dotPath

int item = Math.random() * 100000

def doCode = { return 'hi' }
Map $_ = [a: 1, b: ['c d': [z: item, doit: doCode]]]

use (UtilCategory) {

    assert $_.dotPath() == [a: 1, b: ['c d': [z: item, doit: doCode]]]
    assert $_.dotPath('') == [a: 1, b: ['c d': [z: item, doit: doCode]]]
    assert $_.dotPath(null) == [a: 1, b: ['c d': [z: item, doit: doCode]]]
    
    assert $_.dotPath('a') == 1
    assert $_.dotPath('b.c d.z') == item
    assert $_.dotPath('b/c d/z', '/') == item
    
    assert $_.dotPath('b.c d.doit')() == 'hi'
    
    assert $_.dotPath('b.c d.y') == null
    assert $_.dotPath('b.c d.z.z') == null
    assert $_.dotPath('b.c d.z.z.') == null
    assert 1.dotPath('toString') == null

}

//========================================

@Category(Object)
class UtilCategory {
    def dotPath(String path = '', String delimiter = /\./) {
        Object self = this
        
        if (!path) return self
    
        try {
            return path.split(/$delimiter/).inject(self) { x, i->
               x = x[i]
            }
        }
        catch (Exception ex) { return null }
    }
}

Groovy++ Mixin and Category

@Typed
package categoriestest

@Category(List)
@Typed
class Shuffler {
    def shuffleit() {
        def result = new ArrayList(this)
        Collections.shuffle(result)
        result
    }
}

// @Mixin(Shuffler)
@Typed
class Sentence extends ArrayList {
    Sentence(Collection initial) { super(initial) }
}

@Typed
class Main {
    static main(args) {
        def words = ["The", "quick", "brown", "fox"]
        
        Sentence.mixin Shuffler.class

        def sentence = new Sentence(words)
        println sentence.shuffleit()
    }
}

Main.main(null)

http://gppconsole.appspot.com/script/20001

see http://code.google.com/p/groovypptest/downloads/list

Category - Advanced

class StringCategory {
  static String camelize(String self) {
    def newName = self.split("_").collect() { 
      it.substring(0, 1).toUpperCase() + it.substring(1, it.length())
    }.join()
    
    newName[0..<1].toLowerCase() +  newName[1..-1]      
  }
  static Integer getPm(Integer self) {
      (self == 12 ? 12 : self + 12)
  }
  static Integer getAm(Integer self) {
      self == 12 ? 0 : self
  }
}

class SpeakCategory {
    static String shout(String self) {  // Method argument is String, so we can add shout() to String object.
        self.toUpperCase() + '!!'
    }
    static String whisper(String self, boolean veryQuiet = false, Integer a = 0) {
        "${veryQuiet ? 'sssssssh' : 'sssh'}.. $self $a"
    }
    static String army(String self) {
        "$self. Sir, yes sir!"
    }
}

use (StringCategory) {
    println 'Hi_there_fella'.camelize()
    println 12.am + ' o\'clock'
    println 2.pm + ' o\'clock'
}

use (SpeakCategory) {
    println "Pay attention".shout()
    println "Be vewy, vewy, quiet.".whisper()
    println "Be vewy, vewy, quiet.".whisper(true, 1)
    println "Groovy rocks".army()
}

// Or we can use the @Category annotation.
// implicit 'this', no 'self' arg passed in
@Category(String) 
class StreetTalk {
    String hiphop(Integer times = 1) {
        "Yo, yo, here we go! " * times + "${this}"
    }
}

use(StreetTalk) {
    println 'Groovy is dope!'.hiphop(2)
}

http://groovyconsole.appspot.com/script/448002

Category - Simple

class MathCategory {
    static Number sin(Number self, Number add = 0){Math.sin(self) + add}
    static Number cos(Number self, Number add = 0){Math.cos(self) + add}
}

use(MathCategory) {
    println 42.sin()
    println 42.sin(1)
    println 42f.sin()
    println 42f.class.name
    println 42G.cos()
    println 42G.cos(2)
    println 42G.class.name
}

http://groovyconsole.appspot.com/script/451001