/* Don't try to read it on your mobile - you'll hate me ^^ Subject: - Java core: Explanation of the Random algorithm, when a seed is passed Random example taken from: https://www.javacodegeeks.com/2011/10/weird-funny-java.html */ import java.util.Random; import java.util.concurrent.atomic.AtomicLong; public class NotSoRandom { public static void main(String[] args) { // Not so random Part 1 System.out.println("Why don't these statements generate pseudo random \nnumbers? \nThe sequence is equal every time you run the code."); Random random1 = new Random(441287210); for(int i=0;i<10;i++) System.out.print(random1.nextInt(10)+" "); System.out.println(); // Not so random Part 2 Random random2 = new Random(-6732303926L); for(int i=0;i<10;i++) System.out.print(random2.nextInt(10)+" "); System.out.println("\n"); System.out.println("\n The internal algorithm of the Random class\n produces these sequences due to these particular\n seeds. This is necessary for testing purposes.\n"); System.out.println("\nLet's see what's going on - step by step:\n"); System.out.println("1. Random random1 = new Random(441287210);\n"); System.out.println("A new object of the Random class is created.\nThe argument we pass to the constructor is the value \n441287210 of type long.\nIt sets the seed for the pseudo random generator.\nThe seed is the initial value for the generator.\nThe same seed will produce the same sequence every \ntime you run the code.\nThis is helpful for testing purposes.\nTo create a random number (which is not only pseudo\nrandom), the seed itself must be a random number.\nParadox? Not really ;)"); System.out.println(); System.out.println("2. Internally, in the Random class, the statement\n\n(seed ^ 0x5DEECE66DL) & ((1L << 48) - 1) \n\nis executed.\nWhat is the result of this operation for our \narguments? See this:\n"); System.out.println("(441287210^0x5DEECE66DL)&((1L<<48)-1) = " + ((441287210 ^ 0x5DEECE66DL) & ((1L << 48) - 1)) ); System.out.println("(-6732303926L^0x5DEECE66DL)&((1L<<48)-1) = " + ((-6732303926L ^ 0x5DEECE66DL) & ((1L << 48) - 1)) ); System.out.println("\n"); System.out.println("3. The loop is executed.\nIt runs 10 times, so this statement is executed\n10 times:\n"); System.out.println("random.nextInt(10)\n"); System.out.println("The method nextInt(int) internally executes \nthe following statement, if we pass the argument 10):\n"); System.out.println("int bits, val;"); System.out.println("do {"); System.out.println(" bits = next(31);"); System.out.println(" val = bits % 10;"); System.out.println("} while (bits - val + (10-1) < 0);\n"); System.out.println("You see that the method next(31) is called,\nso we have to know the return value of this method\nbefore we understand what the result is."); System.out.println(""); System.out.println("4. Now our seed values play their roles.\nThis is what's happening in next(31):"); System.out.println("(seed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1)"); System.out.println(" return (int)(seed >>> (17))\n"); System.out.println("For our seeds:\nseed1=24773747783 and seed2=281456460289959:\n"); System.out.println("(24773747783 * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1)="+((24773747783L * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1))); System.out.println(" return (int)(nextseed >>> (17)) // returns " + (int)(189008L >>> (17)) + "\n"); System.out.println("(281456460289959 * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1)="+((281456460289959L * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1))); System.out.println(" return (int)(nextseed >>> (17)) // returns " + (int)(93457754414118L >>> (17))); System.out.println("Before the value is returned,\nthe value of nextseed is assigned to seed."); System.out.println(""); System.out.println("5. These return values are used in the nextInt(int) method,\nas mentioned in item 3. So the return values are:"); System.out.println("int bits, val;"); System.out.println("do {"); System.out.println(" // for seed1: bits = 1; for seed2: bits=713026080"); System.out.println(" bits = next(31);"); System.out.println(" val = bits % 10;"); System.out.println("} while (bits - val + (10-1) < 0);\n"); System.out.println("return val;\n"); System.out.println("If you execute the code with the given values,\nthe outputs in each first iteration are:"); int bits, val; do { bits = 1; val = bits % 10; } while (bits - val + (10-1) < 0); System.out.println("For seed1: " + val); do { bits = 713026080; val = bits % 10; } while (bits - val + (10-1) < 0); System.out.println("For seed2: " + val); System.out.println(); System.out.println("6. The next iteration of the for loop is executed:"); System.out.println("For our new seeds:\nseed1=105822562778182 and seed2=93457754414118:\n"); System.out.println("(189008 * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1)="+((105822562778182L * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1))); System.out.println(" return (int)(nextseed >>> (17)) // returns " + (int)(238817611795929L >>> (17)) + "\n"); System.out.println("(93457754414118 * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1)="+((93457754414118L * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1))); System.out.println(" return (int)(nextseed >>> (17)) // returns " + (int)(71543626391609L >>> (17))); System.out.println(""); System.out.println("7. Back the nextInt(int) method again, printed are:"); System.out.println("int bits, val;"); System.out.println("do {"); System.out.println(" // for seed1: bits = 1822033781; for seed2: bits=545834551"); System.out.println(" bits = next(31);"); System.out.println(" val = bits % 10;"); System.out.println("} while (bits - val + (10-1) < 0);\n"); System.out.println("return val;\n"); System.out.println("The output is:"); do { bits = 1822033781; val = bits % 10; } while (bits - val + (10-1) < 0); System.out.println("For seed1: " + val); do { bits = 545834551; val = bits % 10; } while (bits - val + (10-1) < 0); System.out.println("For seed2: " + val); System.out.println(); System.out.println("8. Next iteration: "); System.out.println("No, just kidding,\nI guess you can imagine now what happens\nduring the next iteration ^^"); } }

Not so random Explanation