FactQuoter.java Source code

Introduction

Here is the source code for FactQuoter.java

Source

/*
 * 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.
 */

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

import java.util.ArrayList;

import java.math.BigInteger;

/**
 * This program displays factorials as the user enters values interactively
 */
public class FactQuoter {
    public static void main(String[] args) throws IOException {
        // This is how we set things up to read lines of text from the user.
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        // Loop forever
        for (;;) {
            // Display a prompt to the user
            System.out.print("FactQuoter> ");
            // Read a line from the user
            String line = in.readLine();
            // If we reach the end-of-file,
            // or if the user types "quit", then quit
            if ((line == null) || line.equals("quit"))
                break;
            // Try to parse the line, and compute and print the factorial
            try {
                int x = Integer.parseInt(line);
                System.out.println(x + "! = " + Factorial4.factorial(x));
            }
            // If anything goes wrong, display a generic error message
            catch (Exception e) {
                System.out.println("Invalid Input");
            }
        }
    }
}

/**
 * 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
 */

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));
    }
}