Java immutable classes? [duplicate]

Question

I found an article with an interesting piece of code:

public class Employee {

    private String firstName;
    private String lastName;

    //private default constructor
    private Employee(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public static Employee valueOf (String firstName, String lastName) {
        return new Employee(firstName, lastName);
    }
}

I am really curious in understanding the advantage of creating this kind of classes. I understand that here that an object of this class would be immutable, because there is no way of changing its variable values once initialized. I never did something like this before, and i dont really understand the advantage of it.

• Why is it a good practice?
• Could you name a situation where this approach can be used?
• What about constants or read only variables? Is not that very similar?
• In the article says, that this is not good for the performance of the application. But why?

Answer 1

Immutable classes are:

• thread-safe by default (concurrent write never occurs)
• cachable

You can read a lot about them in the conext of the language Java in Effective Java.

Answer 2

The example you have mentioned is of an Immutable Objects. Its widely used concepts in programming languages.

Quoting from the link above. The advantages are

• are simple to construct, test, and use
• are automatically thread-safe and have no synchronization issues
• do not need a copy constructor
• do not need an implementation of clone
• allow hashCode to use lazy initialization, and to cache its return value
• do not need to be copied defensively when used as a field
• make good Map keys and Set elements (these objects must not change state while in the collection)
• have their class invariant established once upon construction, and it never needs to be checked again
• always have "failure atomicity" (a term used by Joshua Bloch) : if an immutable object - throws an exception, it's never left in an undesirable or indeterminate state