How do you override a method for a java type instance with Groovy meta programming?

Question

I am trying to override the functionality of a method of a java type instance in my Groovy code but I am getting a classcast exception.

I looked at the guide posted her

Answer 1

You are creating an ExpandoMetaClass for java.lang.String, but assigning it to a groovy.util.Proxy. Make a metaClass for groovy.util.Proxy instread, like so:

java.lang.String hey = new java.lang.String("hey")
def proxiedHey = new groovy.util.Proxy().wrap(hey)
ExpandoMetaClass emc = new ExpandoMetaClass( groovy.util.Proxy, false )
emc.substring = {
    "This is not a very good substring implementation"
}
emc.initialize()

proxiedHey.setMetaClass(emc)
printf proxiedHey.toString()
printf proxiedHey.substring(1)

Answer 2

Have you looked at Pimp my Library Pattern which allows you to add using Groovy Categories. You might find it more convenient and easy to understand in your case.

@Category(String)
class StringSubstrCategory {      
    def substring( int n)   { 
        "This is not a very good substring implementation"
    }
}

use (StringSubstrCategory) {
    "hey".substring(1)
}