生成15位随机数时，后3位始终为零

Question

I wanted to generate a random number between 1_000_0000_0000_000 and 9_999_9999_9999_999L using Java so I used the following code,

final long MAX_NUMBER = 9_999_9999_9999_999L;
final long MIN_NUMBER = 1_000_0000_0000_000L;
System.out.print(Long.valueOf(new Random().nextLong() * 
                     (MAX_NUMBER - MIN_NUMBER)));

The problem with the code is, the code does generate 15 digit random number, but the last 3-digits are always zero, and some time it generates negative number.

Can anybody tell me why the last 3-digits are always zero ?

Here are some of the outputs,

7160629848698886104000

-6581075034034719564000

6645937379553681443000

6363833355925386903000

Answer 1

nextLong() may return any long value, which include negative values. Multiplying the result of nextLong() by (MAX_NUMBER - MIN_NUMBER) may cause an overflow and will not give you a result in the required range.

You can use Math.random() instead :

System.out.print((long)(Math.random() * (MAX_NUMBER - MIN_NUMBER))+ MIN_NUMBER);

Answer 2

Strange, I cannot reproduce your problem. Looks like the JVM's random number generation is flawed? You can try SecureRandom instead of Random.

By the way, I just wanted to note that the Math.random() * (max - min) + min pattern does not result in uniform distributed (though it comes close).

You can try out Apache Commons: RandomDataGenerator . It has an nextLong(long lower, long upper) implementation.

https://commons.apache.org/proper/commons-math/apidocs/org/apache/commons/math3/random/RandomDataGenerator.html