random.java

java源代码 请看看啊 提点宝贵的意见

/*
 * @(#)Random.java	1.39 03/01/23
 *
 * Copyright 2003 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.util;
import java.io.*;
import sun.misc.AtomicLong;

/**
 * An instance of this class is used to generate a stream of 
 * pseudorandom numbers. The class uses a 48-bit seed, which is 
 * modified using a linear congruential formula. (See Donald Knuth, 
 * <i>The Art of Computer Programming, Volume 2</i>, Section 3.2.1.) 
 * <p>
 * If two instances of <code>Random</code> are created with the same 
 * seed, and the same sequence of method calls is made for each, they 
 * will generate and return identical sequences of numbers. In order to 
 * guarantee this property, particular algorithms are specified for the 
 * class <tt>Random</tt>. Java implementations must use all the algorithms 
 * shown here for the class <tt>Random</tt>, for the sake of absolute 
 * portability of Java code. However, subclasses of class <tt>Random</tt> 
 * are permitted to use other algorithms, so long as they adhere to the 
 * general contracts for all the methods.
 * <p>
 * The algorithms implemented by class <tt>Random</tt> use a 
 * <tt>protected</tt> utility method that on each invocation can supply 
 * up to 32 pseudorandomly generated bits.
 * <p>
 * Many applications will find the <code>random</code> method in 
 * class <code>Math</code> simpler to use.
 *
 * @author  Frank Yellin
 * @version 1.39, 01/23/03
 * @see     java.lang.Math#random()
 * @since   JDK1.0
 */
public
class Random implements java.io.Serializable {
    /** use serialVersionUID from JDK 1.1 for interoperability */
    static final long serialVersionUID = 3905348978240129619L;

    /**
     * The internal state associated with this pseudorandom number generator.
     * (The specs for the methods in this class describe the ongoing
     * computation of this value.)
     *
     * @serial
     */
    private AtomicLong seed;

    private final static long multiplier = 0x5DEECE66DL;
    private final static long addend = 0xBL;
    private final static long mask = (1L << 48) - 1;

    /** 
     * Creates a new random number generator. Its seed is initialized to 
     * a value based on the current time:
     * <blockquote><pre>
     * public Random() { this(System.currentTimeMillis()); }</pre></blockquote>
     * Two Random objects created within the same millisecond will have
     * the same sequence of random numbers.
     *
     * @see     java.lang.System#currentTimeMillis()
     */
    public Random() { this(System.currentTimeMillis()); }

    /** 
     * Creates a new random number generator using a single 
     * <code>long</code> seed:
     * <blockquote><pre>
     * public Random(long seed) { setSeed(seed); }</pre></blockquote>
     * Used by method <tt>next</tt> to hold 
     * the state of the pseudorandom number generator.
     *
     * @param   seed   the initial seed.
     * @see     java.util.Random#setSeed(long)
     */
    public Random(long seed) {
        this.seed = AtomicLong.newAtomicLong(0L);
        setSeed(seed);
    }

    /**
     * Sets the seed of this random number generator using a single 
     * <code>long</code> seed. The general contract of <tt>setSeed</tt> 
     * is that it alters the state of this random number generator
     * object so as to be in exactly the same state as if it had just 
     * been created with the argument <tt>seed</tt> as a seed. The method 
     * <tt>setSeed</tt> is implemented by class Random as follows:
     * <blockquote><pre>
     * synchronized public void setSeed(long seed) {
     *       this.seed = (seed ^ 0x5DEECE66DL) & ((1L << 48) - 1);
     *       haveNextNextGaussian = false;
     * }</pre></blockquote>
     * The implementation of <tt>setSeed</tt> by class <tt>Random</tt> 
     * happens to use only 48 bits of the given seed. In general, however, 
     * an overriding method may use all 64 bits of the long argument
     * as a seed value. 
     *
     * Note: Although the seed value is an AtomicLong, this method
     *       must still be synchronized to ensure correct semantics
     *       of haveNextNextGaussian.
     *
     * @param   seed   the initial seed.
     */
    synchronized public void setSeed(long seed) {
        seed = (seed ^ multiplier) & mask;
        while(!this.seed.attemptSet(seed));
    	haveNextNextGaussian = false;
    }

    /**
     * Generates the next pseudorandom number. Subclass should
     * override this, as this is used by all other methods.<p>
     * The general contract of <tt>next</tt> is that it returns an 
     * <tt>int</tt> value and if the argument bits is between <tt>1</tt> 
     * and <tt>32</tt> (inclusive), then that many low-order bits of the 
     * returned value will be (approximately) independently chosen bit 
     * values, each of which is (approximately) equally likely to be 
     * <tt>0</tt> or <tt>1</tt>. The method <tt>next</tt> is implemented 
     * by class <tt>Random</tt> as follows:
     * <blockquote><pre>
     * synchronized protected int next(int bits) {
     *       seed = (seed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1);
     *       return (int)(seed >>> (48 - bits));
     * }</pre></blockquote>
     * This is a linear congruential pseudorandom number generator, as 
     * defined by D. H. Lehmer and described by Donald E. Knuth in <i>The 
     * Art of Computer Programming,</i> Volume 2: <i>Seminumerical 
     * Algorithms</i>, section 3.2.1.
     *
     * @param   bits random bits
     * @return  the next pseudorandom value from this random number generator's sequence.
     * @since   JDK1.1
     */
    protected int next(int bits) {
        long oldseed, nextseed;
        do {
          oldseed = seed.get();
          nextseed = (oldseed * multiplier + addend) & mask;
        } while (!seed.attemptUpdate(oldseed, nextseed));
        return (int)(nextseed >>> (48 - bits));
    }

