IT 114: Introduction to Java

IT 114: Introduction to Java
Class 31

Topics

Review

Encapsulation
Data Hiding
Throwing an Exception
Hiding Methods
Time Accessors and Mutators

New Material

Thinking about Objects
Designing a Class
A Practical Example of A Class
Student Setters
Student Name Getters
A New toString Method for Student
The Section Class
Adding Students to a Section
Reading The Lines
Creating a Student Object

Homework 10

Homework 10 is due this Sunday at 11:59 PM.

You will find the assignment here.

If you have a problem or a question, make a post on the Class Discussion Area.

Review

Encapsulation

To keep programmers from mucking about with things they don't understand many languages employ the principle of encapsulation
Encapsulation means hiding the details of how a class works from the clients that use the class
A programmer who is writing a client class does not need to know how a class he or she is calling works
The programmer only needs to know how the general idea of what the class does
This important principle is called abstraction

Data Hiding

To prevent users of a class from doing dangerous things, most object oriented languages practice data hiding
Data hiding means keeping the inner details of a class hidden from programmers writing client code
Ee need to prevent users of our class from changing the field values
We can do this declaring the fields private
The format for creating a private field is
```
private DATA_TYPE FIELD_NAME;
```
A field marked private can only be changed by instance methods inside the class
It is not enough to merely make the fields private
We have to make sure the the constructor does not accept invalid field values

Throwing an Exception

Java lets methods do more than catch an exception
They can also create an exception object
And throw it
These two operations are usually written on single line with the following format
```
throw new EXCEPTION_TYPE()
```
The type of exception thrown depends upon the nature of the runtime error
When you do this, the header must have a throws clause

Hiding Methods

You can hide methods as well as fields
You do this by making them private using the following format
```
private RETURN_TYPE METHOD_NAME(PARAMETER)
```

Time Accessors and Mutators

Since we have made the fields private, there is no way client code can get their values
So we need to create accessors
We also need to create mutators

New Material

Thinking about Objects

There are two ways to think about objects
- How they appear in RAM
- How they represent some thing
In RAM an object is a section of memory holding many values
And methods that work on these values
But an object also represents one particular thing
Like an individual student
Or a car on a dealer's lot
The object holds particular information about a particular student or car
The method let us see the characteristics of the thing they represent
And do the things one does on that object
Like change the major of a student
Or sell a car
There are two parts to solving any problem
- Deciding what to do
- Deciding how to do it
In working on both parts, it is useful to think abstractly
Objects help us do just that

Designing a Class

A heuristic is a way of solving a problem
It may not be perfect, but it gives us practical approach to the work ahead
In other words it is rule of thumb for tackling the job
Here is a list of heuristics for designing a class
1. Each method should have a coherent set of things it has to do
2. No one method should do too large a share of the overall work
3. Dependencies between methods should be minimized
4. The main method should be a summary of the application does
5. Data should be kept at the lowest level possible
What do these mean?
A method is coherent if all the things it does are closely related
So a method that gives a student a grade should not deal with the student's gender
The second guideline means that a method should do one thing and do it well
It should not be like a Swiss Army Knife
The third point has to do with making the program robust
The more a class depends on other classes, the greater the chance that it will eventually break
Software is always changing
This can cause bugs in other classes that depend on them
The 4th point deals with the main method
Class names and method names should always be simple and straightforward
The names should tell a reader what the class or method does
If they are, someone should be able to read it and understand what is going on
The fifth item refers to keeping data as close as possible to where it is used

A Practical Example of A Class

An instructor can use Jave to help with administrative work
Let's create a Student class to do this
The first question is what should be the attributes
We need at least three values for each student
- Student ID
- Last Name
- First Name
This is all the information an instructor has for a student at the start of the semester
This information can be obtained from class roster in WISER
But let's say the instructor want additional information
- Preferred first name
- Phonetic spelling of the name
- Email address
- Unix username
These values will have to be added later
But we still have to give them values in the constructor
So we will set them to the empty string, " "

Here is my first take on the class which consists of a constructor and a toString method

public class Student1 {
    private String student_id;
    private String first_name;
    private String last_name;
    private String prefered;
    private String phonetic;
    private String email;
    private String username;

    public Student1(String first, String last, String id){
        student_id = id;
        first_name = first;
        last_name  = last;
        prefered   = "";
        phonetic   = "";
        email      = "";
        username   = "";
    }

    public String toString(){
        return first_name + " " + last_name + ", " + student_id;
    }
}

Here is the driver

public class Student1Driver {
    public static void main (String[] args){
        Student1 s = new Student1("John", "Smith", "09999999");
        System.out.println(s);
    }
}

$ java Student1Driver
John Smith, 09999999

Student Setters

After class start, instructors should have the information required for the other fields
So we need mutators for these fields
In order to test these methods we also need getters for each field

Here are the setters and getters

    public void set_prefered(String prefered){
        this.prefered = prefered;
    }

    public void set_phonetic(String phonetic){
        this.phonetic = phonetic;
    }

    public void set_email(String email){
        this.email = email;
    }

    public void set_username(String username){
        this.username = username;
    }
    
    public String student_id(){
        return student_id;
    }

    public String prefered(){
        return prefered;
    }

    public String phonetic(){
        return phonetic;
    }

    public String email(){
        return email;
    }

    public String username(){
        return username;
    }

Notice that none of the getter function names begin with "get_"
This makes their names shorter, which makes it easier to write code
We use the driver to test the class

