Does making all fields Final makes the class Immutable in Java?

One of the common misconceptions among many Java Programmer is that a class with all final fields automatically becomes Immutable. This is not correct, you can easily break immutability of certain class if the final field it contains is a mutable one, as we'll see in this article. One of the most common examples of this is a java.util.Date. You have to be extra cautious to keep your class' immutability intact with mutable fields. When you return a reference to a mutable object, you are sharing ownership of that reference with whoever receives it. This can break invariant, such as immutability.

Another example of this kind of pattern which can break immutability is returning collection or array from the getters method .

So, even though, the field which is pointing to Date or Collection or array object is final, you can still break the immutability of the class by breaking Encapsulation by returning a reference to the original mutable object.


There are two ways to avoid this problem, first, don't provide getters to mutable objects if you can avoid it. If you must, then consider returning a copy or clone of the mutable object. If you are returning a collection, you could wrap it as an unmodifiable collection. Since we cannot make an array final or unmodifiable in Java