CPSC 101 Defining Classes, Part I

Computer Science 1
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Defining Classes, Part I

Motivation

Have seen predefined objects and classes (System.out, String, PrintStream)
These are not enough to model all the possible programs

c.f., BasicDataReader to make input easier than predefined
c.f., Trying to write a program to control airspace around GSP

not surprisingly, Java allows us to define our own classes and associated methods

Class definitions

Usually, it is necessary to have a methodical process to define classes

this will be done later. For now, look at these simple classes

class Student {
  public Student () {}
  public void ask () {
    System.out.println ("I don't understand");
  }
}

Note:

void - method returns nothing
public - method is public - for all to see!

After defining this class, we can use it:

Student barney = new Student ();
barney.ask ();

Parameters

Suppose we want the sender of the ask message to ask about a particular topic

provide the topic as an argument to a method...

barney.ask ("cereal");

this is additional information, which we are passing as a string
to establish this additional information in the method, we declare a parameter, which acts like a variable declaration

the parameter is accessible inside the method only

public void ask (String topic) {
  String question = "I don't understand ".concat (topic);
  System.out.println (question);
}

This method would be placed in the class definition, N.B. overloading

Instance variables

Suppose that, when a student is created, we wish to define the preamble for the string, rather than "I don't understand"

Student barney = new Student ("Please, could you explain ");
barney.ask ();
barney.ask ("cereal");

Please could you explain.
Please, could you explain cereal.

The constructor now takes a string, so its signature must contain a String parameter
An instance variable defined in the class

ask needs to have access to the string passed into the constructor, but:
Methods cannot access variables in other methods (they are not in the same context)
We need to define it in the context of the whole class, and this is called an instance variable
An instance variable belongs to the entire object, and can be accessed by all methods
For an instance variable, the declaration is the same as a reference variable, except for the keyword private added to the beginning.

private String defaultPreamble;

Using a class definition

How do we use something that is not a predefined class?

create a file, Student.java, that contains the class definition
don't forget import java.io.*

import java.io.*;
class Student {
  public Student (String preamble) {
    defaultPreamble = preamble;
  }

  public Student () {

    defaultPreamble = "Excuse me, but can you please explain."

   public void ask () {
    System.out.println (defaultPreamble);
  }

  public void ask (String topic) {
    String question = defaultPreamble.concat (topic);
    System.out.println (question);
  }

  private String defaultPreamble;
}

Compile Student.java and, if no errors, Student.class is created.

We can now write programs

import java.io.*;
class Professor {
  public static void main (String [] arg) { 
    BasicDataReader input = new BasicDataReader ();
    Student bazza = new Student ("Please explain ");
    String topic;

    System.out.println ("I am listening to the lecture...");
    System.out.print ("Please enter a topic:");
    topic = input.readLine ();

    bazza.ask (topic);

    bazza.ask ();
    Student boris = new Student ("Yo, describe ");

    System.out.println ("I am listening to the lecture...");
    System.out.print ("Please enter a topic:");
    topic = input.readLine ();

    boris.ask (topic);

    boris.ask ();
  }
}

Code

import cucs.*;
class Professor {
  public static void main (String [] arg) { 
    BasicDataReader input = new BasicDataReader ();
    BasicDataWriter output = new BasicDataWriter ();
    Student bazza = new Student ("Please explain ");
    String topic;

    output.writeln ("I am listening to the lecture...");
    output.write ("Please enter a topic:");
    topic = input.readLine ();

    bazza.ask (top ic);

    bazza.ask ();

    Student boris = new Student ("Yo, describe ");

    output.writeln ("I am listening to the lecture...");
    output.write ("Please enter a topic:");
    topic = input.readLine ();

    boris.ask (topic);

    boris.ask ();
  }
}

import cucs.*;
class Student {
  public Student (String preamble) {
    defaultPreamble = preamble;
  }

  public void ask () {
    System.out.println (defaultPreamble);
  }

  public void ask (String topic) {
    String question = defaultPreamble.concat (topic);
    System.out.println (question);
  }

  private String defaultPreamble;
}

Last modified: 1/24/99