You can think of Java classes in much the same way as you think of C structs or Turing records. The most important difference (so far) is that you can also have methods inside classes; these operate on the fields of the object that the methods belong to.

'//' is the comment symbol in Java. All following characters on the same line are ignored by the compiler.

Read the following code carefully, and trace it through in the debugger.

// Fraction: Has two fields, 'numerator' and 'denominator'.  Also has a method
// to print itself, and a method to multiply itself by another Fraction.
class Fraction {
    public int numerator, denominator;

    // Print my numerator and my denominator in the standard "n/d" form.
    public void print() {
        System.out.println(numerator + "/" + denominator);
    }

    // Multiply myself by f2.
    public void multiply(Fraction f2) {
        numerator = numerator * f2.numerator;
        denominator = denominator * f2.denominator;
    }
}

// FracExample: The class containing the main() method, which manipulates
// Fractions.
class FracExample {
    
    // main: this is where execution starts.
    public static void main(String[] pars) {
    
        Fraction f1; // A reference to a Fraction, NOT a Fraction itself.

        // The next line creates a new Fraction and makes f1 refer to it.
        f1 = new Fraction();

        // Initialize what f1 refers to.
        f1.numerator = 7;
        f1.denominator = 17;

        Fraction f2; // A reference to another Fraction.

        // THIS NEXT LINE WOULD BE AN ERROR!!!  f2 is not a Fraction, only a
        // reference to one.
        // f2.numerator = 16;

        f2 = f1; // Make f2 refer to the same thing that f1 refers to.

        // This line changes the numerator to 16.  Note that f1's numerator is
        // also changed.
        f2.numerator = 16;

        // This next line prints "16/17".
        f1.print();

        // This next line prints "16/17", too.
        f2.print();

        // Create a new Fraction and make f2 point to it.  THIS DOES NOT AFFECT
        // f1.
        f2 = new Fraction();
        f2.numerator = 4;
        f2.denominator = 5;

        // What does this line do?
        f1.multiply(f2);

        // And what gets printed?
        f1.print();
        f2.print();
    }
}