- values of the instance variables never change once they have been set in the constructor
- equality is not based on identity
- two value objects are equal when their values are equal, not necessarily being the same object.
- this protects from aliasing problems, ie. bugs caused by changing an object is being referred in two different places.
Money is a good example of value object:
a = Money.new(5)
b = Money.new(5)
a == b #=> true