如何在 Java 中创建单元测试？

Question

I have a java class that takes an array of numbers and an array of corresponding probabilities and acts as a random number generator that returns one of the numbers from the array according to the corresponding probabilities defined in the other other. How do I write a minimal but effective set of unit tests?

class RandomGen {

    private int[] randomNums;
    private float[] probabilities;
    private float[] total_prob;

    RandomGen(int[] randomNums, float[] probabilites) throws Exception {
        if (this.set_arrays(randomNums, probabilites));
        else {
            throw new Exception("Invalid array format");
        }
    }

    private boolean set_arrays(int[] randomNums, float[] probabilities) {
        if (check_arrays(randomNums, probabilities)) {
            this.randomNums = randomNums;
            this.probabilities = probabilities;
            this.total_prob = new float[this.probabilities.length];

            total_prob[0] = probabilities[0];
            for (int i = 1; i < probabilities.length; i++){
                total_prob[i] = total_prob[i-1] + probabilities[i];
            }
            return true;
        }
        return false;
    }

    public int[] return_randomNums() {
        return this.randomNums;
    }

    public float[] return_probabilites() {
        return this.probabilities;
    }

    private boolean check_arrays(int[] randomNums, float[] probabilities) {
        float total_prob = 0;
        for (int i = 0; i < probabilities.length; i++) {
            if (probabilities[i] < 0) return false;
            total_prob += probabilities[i];
        }
        if (randomNums.length != probabilities.length || total_prob != 1) return false;
        return true;
    }

    public int nextNum() {
        float random = (float) Math.random();
        if (random <= total_prob[0]) {
            return randomNums[0];
        }
        for (int i=1; i < total_prob.length; i++) {
            if (total_prob[i-1] < random  && random <= total_prob[i]) return randomNums[i];
        }
        return 0;
    }
}

Answer 1

For learning unit testing this class might be a little too complex. We can also just scratch the surface here, as this is not something you can learn from a few examples. I urge you to get a book on the topic.

But since we are already here, let's have a look at your class.

The public interface of your class consists of the constructor, nextNum() and the two getters (methods that just return fields are called getters, because they are usually named getFieldName() , eg getRandomNums() ).

The getters do not contain any logic. Therefore I would refrain from testing them.

Your constructor is actually doing quite a lot. At first it checks if the arrays are valid (same length and the probabilities add up to 1) and then it stores the array and build a new array with the cumulative probabilities. For testing it, each test case would create a new instance of RandomGen with different arrays that constitute different situations. The three situations that are apparent are:

• the probabilities add up to one, but the two arrays have different length
• the two array have the same length, but the probabilities arrays elements do not add up to 1
• the two array have the same length and the probabilities add up to one

The first two cases are error cases and would expect an exception to be thrown and the last one is the "sunshine case" where you could check if the two arrays are now returned by your getters. The cumulative probabilities ( total_prob ) can only be observed by using nextNum() .

Then there is the nextNum() method. This is already a bit advanced, since you can not predict the results of Math.random() . Possible solutions are either to use a predefined seed (as Victor Polo De Gyves Montero suggested) or to wrap and inject Math.random() . In your case I would skip that and first read a book and get some experiences with unit testing.