How to handle groovy ambiguous method overload gracefully - groovy

Question

I know there are similar questions but the answers are not satisfying.
I get an Groovy ambiguous method overload error when calling a method with null as parameter.
e.g.:
class A{
sampleMethod (B bObj){
if(bObj == null) {
handleNullArgumentGracefully()
}
... do some cool stuff ...
}
sampleMethod (C cObj){
... do some other cool stuff ...
}
}
now when i call sampleMethod(null) groovy does not know which method it should invoke. Thats clear but is there a possibility to set one method of these two as default method to handle such null calls? I want to handle this on the callee side and not on the caller side (i do not want to cast something on the caller side)
UPDATE:
i found a solution how it could work but i don´t know why: convert the non default method to a closure property
class app {
static void main(String[] args) {
def a = new A()
a.sampleMethod(new B())
a.sampleMethod(new C())
a.sampleMethod(null)
}
}
class A {
def sampleMethod(B bObj = null) {
if (bObj == null) {
println("handle null")
}
println("1")
}
def sampleMethod = { C cObj ->
println("2")
}
}
class B {
}
class C {
}

Answer 1

the following will fail with Ambiguous method overloading for method A#sampleMethod
class A{
def sampleMethod (Number o=null){
println "num $o"
}
def sampleMethod (String o){
println "str $o"
}
}
new A().sampleMethod(null)
this one will work (Object will be called for null):
class A{
def sampleMethod (Number o=null){
println "num $o"
}
def sampleMethod (String o){
println "str $o"
}
def sampleMethod(Object o){
println "obj $o"
}
}
new A().sampleMethod(null)
but i like this one:
class A{
def _sampleMethod (Number o){
println "num $o"
}
def _sampleMethod (String o){
println "str $o"
}
def sampleMethod(Object o){
if(o==null){
println "null"
return null
}else if(o instanceof Number){
return _sampleMethod ((Number) o)
}else if(o instanceof String){
return _sampleMethod ((String) o)
}
throw new IllegalArgumentException("wrong argument type: ${o.getClass()}")
}
}
new A().sampleMethod(null)