Looking to extend inner classes in Groovy - groovy

Question

I'm looking to extend an inner class in Groovy, it's part of a mix-in pattern that needs a groovy calculation method to be added after class construction but natively use the accessors of the main class. Works in Java, not so much in Groovy. Help appreciated.
p.s. realize that with { } groovy construct may be an alternative... trying to get this to work
Base class (in Java):
public class JavaClass {
private String foo = "Foo";
private Calculation calculation;
public JavaClass() {
calculation = new Calculation();
}
public JavaClass(Calculation calculation) {
this.calculation = calculation;
}
public String getFoo() {
return foo;
}
public String getBar() {
return calculation.getBar();
}
public class Calculation {
public String getBar() {
return null;
}
}
}
I create a Groovy implementation: (I know it's a bit hokey, bear with me)
class GroovyCalculationClass extends JavaClass {
public GroovyCalculationClass() { }
public Calculation getCalculation() {
return new Calculation();
}
public class Calculation extends JavaClass.Calculation {
public String getBar() {
return getFoo() + "Bar";
}
}
}
Run this test:
class TestCalculation {
#Test
public void testCalculation() {
JavaClass javaClass = new JavaClass(
new GroovyCalculationClass().getCalculation());
Assert.assertEquals("FooBar", javaClass.getBar());
}
}
And get this:
java.lang.ArrayIndexOutOfBoundsException: 0
at com.modelshop.datacore.generator.GroovyCalculationClass$Calculation.(GroovyCalculationClass.groovy)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:525)
at org.codehaus.groovy.reflection.CachedConstructor.invoke(CachedConstructor.java:77)
at org.codehaus.groovy.runtime.callsite.ConstructorSite$ConstructorSiteNoUnwrapNoCoerce.callConstructor(ConstructorSite.java:102)
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCallConstructor(CallSiteArray.java:57)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callConstructor(AbstractCallSite.java:182)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callConstructor(AbstractCallSite.java:190)
at com.modelshop.datacore.generator.GroovyCalculationClass.getCalculation(GroovyCalculationClass.groovy:12)
at com.modelshop.datacore.generator.GroovyCalculationClass$getCalculation.call(Unknown Source)
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:45)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:108)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:112)
at com.modelshop.datacore.generator.TestCalculation.testCalculation(TestCalculation.groovy:17)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:45)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:42)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:263)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:68)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:47)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:231)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:60)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:229)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:50)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:222)
at org.junit.runners.ParentRunner.run(ParentRunner.java:300)
at org.junit.runner.JUnitCore.run(JUnitCore.java:157)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:77)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:195)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:63)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)
Replace GroovyCalculationClass with identical JavaCalculationClass and everything works.