I was messing around with PicoContainer and inadvertently got it to throw a new RuntimeException with the message, "Should never get here." Here are the two files: Bowl.java: import org.picocontainer.MutablePicoContainer; import org.picocontainer.defaults.DefaultPicoContainer; class Bowl { private final Fish[] fishes; private final Cod[] cods; public Bowl(Fish[] fishes, Cod[] cods) { this.fishes = fishes; this.cods = cods; } public Fish[] getFishes() { return fishes; } public Cod[] getCods() { return cods; } public static void main( String[] args ) { MutablePicoContainer pico = new DefaultPicoContainer( ) ; pico.registerComponentImplementation(Shark.class); pico.registerComponentImplementation(Cod.class); pico.registerComponentImplementation(Bowl.class); Bowl bowl = (Bowl) pico.getComponentInstance(Bowl.class); Fish[] fishes = bowl.getFishes( ) ; for( int i = 0 ; i < fishes.length ; i++ ) System.out.println( "fish["+i+"]="+fishes[i] ) ; } } Fish.java: public interface Fish { } class Cod implements Fish { static int i = 0 ; int instanceNum ; public Cod( ) { instanceNum = i++ ; } ; public String toString( ) { return "Cod #" + instanceNum ; } } class Shark implements Fish { static int i = 0 ; int instanceNum ; public Shark( ) { instanceNum = i++ ; } ; public String toString( ) { return "Shark #" + instanceNum ; } } Here is the stack trace: java.lang.RuntimeException: Should never get here at org.picocontainer.defaults.ConstructorInjectionComponentAdapter.instantiateComponent(ConstructorInjectionComponentAdapter.java:210) at org.picocontainer.defaults.InstantiatingComponentAdapter.getComponentInstance(InstantiatingComponentAdapter.java:45) at org.picocontainer.defaults.DecoratingComponentAdapter.getComponentInstance(DecoratingComponentAdapter.java:42) at org.picocontainer.defaults.CachingComponentAdapter.getComponentInstance(CachingComponentAdapter.java:37) at org.picocontainer.defaults.GenericCollectionComponentAdapter.getArrayInstance(GenericCollectionComponentAdapter.java:79) at org.picocontainer.defaults.GenericCollectionComponentAdapter.getComponentInstance(GenericCollectionComponentAdapter.java:66) at org.picocontainer.defaults.ConstructorInjectionComponentAdapter.getConstructorArguments(ConstructorInjectionComponentAdapter.java:220) at org.picocontainer.defaults.ConstructorInjectionComponentAdapter.instantiateComponent(ConstructorInjectionComponentAdapter.java:195) at org.picocontainer.defaults.InstantiatingComponentAdapter.getComponentInstance(InstantiatingComponentAdapter.java:45) at org.picocontainer.defaults.DecoratingComponentAdapter.getComponentInstance(DecoratingComponentAdapter.java:42) at org.picocontainer.defaults.CachingComponentAdapter.getComponentInstance(CachingComponentAdapter.java:37) at org.picocontainer.defaults.DefaultPicoContainer.getComponentInstance(DefaultPicoContainer.java:297) at tutorial.Bowl.main(Bowl.java:31) Exception in thread "main" The IllegalAccessException caught in line 210 of ConstructorInjectionComponentAdapter is: java.lang.IllegalAccessException: Class org.picocontainer.defaults.ConstructorInjectionComponentAdapter can not access a member of class tutorial.Shark with modifiers "public" This is because Shark's constructor has package-level protection (despite what Java says about it being public) ant so it's not accessible to classes in the Pico packages. I guess there's nothing you can do about it, but it can definitely happen and Pico should watch out for it!