Aliasing occurs when the same memory location is accessed through more than one reference. Often this is a good thing, but frequently it occurs in an unexpected way, which leads to confusing bugs.

Here's a simple example of the bug.

Date retirementDate = new Date(Date.parse("Tue 1 Nov 2016")); // this means we need a retirement party Date partyDate = retirementDate; // but that date is a Tuesday, let's party on the weekend partyDate.setDate(5); assertEquals(new Date(Date.parse("Sat 5 Nov 2016")), retirementDate); // oops, now I have to work three more days :-(

What's happening here is that when we do the assignment, the partyDate variable is assigned a reference to the same object that the retirement data refers to. If I then alter the internals of that object (with setDate ) then both variables are updated, since they refer to the same thing.

Although aliasing is a problem in that example, in other contexts it's what I expect.

Person me = new Person("Martin"); me.setPhoneNumber("1234"); Person articleAuthor = me; me.setPhoneNumber("999"); assertEquals("999", articleAuthor.getPhoneNumber());

It's common to want to share records like this, and then if it changes, it changes for all references. This is why it's useful to think of reference objects, which we deliberately share , and Value Objects that we don't want this kind of shared update behavior. A good way to avoid shared updates of value objects is to make value objects immutable.

Functional languages, of course, prefer everything to be immutable. So if we want changes to be shared, we need to handle that as the exception rather than the rule. Immutability is a handy property, one that makes it harder to create several kinds of bugs. But when things do need to change, immutability can introduce complexity, so it's by no means a free breakfast.

Acknowledgements Graham Brooks and James Birnie's comments on our internal mailing list led me to write this post.