Groovy's operator override Test.

opeOverrideTest.groovy

class NumHolder {
  int num = 0
  def plus(int n) { num += n; println 'puls!'; this }
  def minus(int n) { num -= n; println 'minus!'; this }
  def negative() { num = -num; println 'negative!'; this }
}

def nh = new NumHolder()
assert 0 == nh.num
nh += 1
assert 1 == nh.num
nh = -nh
assert -1 == nh.num
nh -= 1
assert -2 == nh.num

puls! negative! minus!