CSCI A202 - Introduction to Programming (II)

Lecture 10: Classes and Objects (II)

We are going to continue from lecture notes 9.

Things to remember:

programs are made of classes
a class has members (fields, methods) and constructors
class members can be static or not
static members are one per class
non-static members are called instance members
instance members are one per object
static methods cannot access non-static members
static members can be accessed from instance methods
access of static members: ClassName.memberReference
access of instance members: objectReference.memberReference
this: an instance method can access the object to which it belongs
static and instance fields are also called (class, instance) variables
they have nothing in common with variables declared and used in methods
formal parameters in methods are like variables declared in the body of the method
if the class defines no constructors (and only then) a default no-arg constructor is provided.
a class declaration creates a (user-defined) type name
objects are accessed via object references whose type is the object's class.
execution of a program always starts in a class's main method.

Things that we will do today:

work some more on Point:
- add one more method: distance
- add a main method for test purposes
- work with the intriguing origin
discuss the main method
look at tester programs vs. unit-testing
develop a few Fractions and lcm
get ready for inheritance next week

Here's the code for Fraction that we developed in class today.

I am including a complete Unix session in which I:

create a source file Test.java
that has a definition for class Fraction inside
I compile Test.java (the source file)
a Fraction.class is created (bytecode format)
I run the class file (java Fraction)
since it has a main method, execution starts there
two fractions are created, added and printed
note the use of toString() method (explained in lecture notes 9)

Here's the complete session:

tucotuco.cs.indiana.edu% vi Test.java
tucotuco.cs.indiana.edu% cat Test.java
class Fraction {
  public int num, den; 
  Fraction (int num, int den) { 
    this.num = num; 
    this.den = den; 
  }   
  public Fraction add (Fraction anotherFraction) {
    return new Fraction(num * anotherFraction.den + 
                        den * anotherFraction.num, 
                        den * anotherFraction.den); 
  } 
  public String toString () {
    return "(" + num + "/" + den + ") ";  
  } 
  public static void main(String[] args) {
    Fraction f = new Fraction(1, 3); 
    Fraction g = new Fraction(1, 2); 
    Fraction h = f.add(g); 
    System.out.println( f + " + " + g + " = " + h); 
  } 
} 
tucotuco.cs.indiana.edu% ls -l
total 2
-rw-r--r--   1 dgerman  students     598 Feb 11 14:39 Test.java
tucotuco.cs.indiana.edu% javac Test.java
tucotuco.cs.indiana.edu% ls -l
total 6
-rw-r--r--   1 dgerman  students    1180 Feb 11 14:39 Fraction.class
-rw-r--r--   1 dgerman  students     598 Feb 11 14:39 Test.java
tucotuco.cs.indiana.edu% java Fraction 
(1/3)  + (1/2)  = (5/6) 
tucotuco.cs.indiana.edu%

Note that

I don't check for den > 0 when I create a new Fraction
I don't keep the Fractions in their lowest terms

Your solution to problem 10 on the assignment should check for proper denominators and use gcd to reduce their fractions to their lowest terms. gcd is explained on page 149 in your textbook. Try to implement it if you have everything else working in your Fraction class.

Here's now the other code that we wrote when we discussed main, again in a complete Unix session. The steps are similar to the ones above, in that we first create three source files (this time we use the same names as the classes themselves) and compile them all. Then we can start with any main method: A's, B's or C's.

tucotuco.cs.indiana.edu% ls -l
total 12
-rw-r--r--   1 dgerman  students     118 Feb 11 14:59 A.java
-rw-r--r--   1 dgerman  students     118 Feb 11 14:59 B.java
-rw-r--r--   1 dgerman  students      99 Feb 11 14:58 C.java
-rw-r--r--   1 dgerman  students    1180 Feb 11 14:39 Fraction.class
-rw-r--r--   1 dgerman  students     598 Feb 11 14:39 Test.java
tucotuco.cs.indiana.edu% cat A.java
class A {
  public static void main(String[] args) {
    System.out.println("This is A"); 
    B.main(args); 
  } 
} 
tucotuco.cs.indiana.edu% cat B.java
class B {
  public static void main(String[] args) {
    System.out.println("This is B"); 
    C.main(args); 
  } 
} 
tucotuco.cs.indiana.edu% cat C.java
class C {
  public static void main(String[] args) {
    System.out.println("This is C"); 
  } 
} 
tucotuco.cs.indiana.edu% javac ?.java # one char followed by dot java 
tucotuco.cs.indiana.edu% ls -l ?.*    # one char dot extension 
-rw-r--r--   1 dgerman  students     476 Feb 11 15:00 A.class
-rw-r--r--   1 dgerman  students     118 Feb 11 14:59 A.java
-rw-r--r--   1 dgerman  students     476 Feb 11 15:00 B.class
-rw-r--r--   1 dgerman  students     118 Feb 11 14:59 B.java
-rw-r--r--   1 dgerman  students     451 Feb 11 15:00 C.class
-rw-r--r--   1 dgerman  students      99 Feb 11 14:58 C.java
tucotuco.cs.indiana.edu% java B
This is B
This is C
tucotuco.cs.indiana.edu% java C
This is C
tucotuco.cs.indiana.edu% java A
This is A
This is B
This is C
tucotuco.cs.indiana.edu%

This last part can be done in Cafe by setting the Main Class to either A, B or C in the Project menu (choose Project, then Settings and then type in the name of the Main Class and click OK) after which you can rebuild the project and run it. When you do that the Main Class's main method will be executed.

I'll add the code for Point a bit later.