factorial4.java

java 的164个例子的源代码,包括了 很多方面,供包括了19个方面的 源代码

/*
 * Copyright (c) 2000 David Flanagan.  All rights reserved.
 * This code is from the book Java Examples in a Nutshell, 2nd Edition.
 * It is provided AS-IS, WITHOUT ANY WARRANTY either expressed or implied.
 * You may study, use, and modify it for any non-commercial purpose.
 * You may distribute it non-commercially as long as you retain this notice.
 * For a commercial use license, or to purchase the book (recommended),
 * visit http://www.davidflanagan.com/javaexamples2.
 */
package com.davidflanagan.examples.basics;

// Import some other classes we'll use in this example.
// Once we import a class, we don't have to type its full name.
import java.math.BigInteger;  // Import BigInteger from java.math package
import java.util.*;  // Import all classes (including ArrayList) from java.util

/**
 * This version of the program uses arbitrary precision integers, so it does
 * not have an upper-bound on the values it can compute.  It uses an ArrayList
 * object to cache computed values instead of a fixed-size array.  An ArrayList
 * is like an array, but can grow to any size.  The factorial() method is
 * declared "synchronized" so that it can be safely used in multi-threaded
 * programs.  Look up java.math.BigInteger and java.util.ArrayList while 
 * studying this class.  Prior to Java 1.2, use Vector instead of ArrayList
 **/
public class Factorial4 {
    protected static ArrayList table = new ArrayList(); // create cache
    static { // Initialize the first element of the cache with !0 = 1.
	table.add(BigInteger.valueOf(1));
    }

    /** The factorial() method, using BigIntegers cached in a ArrayList */
    public static synchronized BigInteger factorial(int x) {
        if (x<0) throw new IllegalArgumentException("x must be non-negative.");
        for(int size = table.size(); size <= x; size++) {
            BigInteger lastfact = (BigInteger)table.get(size-1);
            BigInteger nextfact = lastfact.multiply(BigInteger.valueOf(size));
            table.add(nextfact);
        }
        return (BigInteger) table.get(x);
    }

    /**
     * A simple main() method that we can use as a standalone test program
     * for our factorial() method.  
     **/
    public static void main(String[] args) {
        for(int i = 0; i <= 50; i++)
	    System.out.println(i + "! = " + factorial(i));
    }
}