1. Barb Ericson Georgia Institute of Technology Oct 2005
Creating Classes part 3
Barb Ericson
Georgia Institute of Technology
Oct 2005
Georgia Institute of Technology

2. Georgia Institute of Technology
Learning Goals
Computing concepts
Adding a method
To calculate the average of the grades
Creating accessors and modifiers
That protect the fields in the object
Declaring a main method
To execute the desired task
Georgia Institute of Technology

3. 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
Georgia Institute of Technology

4. Create a Method Exercise
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
Modify the toString method to call the getAverage method and add the average to the returned string
Use the debugger to check that this is working correctly
Stop in the getAverage method
Georgia Institute of Technology

5. 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
Unless it is declared with public visibility
Georgia Institute of Technology

6. 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; }
Georgia Institute of Technology

7. Georgia Institute of Technology
Naming Conventions
Accessors – also called Getters
Use getFieldName for non boolean fields
Use isFieldName for boolean fields
Modifiers – also called Setters and Mutators
Use setFieldName
Sometimes return a boolean value to indicate if the value was set successfully
Examples
getName and setName
Georgia Institute of Technology

8. 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];}
Georgia Institute of Technology

9. 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 and >=0
Setting a name
We can decide if this can be changed or not once it isn’t null
Georgia Institute of Technology

10. Georgia Institute of Technology
Name Modifier
Setting the name only if currently null
public boolean setName(String theName) {
boolean result = false;
if (this.name == null)
this.name = theName;
result = true; }
return result;
Georgia Institute of Technology

11. Georgia Institute of Technology
Grade Modifier
public boolean setGrade(int index, double newGrade) {
boolean result = false;
if (newGrade >= 0 ||
this.gradeArray != null ||
index < this.gradeArray.length ||
index >= 0) {
this.gradeArray[index] = newGrade;
result = true; }
return result;
Georgia Institute of Technology

12. Georgia Institute of Technology
Add a Field Exercise
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
Georgia Institute of Technology

13. 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.
Georgia Institute of Technology

14. Georgia Institute of Technology
Main Method
Add a main method to Student
Put the statements that you have been doing in the interactions pane in the main method
public static void main(String[] args) {
Student student1 = new Student();
System.out.println(student1);
Student student2 = new Student(“Sue Clark”);
System.out.println(student2); }
Georgia Institute of Technology

15. 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
Georgia Institute of Technology

16. Georgia Institute of Technology
Summary
To calculate an average
Sum all the values and divide by the number of values
Be careful not to divide by 0
Be careful not to access an array if it is null
Fields are usually declared to be private
To protect the data from misuse by other classes
So you need to provide public accessor (getter) and modifier (setter) methods
That still protect the data
Use a main method to begin execution
public static void main(String[] args) {}
Georgia Institute of Technology