I am trying to create a subclass of HashMap with its own string representation. I figured that I just needed to override toString() in the subclass:

class Foo extends HashMap {
  String toString() {
    "foo"
  }
}

This works, insofar as I can do (new Foo()).toString() and it returns the string foo. However, in a string interpolation context (or when using as String), the toString implementation of HashMap seems to rule:

Foo test = new Foo()
test["abc"] = 123

println "${test.toString()}"  // Prints: foo
println "${test}"             // Prints: [abc:123]
println (test as String)      // Prints: [abc:123]

I’m curious why this happens and what — if anything — I can do to get around it?