Computer Science 202

Introduction to Programming

Fall 2012, The College of Saint Rose

Ratios BlueJ Project

Click here to download a BlueJ project for Ratios.

Ratios Source Code

The Java source code for Ratios is below. Click on a file name to download it.

/* * Example Ratios -- a main method that demonstrates the use of * the Ratio class (also provided in this project) * * Jim Teresco, The College of Saint Rose, CSC 202, Fall 2012 * * $Id: Ratios.java 2003 2012-12-04 01:42:42Z terescoj $ */ public class Ratios { public static void main(String[] args) { // Now to represent ratios, we create an instance of Ratio // for each. The "new" results in a call to the constructor // of the class. Ratio a = new Ratio(4, 6); Ratio b = new Ratio(2, 4); // print some information about these. If we try to "print out" // an instance of a class, Java will implicity call the class's // toString method. We can also call it explicitly, as in the // second example. System.out.println("Ratio a is " + a); System.out.println("Ratio b is " + b.toString()); // we can print their decimal equivalents, which is now done by // the Ratio class by calling its getDecimalValue method System.out.println("a as a decimal is " + a.getDecimalValue()); System.out.println("b as a decimal is " + b.getDecimalValue()); // let's change the ratios a bit, this time by calling mutator // methods of our Ratio instances, then do the printouts again a.setNumerator(1); b.setDenominator(10); System.out.println("Ratio a is " + a); System.out.println("Ratio b is " + b); System.out.println("a as a decimal is " + a.getDecimalValue()); System.out.println("b as a decimal is " + b.getDecimalValue()); } }

/* * Example Ratios -- the Ratio class * * This class encapsulates a numerator and denominator and * includes the capability to set the numerator or denominator, * retrieve the numerator or denominator, retrieve the decimal * equivalent of the ratio, and return a "pretty" String * representation of the ratio * * Jim Teresco, The College of Saint Rose, CSC 202, Fall 2012 * * $Id: Ratio.java 2003 2012-12-04 01:42:42Z terescoj $ */ public class Ratio { // We first define the fields -- also called instance variables // that define the state of a ratio. // In this case, we need only two integers, one each for the numerator // and denominator. // Note that these look like local variables in a method, except we // add the qualifier "private" to indicate that no one except the // methods of this class can access these variables. This is the // most common qualifier for instance variables. // Instance variables have a class scope -- they are visible inside all // methods we write within this class. private int numerator; private int denominator; // We next provide the methods that operate on a Ratio. The first is usually // a special method that gets called when we create a Ratio, called the // constructor. It looks like the method calls we have seen earlier in // the semester, except we do not need "static" or "void". The name of // the constructor must match the name of the class, and it should take // any parameters needed to give the class its initial value. public Ratio(int num, int den) { // Inside the constructor, we initialize our instance variables. In // this case, we initialize them based entirely on the parameters. // In some other cases, the initialization might be to constant values // or may use more complex expressions. Note that "num" and "den" are // parameters whose scope is only this constructor -- they will not // exist when we get to other methods later on. While "numerator" // and "denominator" are instance variables that will remain in scope // and retain their values in other methods we call later. numerator = num; denominator = den; } // We next provide two "mutator" methods. These methods are capable of // modifying the "private" instance variables based on the values in the // parameters. Mutator methods modify the state of an object, but do // not usually return any information, so they are specified with a "void" // qualifier in addition to "public". Note that we do not specify "static" // on any of our methods in the class now that we are defining our own // classes that represent objects. public void setNumerator(int num) { numerator = num; } public void setDenominator(int den) { denominator = den; } // Objects aren't useful if we cannot get information back out from them. // This is done with "accessor" methods -- methods that allow us to access // the private instance variables within the class describing our object. // We start with 2 simple accessors -- ones that allow us to retrieve the // numerator and denominator. Since those are both integer values, we specify // "int" return values on our methods. public int getNumerator() { return numerator; } public int getDenominator() { return denominator; } // Methods can also compute and return something interesting. In this case, // we add the capability to compute and return the "decimal" equivalent // of the ratio. This will be a double. public double getDecimalValue() { return 1.0 * numerator / denominator; } // All Java classes provide a mechanism for printing a meaningful // representation of each instance of the class. It is always in a // method named toString that returns a String. If we do not provide // such a method, Java will use a builtin one (which will work but // which probably does not do quite what we'd like). So we provide one // here to print out our Ratio in a "nice" format. public String toString() { return numerator + "/" + denominator; } }