Lab2

1）this的用法；

2）static

3）gcd求最大公约数

代码：

	/* Fraction.java */
	  
	import java.io.*;

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

	  /* private fields within a Fraction. */
	  static private int numberOfFractions = 0;// static

	  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);   //call the two-parameter constructor
	  }

	  /** 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);//gcd最大公约数
		  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 gcd(y, x % y);
		  else return x;         //递归法
	  }

	  /** 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("
Testing 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("
Testing 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("
Testing 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("
Testing 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));
	  }
	}	
	

 运行结果：