How to Assert Two doubles in JUnit Using assertEquals with Epsilon?

4
votes

assertEquals for doubles is deprecated. I found that the form with Epsilon should be used. It is because of impossible 100% strictness of doubles. But anyway I need to compare two doubles (expected and actual result), but I don't know how to do it.

At the moment my test looks like:

@Test
public void testCalcPossibleDistancePercentageCount() {
    int percentage = 100;
    assertEquals("Wrong max possible value for %" + percentage, 110.42, processor.calcPossibleValue(percentage));
    percentage = 75;
    /*corresponding assertions*/
}

Here are 3 double values I receive and which I want to check with JUnit: 110.42, 2760.5 and 10931.58. How should JUnit test look like with assertions for them? I receive them as a result of calculation in a method:

processor.calcPossibleValue(allowed_percentage){return /*Some weird formulae here*/;}
4
javajunit

4 Answers

11
votes

You need to add a fourth parameter to the assertEquals call: the threshold within which two doubles should be considered "equal". Your call should look like this:

assertEquals("Wrong max possible value for %" + percentage, 110.42,
        processor.calcPossibleValue(percentage), 0.01);

The above call would indicate that if the value returned by processor.calcPossibleValue(percentage) is within ± 0.01 of 110.42, then the two values are considered equal. You can change this value to make it as small as is necessary for your application.

See the JUnit documentation for more information.

1
votes

A Java double uses the IEEE 754 64-bit format. This format has 52 mantissa bits. When comparing 2 values the epsilon should take into account the magnitude of the expected value.

For example, 0.01 might work okay for 110.42 however it won't work if the expected value is > 252. The magnitude of 252 is so large that 0.01 would be lost due to precision (i.e. only 52-bit mantissa bits). For example, 252 + 0.01 == 252.

With that in mind, epsilon should be scaled to the expected value. For example, expected value ÷ 252 - 3 or 110.42 ÷ 252 - 3 = 1.96... x 10-13. I chose 252 - 3 since this will give a tolerance in the 3 least significant bits in the mantissa.

One caution is that if the expected value is 0.0 then this formula computes epsilon as 0.0 which may be too strict for a particular case.

Another caution is that NaN and ±∞ are not handled.

0
votes
Assert.assertTrue("Not equals", expectedDouble -  actualDouble == 0);

There is no need to involve epsilon or delta.

0
votes

Assert is Deprecated

assertTrue("message like not equal ",expectedresult-actual == 0);