How do I get the seed from a Random in Java?

Question

I am creating a deep clone for some object. The object contains a Random.

Is it good practice to retrieve the seed from the Random? If so,

Answer 1

This can be done via reflection, although there is a small quirk:

Random r = ...;  //this is the random you want to clone
long theSeed;
try
{
    Field field = Random.class.getDeclaredField("seed");
    field.setAccessible(true);
    AtomicLong scrambledSeed = (AtomicLong) field.get(r);   //this needs to be XOR'd with 0x5DEECE66DL
    theSeed = scrambledSeed.get();
}
catch (Exception e)
{
    //handle exception
}
Random clonedRandom = new Random(theSeed ^ 0x5DEECE66DL);

The magic number 0x5DEECE66DL comes from the source code of Random.java, where seeds are given an "initial scramble" of:

private static final long multiplier = 0x5DEECE66DL;
private static final long mask = (1L << 48) - 1;
//...
private static long initialScramble(long seed) {
    return (seed ^ multiplier) & mask;
}

which XOR's them with a random number and truncates them to 48 bits. Thus, to recreate the seed state, we have to XOR the seed we pulled.