Put a key named metaClass in a Map

Question

Hi, I've written a small Groovy script that exposes a very weird behaviour. can anyone explain it ?

// Creating a groovy map 
def map = [:] 
// Putting a value in 
map["a"]="b"
// Render it without trouble 
println map["a"] 
// Putting another value in (yup, this one has THE name) 
map["metaClass"]="c"
// Failing to render it 
println map["metaClass"]

My question in that case is quite simple : why does the last instruction will throw the exception below :

org.codehaus.groovy.runtime.typehandling.GroovyCastException: Cannot cast object 'c' with class 'java.lang.String' to class 'groovy.lang.MetaClass'
at Script1.run(Script1.groovy:8)

Answer 1

The problem is that:

map["metaClass"]="c"

is the same as writing:

map.metaClass = "c"

I am guessing that before it delegates to the Map.put(x,y) method, it checks to see if a setXxxx method exists on the object.

As there is a method (in every object in Groovy) called setMetaClass(), it then calls this method instead of setting the property in the map (and fails to cast "c" to a metaClass object, as you have seen)

Workarounds:

• Don't have a key called metaClass
• Use map.put( 'metaClass', 'c' ) instead of the groovy magic