Meaning of delta or epsilon argument of assertEquals for double values

Question

I have a question about JUnit assertEquals to test double values  Reading the API doc I can see    Deprecated public static void assertEquals double expected  double actual   Deprecated  Use assertEquals double expected  double actual  double delta  instead    Note  in older documentation versions  the delta parameter is called epsilon  What does the delta  or epsilon  parameter mean

User · Answer

The thing is that two double may not be exactly equal due to precision issues inherent to floating point numbers. With this delta value you can control the evaluation of equality based on a error factor.

Also some floating-point values can have special values like NAN and -Infinity/+Infinity which can influence results.

If you really intend to compare that two doubles are exactly equal it is best compare them as an long representation

Assert.assertEquals(Double.doubleToLongBits(expected), Double.doubleToLongBits(result));

Or

Assert.assertEquals(0, Double.compareTo(expected, result));

Which can take these nuances into account.

I have not delved into the Assert method in question, but I can only assume the previous was deprecated for this kind of issues and the new one does take them into account.

User · Answer

Floating point calculations are not exact - there is often round-off errors  and errors due to representation   For example  0 1 cannot be exactly represented in binary floating point    Because of this  directly comparing two floating point values for equality is usually not a good idea  because they can be different by a small amount  depending upon how they were computed    The  delta   as it s called in the JUnit javadocs  describes the amount of difference you can tolerate in the values for them to be still considered equal  The size of this value is entirely dependent upon the values you re comparing  When comparing doubles  I typically use the expected value divided by 10 6

User · Answer

Assert assertTrue Math abs actual-expected     0

User · Answer

I just want to mention the great AssertJ library  It s my go to assertion library for JUnit 4 and 5 and also solves this problem elegantly  assertThat actual  isCloseTo expectedDouble  within delta

User · Answer

Note that if you re not doing math  there s nothing wrong with asserting exact floating point values  For instance   public interface Foo       double getDefaultValue       public class FooImpl implements Foo       public double getDefaultValue     return Double MIN VALUE        In this case  you want to make sure it s really MIN VALUE  not zero or -MIN VALUE or MIN NORMAL or some other very small value  You can say  double defaultValue   new FooImpl   getDefaultValue    assertEquals Double MIN VALUE  defaultValue     but this will get you a deprecation warning  To avoid that  you can call assertEquals Object  Object  instead      really you just need one cast because of autoboxing  but let s be clear assertEquals  Object Double MIN VALUE   Object defaultValue     And  if you really want to look clever   assertEquals      Double doubleToLongBits Double MIN VALUE        Double doubleToLongBits defaultValue       Or you can just use Hamcrest fluent-style assertions      equivalent to assertEquals  Object Double MIN VALUE   Object defaultValue   assertThat defaultValue  is Double MIN VALUE      If the value you re checking does come from doing some math  though  use the epsilon

User · Answer

Epsilon is a difference between expected and actual values which you can accept thinking they are equal  You can set  1 for example

User · Answer

Which version of JUnit is this  I ve only ever seen delta  not epsilon - but that s a side issue   From the JUnit javadoc      delta - the maximum delta between expected and actual for which both   numbers are still considered equal    It s probably overkill  but I typically use a really small number  e g   private static final double DELTA   1e-15    Test public void testDelta        assertEquals 123 456  123 456  DELTA       If you re using hamcrest assertions  you can just use the standard equalTo   with two doubles  it doesn t use a delta   However if you want a delta  you can just use closeTo    see javadoc   e g   private static final double DELTA   1e-15    Test public void testDelta        assertThat 123 456  equalTo 123 456        assertThat 123 456  closeTo 123 456  DELTA        FYI the upcoming JUnit 5 will also make delta optional when calling assertEquals   with two doubles  The implementation  if you re interested  is    private static boolean doublesAreEqual double value1  double value2        return Double doubleToLongBits value1     Double doubleToLongBits value2

User · Answer

Epsilon is the value that the 2 numbers can be off by   So it will assert to true as long as Math abs expected - actual   lt  epsilon

[java] Meaning of delta or epsilon argument of assertEquals for double values

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