MyRandom.java

import java.security.SecureRandom;

// Targeting Android SDK 19
// Goal is a (cryptographic) random long between 2^32 ..< 2^62.
// Ultimately I want to pick ~10M values with predictible and very low liklihood of collision (~0.001% Birthday Attack).
// Approach is to compute a random upper 32 bits between 1..<2^30 and add a lower 32-bits 0..<2^32.
// Need to be careful of lower 32 bits being treated as negative, since that would skew the results out of range.

public class MyRandom {
    public static void main(String args[]) {
        SecureRandom random = new SecureRandom();

        // Compute upper 32 bits
        int lowerBound = 1; // Don't allow zero; the minimum is 2^32, not 2^32 - 1
        int upperBound = 1<<30;  // Don't use upper two bits (2^62, not 2^64)
        // Since we're not allowing upper values above 2^30, don't have to worry about signed/unsigned.
        long upper = random.nextInt(upperBound - lowerBound) + lowerBound;

        // Compute lower 32 bits. Want the entire 0..<2^32 range, not negatives.
        long lower = Integer.toUnsignedLong(random.nextInt());

        // Put them together. Remember + is higher precedence than <<.
        long id = (upper << 32) + lower;

        System.out.println(upper);
        System.out.println(upper << 32);
        System.out.println(lower);
        System.out.println(id);
        System.out.println(id >= (long)1<<32);
        System.out.println(id < (long)1<<62);
    }
}