Spring 2001 CS100M  
Exercise E13: Object Lesson
Due: hard copy handed in at the *start* of lecture, Tuesday, April 24.
     no late assignments will be accepted.

===============================================================================
goals
===============================================================================

this exercise is to make sure you start to learn how to trace Java programs:
+ $public$ = everyone is allowed to see the variable or method
+ $private$ = only code within the same class is allowed to see 
              the variable or method
+ $static$ = exactly one copy, inside the class
+ non-static = exactly one copy per object inside the class
+ $new CLASS()$ = create a new object inside class $CLASS$ 
                  and return a reference to it
+ $COND ? YES : NO$ = if $COND$ is true, then value $YES$, else value $NO$
+ value = a scalar (primitive) type or a reference to an object
+ copying a reference does *not* copy the object it points to
+ in java, there is never a reference pointing to a variable
+ earlier rules still apply, e.g.
  + draw activation records in the same scope as where you found the method
  + if you don't see an identifier in the current scope, 
    go to the next enclosing scope

note: the 4/24 lecture will go over the solutions to this quiz.

===============================================================================
instructions
===============================================================================

trace the program below (E13 is the main class) using box scope diagrams:
+ if a line of code is illegal, pretend it does not exist
+ record output
+ run the code to double-check your answer (comment out illegal lines of code)
  -- you should also make changes to the code and make note of the results 
     to see if you understand them or are confused by them

submission
+ turn in a printout of the output from running the program
+ turn in your hand tracing, but keep a copy for reference during lecture
+ you must turn in hard copy at the start of lecture
  -- no late assignments will be accepted

===============================================================================
code
===============================================================================

/* complex numbers $a + i * b$ */
public class Complex {
    private double a, b; /* $a$ = real part, $b$ = imaginary part */
    
    /* create complex number $real + i * imaginary$ */
    public static Complex makeComplex(double real, double imaginary) {
        Complex c = new Complex();
        c.a = real;
        c.b = imaginary;
        System.out.println(str(a) + "i" + str(b));
        return c;
    }
    
    /* sum of self with $c$ */
    public Complex add(Complex c) {
        return makeComplex(a + c.a, b + c.b);
    }

    /* product of self with $c$ */
    public Complex mul(Complex c) {
        return makeComplex(a * c.a - b * c.b, a * c.b + b * c.a);
    }
    
    /* description of self */
    public String toString() {
        String s = b == 1  ? "i" 
                 : b == -1 ? "-i"
                 : b < 0   ? "-" + str(-b) + "i"
                           : "+" + str(b) + "i";
        return b == 0 ? str(a) + s : s;
    }
    
    /* string representation of $n$ */
    private String str(double n) {
        int i = (int) n;
        return i == n ? "" + i : "" + n;
    }
}

public class E13 {
    public Complex a = Complex.makeComplex(1.0, 2.0);

    public static Complex b = Complex.makeComplex(1.0, -2.0);

    public static void main(String[] args) {
        b.b++;
        Complex c = b.makeComplex(0.0, 1.0);
        Complex d = c;
        c = b;
        b = d;
        d = c.mul(b);
        E13 e = new E13();
        System.out.println(b.add(c).toString());

        System.out.println(a.toString());
        System.out.println(e.a.toString());
        System.out.println(b.toString());
        System.out.println(e.b.toString());
        System.out.println(c.toString());
        System.out.println(d.toString());
    }
}