How to design a Pseudorandom Number Generator-Pattern which can be used for testing?

I currently develop a fairly large application which, amongst other things, calculates mathematical problems. Calculations are made in classes, let’s call them A and B, which have some random final attributes and a (unique) final int id as instance variables.

I want to be able to run the program in a test mode, where in each execution, the random variables are exactly the same and allow me, for example, to compare the results with hand-made calculations, using Junit tests. Obviously, I don't want to re-calculate my sample solutions after every code change (for example, if an earlier access to the prng is inserted which would shift all random numbers).

Note that I currently use Java.util.Random as prng, but am open to suggestions.

The question is now: How should I structure the instantiation and accessing of the prng? To point out the problem, note that following would be very bad approaches:

• I cannot make it a singleton and access from everywhere, as then, the random numbers within the classes would depend on the order of instantiation.
• I obviously cannot instantiate a new Random(seed) with a hardcoded seed, as the numbers would not be different if the programm is run multiple times (outside of test mode).

I came up with the following solution (largely based on this answer), but not identical, as the programming language and test seed generation is different.

public class PRNGeneratorGenerator{ //Make class a singleton private PRNGeneratorGenerator instance; private PRNGeneratorGenerator() {} public PRNGeneratorGenerator getInstance(){ if (instance == null) instance = new PRNGeneratorGenerator(); return instance; } //RandomNumberGenerator-Methods and Attributes private boolean isTestMode = false; private Random seedRng = new Random(); public void setTestMode(boolean testMode){ isTestMode = testMode; } public Random getPseudorandomNumberGenerator(long testSeed){ if(isTestMode) return new Random(testSeed); return new Random(seedRng.nextLong()); } } 

The classes (named A and B above) with the final random numbers would then look as follows:

public class A{ private final static long CLASS_SEED = 872349; private final int randomNumberOne; private final int randomNumberTwo; private final int id; public A(int id){ this.id = id; long testSeed = CLASS_SEED + id; Random rnd = PRNGeneratorGenerator.getInstance().getPseudorandomNumberGenerator(testSeed); randomNumberOne = rnd.nextInt(); randomNumberTwo = rnd.nextInt(); } } 

In the test mode, I would call PRNGeneratorGenerator.getInstance().setTestMode(true) before starting any instantiations.

Is this a good way to solve my problem in java or are there any downsides of this approach? I have read a number of similiar questions, but have found no equivalent ones which answers my question. Thanks in advance for your answer.