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("\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));
}
}
运行结果：