The setter for this field is simple

public class Student2Driver {
    public static void main (String[] args){
        Student2 s = new Student2("William", "Dimetrio", "07777777");
        System.out.println(s);
        System.out.println("Student ID: " + s.student_id());
        s.set_prefered("Bill");
        System.out.println("Prefered: " + s.prefered());
        s.set_phonetic("di MEET tree oh");
        System.out.println("Phonetic: " + s.phonetic());
        s.set_email("bill.dimetrio001@umb.edu");
        System.out.println("Email: " + s.email());
        s.set_username("bdimetrio");
        System.out.println("Username: " + s.username());
    }
}

$ java Student2Driver
William Dimetrio, 07777777
Student ID: 07777777
Prefered: Bill
Phonetic: di MEET tree oh
Email: bill.dimetrio001@umb.edu
Username: bdimetrio

Student Name Getters

We need two additional getters
One to return the full name

full_name will use prefered if it has a value

public String full_name() {
        if (prefered.length() > 0){
            return prefered + " " + last_name;
        } else {
            return first_name + " " + last_name;
        }
    }

And one to return the name as it appears in WISER roster

Here last_name appears first, followed by ", " and first_name

public String wiser_name(){
        return last_name + ", " + first_name;
    }

We test them with a driver

public class Student3Driver {
    public static void main (String[] args){
        Student3 s = new Student3("William", "Dimetrio", "07777777");
        s.set_prefered("Bill");
        System.out.println(s.full_name());
        System.out.println(s.wiser_name());
    }
}

$ java Student3Driver
Bill Dimetrio
Dimetrio, William

A New toString Method for Student

Now that we have all these new setters, we should rewrite toString to print more field data

	public String toString(){
        return full_name() + ", " + student_id + ", " + email + ", " + username
            + "  " + phonetic;
    }

There are two things a little different about this method
First, a single statement is written on more than one line of the code
We can do this in Java because a statement does not end until you write ;
Also note there is just a space after phonetic
Most students will not have an entry in this field
So a comma with nothing after it would not look good

Here is the driver

public class Student4Driver {
    public static void main (String[] args){
        Student4 s = new Student4("William", "Dimetrio", "07777777");
        s.set_prefered("Bill");
        s.set_phonetic("di MEET tree oh");
        s.set_email("bill.dimetrio001@umb.edu");
        s.set_username("bdimetrio");
        System.out.println(s);
    }
}

$ java Student4Driver
Bill Dimetrio, 07777777, bill.dimetrio001@umb.edu, bdimetrio  di MEET tree oh

You can see the final version of Student here

The Section Class

There are many things an instructor has to do that can be automated
Like taking attendance
These tasks have to be done for each section
Let's create a Section class where an instructor can store the student objects
And perform administrative takes
This class will need 5 fields
- class_id
- class_name
- section_no
- students
- grades
students will hold a Student objects
grades will hold the grade for each student

The first version of Section just has a constructor and a toSpring method

import java.util.ArrayList;

public class Section1 {
    private String class_id;
    private String class_name;
    private String section_no;
    private ArrayList<Student> students;
    private ArrayList<String> grades;

    public Section1(String class_id, String class_name, String section_no){
        this.class_id   = class_id;
        this.class_name = class_name;
        this.section_no = section_no;
        students        = new ArrayList<>();
        grades          = new ArrayList<>();
    }

    public String toString(){
        return class_id + "-" + section_no + ": " + class_name;
    }
}

We test this with a driver

public class Section1Driver {
    public static void main (String[] args){
        Section1 section = new Section1("IT 114", "Introduction to Java", "1");
        System.out.println(section);
    }
}

$ java Section1Driver
IT 114-1: Introduction to Java

Adding Students to a Section

A Student object will be created for each student in the section
Each object will be stored the the ArrayList students
The data for creating the student object will come from a file with information from the WISER roster for a class
The file will contain three pieces of information
- Student ID
- Last name
- First name

The file will look like this

07777777,Dimetrio,William
06666666,Smith,Andrew
05555555,Sanders,Joan

Creating the entries in students has two parts
- Reading each line of the file
- Creating a Student object for each line and storing it in the ArrayList
We will create a method for each

Reading The Lines

The add_students method will have three things to do
- Create a Scanner object on the file or print an error message if it can't
- Read each line in the file
- Call a method to create the Student object

Here is the method

    public void add_students(String filename){
        try {
            File file = new File(filename);
            Scanner input = new Scanner(file);
            while (input.hasNext()){
                add_student(input.next());
            }
        } catch (Exception e) {
            System.out.println("Could not open " + filename);
        }
    }

But we have to create a "dummy" add_student method to make this work

It will simply print the line

private String toString(){
        return class_id + "-" + section_no + ": " + class_name;
    }

Notice we have made this method private
Since add_students is the only code that will call it

Now we test it with the driver

    public static void main (String[] args){
        Section2 section = new Section2("IT 114", "Introduction to Java", "1");
        System.out.println(section);
        section.add_students("it114-1_student.txt");
    }
}

$ java Section2Driver
IT 114-1: Introduction to Java
07777777,Dimetrio,William
06666666,Smith,Andrew
05555555,Sanders,Joan

Creating a Student Object

add_student has to do three things
- Break up the line into fields
- Create a Student object from the fields
- Add the object to students

Here is the code

    public void add_student(String line){
        String [] fields = line.split(",");
        Student s = new Student(fields[2], fields[1], fields[0]);
        students.add(s);
    }

The driver shows that the code works

import java.util.ArrayList;

public class Section3Driver {
    public static void main (String[] args){
        Section3 section = new Section3("IT 114", "Introduction to Java", "1");
        System.out.println(section);
        section.add_students("it114-1_student.txt");
        ArrayList students = section.students();
        for (int i = 0; i < students.size(); i++){
            System.out.println(students.get(i));
        }
    }
}

$ java Section3Driver
IT 114-1: Introduction to Java
William Dimetrio, 07777777, ,   
Andrew Smith, 06666666, ,   
Joan Sanders, 05555555, ,