Mutable and immutable objects are both common. In Java, ArrayList is mutable, but String is not. If you concatenate strings, it makes a new string. It never

Mutable and immutable objects are both common. In Java, ArrayList is mutable, but String is not. If you concatenate strings, it makes a new string. It never changes the existing one. This maps pretty well to peoples' intuitions when using strings. If you have a reference to a string, its' contents can never change. You can assign a new string to your variable, but that's different. In JS, the contents of a Date object can change. This is widely regarded to be a mistake, and I agree with that.