CS61b lab2 打卡

修改后的代码：

import java.io.*;

    /** The Fraction class implements nonnegative fractions (rational numbers).
     */
    public class Fraction {

      /* private fields within a Fraction. */
      private static int numberOfFractions = 0;//change type to static,part 3

      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);//part1 更改过后的constructor
      }

      /** Copies the Fraction "original".
       */
      public int getNumerator(){
          return numerator;
      }
      public int getDenominator(){
          return denominator;
      }
      
      public Fraction(Fraction original) {
        this(original.getNumerator(),original.getDenominator());
      }//part2 修改后的constructor

      /** 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;//change numerator to this.numerator,part3
      }

      /** 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);
      }//part 4,the greatest common divisor of numerator and denominator;

      /** 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.part2
        Fraction sumOfThree = f0.add(f1).add(f2);             // Sum of f0, f1, and f2.part2

        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));
      }
    }

主要复习了constructor的应用，this的应用，以及辗转相除法求最大公约数；

运行结果：