one in Object was executed
Which prints the hash code for the object
The Student class inherited the toString method from the Object class
The compiler will call the toString method on any object you print with System.out.println().
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How Inheritance Works
When a method is invoked on an object
We first check for that method in the object that defines the object’s class
If it isn’t there we look in the parent of that class
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13. All Classes Inherit from Object
If you don’t specify the parent class when you declare a class
The class with inherit from java.lang.Object
You can specify the parent class
Add extends Parent to the class declaration
A declaration of
public class Student
Is the same as
public class Student extends Object
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14. Georgia Institute of Technology
Getting the Class
An object keeps a reference to the class that created it
You can get this class with
getClass();
Each class keeps a reference to the parent class
getSuperclass();
Try:
Student student1 = new Student();
Class studentClass = student1.getClass();
Sytem.out.println(studentClass);
Class parentClass = studentClass.getSuperclass();
System.out.println(parentClass);
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15. Overriding an Inherited Method
If a class defines a method with the same name and parameter list as an inherited method
This method will be called instead of the parent method
To override Object’s toString add this one to Student:
public String toString() {
return “Student object named “ + this.name; }
Test this new method:
Student studentObj = new Student();
System.out.println(studentObj);
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Constructors
Are used to initialize the fields of an object
To other than the default values or assigned value
You can have more than one constructor
As long as the parameter lists are different
This is called overloading constructors
Syntax
visibility ClassName(paramList) {}
Example
public Student(String theName)
{ this.name = theName;}
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17. Georgia Institute of Technology
Add a Constructor
Add the following after the field declarations:
public Student(String theName) {
this.name = theName; }
Compile and test
Student student1 = new Student();
System.out.println(student1);
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Why did you get an Error?
We hadn’t declared any constructors before we added this one
But a constructor is called each time a new object is created
We didn’t provide one so the compiler added a no-argument constructor
One that takes no parameters and leaves the fields with their default or assigned values
But once you add a constructor
The compiler will not add any for you
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19. Adding a No-Argument Constructor
Add the following constructor to the Student class
public Student() {}
Now test it again with:
Student student1 = new Student();
System.out.println(student1);
Also try:
Student student2 = new Student(“Sue Clark”);
System.out.println(student2);
When we create student1 the no-argument constructor will be called since we haven’t passed any parameters. When we create student2 the constructor will be called that takes a string.
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Tracing Execution
One way to trace what is happening in your program is
To add System.out.println() statements
Add these to print out the value of the current object’s name before and after the name is initialized in the constructor that takes a name
System.out.println(this.name);
this.name = theName;
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21. Georgia Institute of Technology
Debuggers
You can use a debugger to find the cause of bugs (errors in your program)
A moth caused one bug
And to trace execution to see what is happening
Which constructor is executed or what method is executed
What values are in the fields
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DrJava’s Debugger
Click on Debugger in the menu
Then check the Debug Mode checkbox
Stack and
Threads
Area
Watches
and
Breakpoints
Area
Check values here
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23. Georgia Institute of Technology
Setting a Breakpoint
When you use a debugger you often want to set places to stop execution
Each place to stop at is a breakpoint
Once execution has stopped there
You can check the value of parameters and fields
To set a breakpoint
Right click on a line of code
Pick “Toggle Breakpoint”
It will be highlighted in red
All debuggers let you set breakpoints.
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Showing a Breakpoint
Lines with breakpoints are highlighted in red in DrJava
Set a breakpoint at the line that sets the name in the constructor that takes a name as a parameter
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Testing a Breakpoint
Type the following in the interactions pane
Student student1 = new Student(“Sue Clark”);
Execution should stop at the breakpoint
And the color change to blue
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Checking Values
Execution stops before the breakpoint line is executed
So the name hasn’t been set to the passed value yet
Check this by printing out the value of it in the interactions pane
this.name
this.gradeArray
Then click on the Step Over button
To let the current line of code be executed
And check the values again
Remember that in DrJava if you type something without a semicolon at the end it will print the value of the expression.
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Debugging Options
Step Over
Execute the current line of code and then stop again before you execute the next line of code
Step Into
If the line of code that we are stopped at has a method call in it stop at the first line in the called method
Resume
Continue execution at the current point
Until the next breakpoint
Or the program ends
Step Out
Execute the rest of the current method and stop at the first line after the call to this method
You can quit debugging by clicking on the X
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Challenge
Create another constructor in the Student class
That takes both the name and an array of grades
To create an array of doubles for the grade array use:
double[] gradeArray = {90.3, 85.2, 89.7, 95.3};
Use the debugger to check what happens during execution of this constructor
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29. Calculating the Grade Average
Now that a student has an array of grades one of the things we probably would like is to
Show the average when you print information about the student
To calculate an average
Sum the grades
Divide by the number of grades
The length of the array
We need to be careful of
A null gradeArray
A 0 length gradeArray
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30. Georgia Institute of Technology
Challenge
Create a method (getAverage) that will calculate and return the average of the grades in the grade array
It will return 0 if the grade array is null
It will return 0 if the grade array length is 0
Otherwise it will return the sum of the grades / the number of grades
Add to the toString method a call to this method
Use the debugger to check that this is working correctly
Stop in the getAverage method
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31. Accessing Fields from Other Classes
Fields are usually declared to be private
So that code in other classes can’t directly access and change the data
Try this in the interactions pane
Student student1 = new Student(“Sue Clark”);
System.out.println(student1.name);
You will get an exception
Short for exceptional event – error
Outside classes can not use object.field to access the field value
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32. Accessors and Modifiers
Are public methods that return data
In such a way as to protect the data for this object
Syntax
public fieldType getFieldName()
Example
public String getName() { return this.name;}
Modifiers
Are public methods that modify data
public returnType setFieldName(type name);
public void setName(String theName)
{this.name = theName; }
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33. Creating Student Accessors
Add a method to get the name
public String getName() { return this.name;}
What about a method to get the array of grades?
