I have to generate unique serial numbers for users consisting of 12 to 13 digits. I want to use random number generator in Java giving the system. Time in milliseconds as a seed to it. Please let me know about the best practice for doing so. What I did was like this

Random serialNo = new Random(System.currentTimeMillis());
System.out.println("serial number is " serialNo);

Output came out as: serial number is java.util.Random@13d8cc98

CodePudding user response：

Create a random number generator using the current time as seed (as you did)

long seed = System.currentTimeMillis();
Random rng = new Random​(seed);

Now, to get a number, you have to use the generator, rng is NOT a number.

long number = rng.nextLong();

According to the documentation, this will give you a pseudorandom number with 281.474.976.710.656 different possible values.

Now, to get a number with a maximum of 13 digits:

long number = rng.nextLong() % 10000000000000;

And to get a number with exactly 13 digits:

long number = (rng.nextLong() % 9000000000000)   1000000000000;

CodePudding user response：

For a bit better algorithm pick SecureRandom. You passed a seed to the random constructor. This will pick a fixed sequence with that number. A hacker knowing the approximate time of calling, might restrict the number of attempts. So another measure is not using the constructor and nextLong at the same spot.

SecureRandom random = new SecureRandom​();
long n = random.nextLong();

A symmetric bit operation might help:

n ^= System.currentMillis();

However there is a unique number generation, the UUID, a unique 128 bits number, two longs. If you xor them (^) the number no longer is that unique, but might still be better having mentioned the circumstantial usage of random numbers.

UUID id = UUID.randomUUID();
long n = id.getLeastSignificantBits() ^ id.getMostSignificantBits();

CodePudding user response：

Random random = new Random(System.currentTimeMills()); but i don't know this code

CodePudding user response：

First, import the Random class:

import java.util.Random;

Then create an instance of this class, with the current milliseconds as its seed:

Random rng = new Random(System.currentTimeMillis());

This line would generate an integer that can have up to 13 digits:

long result = rng.nextLong() % 10000000000000;

This line would generate an integer that always have 13 digits:

long result = rng.nextLong() % 9000000000000   1000000000000;

CodePudding user response：

There are three ways to generate Random numbers in java

1. java.util.Random class We can generate random numbers of types integers, float, double, long, booleans using this class.

Example : //Random rand = new Random(); // rand_int1 = rand.nextInt(1000)

2. Math.random method : Can Generate Random Numbers of double type. random(), this method returns a double value with a positive sign, greater than or equal to 0.0 and less than 1.0.

Example : Math.random()); //Gives output 0.15089348615777683

3. ThreadLocalRandom class This class is introduced in java 1.7 to generate random numbers of type integers, doubles, booleans etc

Example :

    //int random_int1 = ThreadLocalRandom.current().nextInt();
    // Print random integers
    //System.out.println("Random Integers: "   random_int1);