    private static final int BITS_PER_BYTE = 8;
    private static final int BYTES_PER_INT = 4;

    /**
     * Generates random bytes and places them into a user-supplied 
     * byte array.  The number of random bytes produced is equal to 
     * the length of the byte array.
     * 
     * @param bytes  the non-null byte array in which to put the 
     *               random bytes.
     * @since   JDK1.1
     */
    public void nextBytes(byte[] bytes) {
	int numRequested = bytes.length;

	int numGot = 0, rnd = 0;

	while (true) {
	    for (int i = 0; i < BYTES_PER_INT; i++) {
		if (numGot == numRequested)
		    return;

		rnd = (i==0 ? next(BITS_PER_BYTE * BYTES_PER_INT)
		            : rnd >> BITS_PER_BYTE);
		bytes[numGot++] = (byte)rnd;
	    }
	}
    }

    /**
     * Returns the next pseudorandom, uniformly distributed <code>int</code>
     * value from this random number generator's sequence. The general 
     * contract of <tt>nextInt</tt> is that one <tt>int</tt> value is 
     * pseudorandomly generated and returned. All 2<font size="-1"><sup>32
     * </sup></font> possible <tt>int</tt> values are produced with 
     * (approximately) equal probability. The method <tt>nextInt</tt> is 
     * implemented by class <tt>Random</tt> as follows:
     * <blockquote><pre>
     * public int nextInt() {  return next(32); }</pre></blockquote>
     *
     * @return  the next pseudorandom, uniformly distributed <code>int</code>
     *          value from this random number generator's sequence.
     */
    public int nextInt() {  return next(32); }

    /**
     * Returns a pseudorandom, uniformly distributed <tt>int</tt> value
     * between 0 (inclusive) and the specified value (exclusive), drawn from
     * this random number generator's sequence.  The general contract of
     * <tt>nextInt</tt> is that one <tt>int</tt> value in the specified range
     * is pseudorandomly generated and returned.  All <tt>n</tt> possible
     * <tt>int</tt> values are produced with (approximately) equal
     * probability.  The method <tt>nextInt(int n)</tt> is implemented by
     * class <tt>Random</tt> as follows:
     * <blockquote><pre>
     * public int nextInt(int n) {
     *     if (n<=0)
     *		throw new IllegalArgumentException("n must be positive");
     *
     *     if ((n & -n) == n)  // i.e., n is a power of 2
     *         return (int)((n * (long)next(31)) >> 31);
     *
     *     int bits, val;
     *     do {
     *         bits = next(31);
     *         val = bits % n;
     *     } while(bits - val + (n-1) < 0);
     *     return val;
     * }
     * </pre></blockquote>
     * <p>
     * The hedge "approximately" is used in the foregoing description only 
     * because the next method is only approximately an unbiased source of
     * independently chosen bits.  If it were a perfect source of randomly 
     * chosen bits, then the algorithm shown would choose <tt>int</tt> 
     * values from the stated range with perfect uniformity.
     * <p>
     * The algorithm is slightly tricky.  It rejects values that would result
     * in an uneven distribution (due to the fact that 2^31 is not divisible
     * by n). The probability of a value being rejected depends on n.  The
     * worst case is n=2^30+1, for which the probability of a reject is 1/2,
     * and the expected number of iterations before the loop terminates is 2.
     * <p>
     * The algorithm treats the case where n is a power of two specially: it
     * returns the correct number of high-order bits from the underlying
     * pseudo-random number generator.  In the absence of special treatment,
     * the correct number of <i>low-order</i> bits would be returned.  Linear
     * congruential pseudo-random number generators such as the one
     * implemented by this class are known to have short periods in the
     * sequence of values of their low-order bits.  Thus, this special case
     * greatly increases the length of the sequence of values returned by
     * successive calls to this method if n is a small power of two.
     *
     * @param n the bound on the random number to be returned.  Must be
     *	      positive.
     * @return  a pseudorandom, uniformly distributed <tt>int</tt>
     *          value between 0 (inclusive) and n (exclusive).
     * @exception IllegalArgumentException n is not positive.
     * @since 1.2
     */

    public int nextInt(int n) {
        if (n<=0)
            throw new IllegalArgumentException("n must be positive");

        if ((n & -n) == n)  // i.e., n is a power of 2