If someone gets the array they can directly change the grades in the array
It is safer to return the grade at an index
Then other classes can’t directly change the grade
public double getGrade(int index)
{ return this.gradeArray[index];}
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34. Creating Student Modifiers
We need public methods
That let other classes ask for the grade to change or the name to change
Our class is responsible for making sure this only happens in such a way
as to keep the data valid and not cause errors
Setting a grade
The grade must be >= 0
The gradeArray must not be null
The index must be < the length of the array
Setting a name
We can decide if this can be changed or not once it isn’t null
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Name Modifier
Setting the name only if currently null
public boolean setName(String theName) {
if (this.name == null)
this.name = theName;
return true; }
else return false;
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Grade Modifier
public boolean setGrade(int index, double newGrade) {
if (newGrade < 0 || this.gradeArray == null ||
this.gradeArray.length() <= index ||
index < 0)
return false;
else
this.gradeArray[index] = newGrade;
return true; }
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37. Georgia Institute of Technology
Challenge
Add a picture field to student that will hold a Picture object of a student
Add an accessor to get the value of this field
Add a modifier to set the value of this field
Add a method (show) that will show the picture
If it isn’t null
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38. Georgia Institute of Technology
Adding a Main Method
We have been typing stuff in the interactions pane in DrJava
To try out Java code and to try methods
Most development environments make you write a main method to start execution
DrJava allows this too
Each class can have a main method declared as follows:
public static void main(String[] args)
It is public so that it can be called by other classes
It is static because no object of the class exists when it is executed
It doesn’t return anything so the return type is void
You can pass several arguments to the main method and these are put in an array of strings
One of the reasons why we use DrJava is to let students use the interactions pane to try things out without having to use a main. Many books start with a main method but don’t ever explain it.
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Main Method
Add a main method to Student
Do it in what you have been doing in the interactions pane
public static void main(String[] args) {
Student student1 = new Student();
System.out.println(student1);
Student student2 = new Student(“Sue Clark”);
System.out.println(student2); }
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40. Execute the Main Method
In DrJava you can run the main method in the class that is displayed in the definitions pane
By clicking on Tools then Run Document’s Main Method (or press key F2)
It will do
java Student
In the interactions pane
Which executes the main in the Student class
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Comments
You should add comments to your code
To make it easier to read and change
Comments are ignored by the complier
Not added to the byte codes
Java has 3 kinds of comments
// comment ends at the end of this line
/* comment ends with next */
/** Javadoc comment that ends with */
can be used by the javadoc utility to create HTML documentation
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Javadoc Comments
Add a comment before the class definition
That explains the purpose of this class
And says who wrote it
@author Barb Ericson
/**
* Class that describes a student. A student has a
* name and an array of grades. You can get
* information about a student such as her/his
* name and grade average. *
Barb Ericson */
public class Student
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43. Georgia Institute of Technology
Method Comments
Add a comment before each method
What the parameters are
@param name info
What is returned
@return info
/**
* Method to set the name for this student
theName the new name to use
true if success else false */
public boolean setName(String theName)
Add comments to constructors that are like the method comments. Just realize the constructors don’t return anything.
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44. Generating Javadoc based HTML
In DrJava click on the Javadoc button
to create the HTML documentation
based on the Javadoc comments
This will generate HTML for all files in the same directory as all open files
Generates an index.html as a starting point
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45. Georgia Institute of Technology
Challenge
Add a class javadoc comment and method javadoc comments to the Student class
Execute Javadoc and check out the created documentation
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46. Georgia Institute of Technology
Create a ClassPeriod
A teacher has students in each class period
Each period can have different students
For a class period we might want to know
The teacher’s name
The period number
The students in that period
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UML Class Diagram
There is a standard way to diagram object-oriented classes
It is a UML class diagram
It shows the classes as boxes and the relationships between them
UML means Uniform Modeling Language. This diagram shows that both the Student class and ClassPeriod class inherit from Object and that a ClassPeriod object has 0 to many Student objects associated with it. It also shows that a Student object has 5 to 8 ClassPeriod objects associated with it.
Shows inheritance
Has a or association
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48. Creating a ClassPeriod class
We want fields for the
Teacher’s name
Period number
Students in the period
What type should each of these be?
A name can be a string
A period number can be an integer
The students in the period can be an array of Student objects
With a max of 35 students in a class period
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49. Georgia Institute of Technology
Challenge
Declare a new class ClassPeriod in the file ClassPeriod.java
Add the fields:
private String teacherName;
private int periodNumber;
private Student[] studentArray = new Student[35];
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50. Georgia Institute of Technology
Challenge
Add constructors to ClassPeriod
Add the no argument constructor
Add a constructor that takes the teacher’s name and the period number
Remember that constructors are declared as
public ClassName(paramList) { }
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51. Add Accessors and Modifiers
Add methods to get and set the fields in the class period
public String getTeacherName() {
return this.teacherName; }
public void setTeacherName(String theName)
this.teacherName = theName;
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Override toString
Add the following method to ClassPeriod
public String toString() { }
Add code to the method to return a String with the teacher’s name, the period number, and the number of students in the class period
The length of the array isn’t a count of the actual students
Find out how many in the array are not null
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53. Georgia Institute of Technology
Summary
Object-oriented programs
Have interacting objects
Classes define and create objects
The contain the fields, constructors, and methods
Some methods are called accessor and modifier methods since they give you access to private fields and let you modify private fields
A class inherits methods from a parent class
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