An immutable object is an object that cannot be changed (its state is fixed) once it is created.

In Java, the following applies:

• Primitive types are immutable.
• The famous Java String class is immutable (other classes are immutable as well, for example, Pattern, and LocalDate)
• Arrays are not immutable.
• Collections can be mutable, unmodifiable, or immutable.

An unmodifiable collection is not automatically immutable. It depends on which objects are stored in the collection. If the stored objects are mutable, then the collection is mutable and unmodifiable. But if the stored objects are immutable, then the collection is effectively immutable.

Immutable objects are useful in concurrent (multithread) applications and streams. Since immutable objects cannot be changed, they are impassible to concurrency issues and they don't risk being corrupted or inconsistent.

One of the main concerns of using immutable objects is related to the penalties of creating new objects, instead of managing the state of a mutable object. But keep in mind that immutable objects take advantage of special treatment during garbage collection. Moreover, they are not prone to concurrency issues and eliminate the code needed for managing the state of the mutable objects. The code necessary to manage the state of mutable objects is prone to be slower than the creation of new objects.

Looking at the following problems will allow us to dive deeper into object immutability in Java.