Reverse engineering of the java.util.Random class, finds the seed from the first two consecutive integers generated by a Random.

gistfile1.java

import java.util.Random;


public class RandomReverse {
  
	public static void main(String[] args) {
		Random rng = new Random(1337L);
		
		int i1 = rng.nextInt();
		int i2 = rng.nextInt();
		
		long internalSeed = getSeed(i1, i2);
		long firstInternalSeed = previousSeed(previousSeed(internalSeed));
		
		/* 
		 * If the original seed was larger than 2^48
		 * the seed produced will be different but
		 * produce the same pseudo-random sequence.
		 * */
		long originalSeed = (firstInternalSeed ^ 0x5DEECE66DL);
		
		System.out.println(originalSeed);
	}
	
	public static long parseSeed(long seed) {
		return ((seed ^ 0x5DEECE66DL) & ((1L << 48) - 1));
	}
	
	public static long previousSeed(long seed) {
		seed -= 0xBL;
		long result = 0;
		for (int i = 0; i < 48; i++)
		{
		    long mask = 1L << i;
		    long bit = seed & mask;
		    result |= bit;
		    if (bit == mask)
		    {
		        seed -= 0x5DEECE66DL << i;
		    }
		}
		return result;
	}
	
	public static long getSeed(int x1, int x2) {
		long i1 = x1 & 0xFFFFFFFFL;
		long i2 = x2 & 0xFFFFFFFFL; 
		long seed = i1 << 16;
		for (int i = 0; i < 65536; i++) {
			long tseed = seed + i;
			tseed = (tseed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1);
			if ((tseed >>> 16) == i2) {
				return tseed;
			}
		}
		throw new AssertionError(); //This should never happen.
	}
}

how are you supposed to use this? I just need to be able to predict the next numbers given other numbers. I also know the range of numbers.