Groovy meta-programming to 'bump' a counter in a map

map_counter_bump.groovy

/**
 * Copying and distribution of this file, with or without modification,
 * are permitted in any medium without royalty provided the copyright
 * notice and this notice are preserved.  This file is offered as-is,
 * without any warranty.
 *
 * @author Robin Bramley (c) 2011
 *
 * Groovy meta-programming to 'bump' a counter in a map for conciseness.
 */

Map.metaClass.bump = { key ->
     if(delegate.containsKey(key)) {
        delegate.put(key, 1 + delegate.get(key))
    } else {
        delegate.put(key,1)
    }
}

m1 = [:]
m1.bump('foo')
assert m1['foo'] == 1
m1.bump('foo')
assert m1['foo'] == 2

m2 = [:]
m2 << m1
m2.bump('bar')
assert m2['foo'] == 2
assert m2['bar'] == 1