FloatingPointAssertionWithinEpsilon

Both JUnit and Truth allow for asserting equality of floating point numbers with an absolute tolerance. For example, the following statements are equivalent,

double EPSILON = 1e-20;
assertThat(actualValue).isWithin(EPSILON).of(Math.PI);
assertEquals(Math.PI, actualValue, EPSILON);

What's not immediately obvious is that both of these assertions are checking exact equality between Math.PI and actualValue, because the next double after Math.PI is Math.PI + 4.44e-16.

This means that using the same tolerance to compare several floating point values with different magnitude can be prone to error,

float TOLERANCE = 1e-5f;
assertThat(pressure).isWithin(TOLERANCE).of(1f); // GOOD
assertThat(pressure).isWithin(TOLERANCE).of(10f); // GOOD
assertThat(pressure).isWithin(TOLERANCE).of(100f); // BAD -- misleading equals check

A larger tolerance should be used if the goal of the test is to allow for some floating point errors, or, if not, isEqualTo makes the intention more clear.