Lab 2

/* Fraction.java */import java.io.*;/** The Fraction class implements nonnegative fractions (rational numbers). */public class Fraction {      /* private fields within a Fraction. */      private static int numberOfFractions = 0;      private int numerator;      private int denominator;      /** Constructs a Fraction n/d.        *  @param n is the numerator.  Must be nonnegative.       *  @param d is the denominator.  Must be positive.       */      public Fraction(int n, int d) {        if (n < 0) {          System.out.println("Fatal error:  Negative numerator.");          System.exit(0);        }        if (d < 1) {          System.out.println("Fatal error:  Nonpositive denominator.");          System.exit(0);        }        numberOfFractions++;        numerator = n;         denominator = d;      }      /** Constructs a Fraction n/1.        *  @param n is the numerator.  Must be nonnegative.       */      public Fraction(int n) {        this(n, 1);      }      /** Constructs a Fraction 0/1.        */      public Fraction() {        this(0, 1);      }      /** Copies the Fraction "original".       */      public Fraction(Fraction original) {        this(original.numerator, original.denominator);      }      /** Converts this Fraction to a string format:  "numerator/denominator."       *  Fractions should be printed in reduced form (part of your assignment is       *  to make this true).       *  @return a String representation of this Fraction.       */      public String toString() {        int thisGcd = gcd(numerator, denominator);        return (numerator / thisGcd + "/" + denominator / thisGcd);      }      /** Return the sum of two fractions.       *  @param f2 is the Fraction to be added.       *  @return the result of adding f2 to this Fraction.       */      public Fraction add(Fraction f2) {        Fraction r = new Fraction((numerator * f2.denominator) + (f2.numerator * denominator), denominator * f2.denominator);        return r;      }      /** Replaces this Fraction's numerator with a new value.       *  @param numerator is the new numerator.  Must be nonnegative.       */      public void changeNumerator(int numerator) { // DO NOT CHANGE THIS SIGNATURE!        // Fix the bug that prevents this method from working correctly.        if (numerator < 0) {          System.out.println("Fatal error:  Negative numerator.");          System.exit(0);        }        this.numerator = numerator;      }      /** Returns the number of Fraction objects in existence.       *  @return the number of Fraction objects in existence.       */      public int fracs() {                         // DO NOT CHANGE THIS SIGNATURE!        // Fix the bug that prevents this method from working correctly.        return numberOfFractions;      }      /** Computes the greatest common divisor (gcd) of the two inputs.       * @param x must be nonnegative       * @param y must be nonnegative       * @return the gcd of x and y       */      static private int gcd(int x, int y) {        /* Replace the following line with your solution. */        if(y == 0){            return x;        }else{            return gcd(y, x % y);        }      }      /** Put the Fraction class through some tests.       * @param argv is not used.       */      public static void main(String[] argv) {        /* Test all four contructors and toString. */        Fraction f0 = new Fraction();        Fraction f1 = new Fraction(3);        Fraction f2 = new Fraction(12, 20);        Fraction f3 = new Fraction(f2);        System.out.println("\nTesting constructors and toString():");        System.out.println("The fraction f0 is " + f0.toString());        System.out.println("The fraction f1 is " + f1);    // toString is implicit.        System.out.println("The fraction f2 is " + f2);        System.out.println("The fraction f3 is " + f3 + ", which should equal f2");        /* Test the add method. */        System.out.println("\nTesting add:");        Fraction sumOfTwo = f1.add(f2);              // Sum of f1 and f2.        Fraction sumOfThree = f0.add(f1.add(f2));    // Sum of f0, f1, and f2.        System.out.println("The sum of " + f1 + " and " + f2 + " is " + sumOfTwo);        System.out.println("The sum of " + f0 + ", " + f1 + " and " + f2 + " is " + sumOfThree);        /* Test the methods used in Part III. */        System.out.println("\nTesting changeNumerator and fracs:");        f3.changeNumerator(7);        System.out.println("Now f3 is " + f3 + ", which should be 7/20");        System.out.println("The total number of Fraction objects is " + f3.fracs());        /* Test gcd function (static method). */        System.out.println("\nTesting gcd:");        System.out.println("The gcd of 2 and 10 is: " + gcd(2, 10));        System.out.println("The gcd of 15 and 5 is: " + gcd(15, 5));        System.out.println("The gcd of 24 and 18 is: " + gcd(24, 18));        System.out.println("The gcd of 10 and 10 is: " + gcd(10, 10));        System.out.println("The gcd of 21 and 400 is: " + gcd(21, 400));      }}

(1) this

if the parameters or local variables of a method have the same name as the fields of an object, then the former have priority, and the “this” keyword is needed to refer to the object’s fields.

“this” is a keyword in Java, which normally refers to the object on which a method is invoked. In a constructor, it can be used (as
below) to invoke a constructor from within another constructor.

public class Fraction {      /* private fields within a Fraction. */      private static int numberOfFractions = 0;      private int numerator;      private int denominator;      /** Constructs a Fraction n/d.        *  @param n is the numerator.  Must be nonnegative.       *  @param d is the denominator.  Must be positive.       */      public Fraction(int n, int d) {        if (n < 0) {          System.out.println("Fatal error:  Negative numerator.");          System.exit(0);        }        if (d < 1) {          System.out.println("Fatal error:  Nonpositive denominator.");          System.exit(0);        }        numberOfFractions++;        numerator = n;         denominator = d;      }      /** Constructs a Fraction n/1.        *  @param n is the numerator.  Must be nonnegative.       */      public Fraction(int n) {        this(n, 1);      }      /** Constructs a Fraction 0/1.        */      public Fraction() {        this(0, 1);      }       /** Copies the Fraction "original".       */      public Fraction(Fraction original) {        this(original.numerator, original.denominator);      }}

a good software engineering: reusing code!
reduced duplicate code, the program is shorter, and more importantly, if we later find a bug in the constructor, we might only need to fix the first constructor to fix all of them.

(2) toString

any object can have a toString method, and if it does, that method will be automatically called when you concatenate the object to a String. (Actually, every object has a toString method, but the default toString isn’t particularly enlightening.)

System.out.println("The fraction f0 is " + f0.toString());System.out.println("The fraction f1 is " + f1);    // toString is implicit.

如果我们想让一个类以我们希望的方式转化成String类型，就必须重写toString()方法。

public String toString() {    int thisGcd = gcd(numerator, denominator);    return (numerator / thisGcd + "/" + denominator / thisGcd);}

(3) greatest common divisor - recursive GCD function

/** Computes the greatest common divisor (gcd) of the two inputs.* @param x must be nonnegative* @param y must be nonnegative* @return the gcd of x and y*/static private int gcd(int x, int y) {/* Replace the following line with your solution. */    if(y == 0){        return x;    }else{        return gcd(y, x % y);    }}

(4) reduced form

public String toString() {    int thisGcd = gcd(numerator, denominator);    return (numerator / thisGcd + "/" + denominator / thisGcd);}