I just hit very strange (to me) behaviour of java. I have following classes:

public abstract class Unit {
public static final Unit KM = KMUnit.INSTANCE;
public static final Unit METERS = MeterUnit.INSTANCE;
protected Unit() {
}
public abstract double getValueInUnit(double value, Unit unit);
protected abstract double getValueInMeters(double value);
}

And:

public class KMUnit extends Unit {
public static final Unit INSTANCE = new KMUnit();

private KMUnit() {
}
//here are abstract methods overriden
}

public class MeterUnit extends Unit {
public static final Unit INSTANCE = new MeterUnit();

private MeterUnit() {
}

///abstract methods overriden
}

And my test case:

public class TestMetricUnits extends TestCase {

@Test
public void testConversion() {
    System.out.println("Unit.METERS: " + Unit.METERS);
    System.out.println("Unit.KM: " + Unit.KM);
    double meters = Unit.KM.getValueInUnit(102.11, Unit.METERS);
    assertEquals(0.10211, meters, 0.00001);
}
}

1) MKUnit and MeterUnit are both singletons initialized statically, so during class loading. Constructors are private, so they can't be initialized anywhere else. 2) Unit class contains static final references to MKUnit.INSTANCE and MeterUnit.INSTANCE

I would expect that:

• KMUnit class is loaded and instance is created.
• MeterUnit class is loaded and instance is created.
• Unit class is loaded and both KM and METERS variable are initialized, they are final so they cant be changed.

But when I run my test case in console with maven my result is:

T E S T S

Running de.audi.echargingstations.tests.TestMetricUnits
Unit.METERS: m
Unit.KM: null
Tests run: 3, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 0.089 sec <<< FAILURE! - in de.audi.echargingstations.tests.TestMetricUnits
testConversion(de.audi.echargingstations.tests.TestMetricUnits) Time elapsed: 0.011 sec <<< ERROR!
java.lang.NullPointerException: null
at
de.audi.echargingstations.tests.TestMetricUnits.testConversion(TestMetricUnits.java:29)

Results :

Tests in error: TestMetricUnits.testConversion:29 NullPointer

And the funny part is that, when I run this test from eclipse via JUnit runner everything is fine, I have no NullPointerException and in console I have: Unit.METERS: m Unit.KM: km

So the question is: what can be the reason that KM variable in Unit is null (and in the same time METERS is not null)