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javaimmutabilityimmutables-library

Mutable items in Immutable

When we create immutable classes using the Immutable objects library, how can we handle mutable members (e.g. j.u.Date)?

NOTE: this is not about the java Date class and totally related to the Immutable Objects java library which will generate some code!

Example:

@Value.Immutable
public interface MyImmutableClass {
    Date creationDateTime();
}

Is there a way to override the getter, so that it returns a copy?

public Date creationDateTime() {
    return new Date(creationDateTime.getTime());
}
Solution

  • From my question in the Immutables issue tracker I learned that the cleanest way to handle mutable objects is to use a custom encoding annotation.

    I've created a little open source project tmtron-immutables encoding to create such an annotation for the java.util.Date class. This should be a good starting point for anyone who wants to create a custom encoding.

    Then you can directly use your mutable class (e.g. java.util.Date) as attribute and still get the same immutability guarantee as for immutable attributes (like String, long, etc.)

    @Value.Immutable
@DateEncodingEnabled
public interface ValueObject {
    Date creationDate();
}

    The @DateEncodingEnabled annotation will make sure that only the immutable long value of the Date object is stored in the ImmutableValueObject class.