不可变的可变项

Question

When we create immutable classes using the Immutable objects library, how can we handle mutable members (eg juDate )?

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());
}

Answer 1

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

Pretty much like you've written it there:

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

(like khelwood pointed out in the comments above).

However, if you want to avoid accidental mutation of creationDateTime inside your class, consider just storing the millis as a final long :

private final creationDateTimeMillis;

public Date creationDateTime() {
    return new Date(creationDateTimeMillis);
}

Whereas you can call Date.setTime() even if the Date is final , thus mutating the internal state, you can't reassign creationDateTimeMillis .

Answer 2

You can make the generated method protected and serve the field only from a cloning getter method:

@Value.Immutable
public abstract class WrapMutable {
    protected abstract Date timestamp();

    public Date getTimestamp() {
        return new Date(timestamp().getTime());
    }
}

All usage of the field is then through its copy-getter, while the timestamp() method is only used to define the setter in the builder:

WrapMutable obj = ImmutableWrapMutable.builder().timestamp(new Date()).build();
System.out.println(obj.getTimestamp());
System.out.println(obj.timestamp()); // Error: timestamp() has protected access in WrapMutable

Answer 3

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 (eg 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.