Lecture 08: Objects and Memory

Overview

1. Finishing and Testing Fraction
2. Objects and Memory

Last Time

We started writing a Fraction class to represent fractional values

public class Fraction {
    private long num; // numerator
    private long den; // denominator

    public Fraction (long numerator, long denominator) {
	num = numerator;
	den = denominator;
    }
	
    // Fraction g, Fraction h then g.add(h) returns "g + h"
    public Fraction add (Fraction f) {

	// (a / b) + (c / d) = (a * d + b * c) / b * d
	long newNum = num * f.den + den * f.num;
	long newDen = den * f.den;

	return new Fraction(newNum, newDen);
    }

    // compute the greatest common divisor of two longs
    public static long gcd (long a, long b) {
	if (b == 0) {
	    return a;
	}

	return gcd(b, a % b);
    }
}

Today

Improving and testing the implementation

1. Storing fractions as reduced fractions
2. Converting to other formats:
   • double
   • String
3. Comparing with BadArithmetic from last week

Conclusion

We can design objects that do better arithmetic!

Objects and Memory

A Question

What does the following code do?

    int a = 10;
    int b = 5;
    
    b = a;
    
    a = 20;

What are the values of a and b at the end of the execution?

Second Question

Consider the method

    void setValue (int a, int value) {
      a = value;
    }

What is the result of

    a = 20;
    setValue(a, 0);

What is the value of a at the end of the execution?

Again, but with objects

Consider the following simple class

class Number {
    public int value;

    Number (int val) {
	value = val;
    }
}

First Question, again

What does the following code do?

    Number aNum = new Number(10);
    Number bNum = new Number(5);
    
    bNum = aNum;
    aNum.value = 20;

What are the values of a and b at the end of the execution?

Second Question, again

Consider the method

    void setNumberValue (Number a, int value) {
      a.value = value;
    }

What is the result of

    setNumberValue(aNum, 0);

What is the value of aNum.value at the end of the execution?