Do Java 8 default methods break source compatibility?

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Doesn’t JDK 1.8 introduce a forward incompatibility for Java source code due to default methods?

Any new method in a superclass or interface can break compatibility. Default methods make it less likely that a change in an interface will break compatibility. In the sense that default methods open the door to adding methods to interfaces, you could say that default methods may contribute to some broken compatibility.

Is this the first such forward incompatible change?

Almost certainly not, since we’ve been subclassing classes from the standard library since Java 1.0.

Was this considered or discussed when default methods were designed and implemented? Is it documented anywhere?

Yes, it was considered. See Brian Goetz’s August 2010 paper “Interface evolution via “public defender” methods”:

  1. Source compatibility

It is possible that this scheme could introduce source incompatibilities to the extent that library interfaces are modified to insert new methods that are incompatible with methods in existing classes. (For example, if a class has a float-valued xyz() method and implements Collection, and we add an int-valued xyz() method to Collection, the existing class will no longer compile.)

Was the (admittedly small) inconvenience discounted versus the benefits?

Before, changing an interface would definitely break compatibility. Now, it might. Going from ‘definitely’ to ‘might’ can be seen either positively or negatively. On the one hand, it makes it feasible to add methods to interfaces. On the other hand, it opens the door to the kind of incompatibility you saw, not just with classes, but with interfaces too.

The benefits are larger than the inconveniences, though, as cited at the top of Goetz’s paper:

  1. Problem statement

Once published, it is impossible to add methods to an interface without breaking existing implementations. The longer the time since a library has been published, the more likely it is that this restriction will cause grief for its maintainers.

The addition of closures to the Java language in JDK 7 place additional stress on the aging Collection interfaces; one of the most significant benefits of closures is that it enables the development of more powerful libraries. It would be disappointing to add a language feature that enables better libraries while at the same time not extending the core libraries to take advantage of that feature.

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