Do Java 8 default methods break source compatibility?

Question

It has generally been the case the Java source code has been forward compatible. Until Java 8, as far as I know, both compiled classes and source have been forward compatible with later JDK/JVM releases. [Update: this is not correct, see comments re 'enum', etc, below.] However, with the addition of default methods in Java 8 this appears to no longer be the case.

For example, a library I have been using has an implementation of java.util.List which includes a List<V> sort(). This method returns a copy of the contents of the list sorted. This library, deployed as a jar file dependency, worked fine in a project being built using JDK 1.8.

However, later I had occasion to recompile the library itself using JDK 1.8 and I found the library no longer compiles: the List-implementing class with its own sort() method now conflicts with the Java 8 java.util.List.sort() default method. The Java 8 sort() default method sorts the list in place (returns void); my library's sort() method - since it returns a new sorted list - has an incompatible signature.

So my basic question is:

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

Also:

• Is this the first such forward incompatible change?
• Was this considered or discussed when default methods where designed and implemented? Is it documented anywhere?
• Was the (admittedly small) inconvenience discounted versus the benefits?

The following is an example of some code that compiles and runs under 1.7 and runs under 1.8 - but does not compile under 1.8:

import java.util.*;  public final class Sort8 {      public static void main(String[] args) {         SortableList<String> l = new SortableList<String>(Arrays.asList(args));         System.out.println("unsorted: "+l);         SortableList<String> s = l.sort(Collections.reverseOrder());         System.out.println("sorted  : "+s);     }      public static class SortableList<V> extends ArrayList<V> {          public SortableList() { super(); }         public SortableList(Collection<? extends V> col) { super(col); }          public SortableList<V> sort(Comparator<? super V> cmp) {             SortableList<V> l = new SortableList<V>();             l.addAll(this);             Collections.sort(l, cmp);             return l;         }      }  } 

The following shows this code being compiled (or failing to) and being run.

> c:\tools\jdk1.7.0_10\bin\javac Sort8.java  > c:\tools\jdk1.7.0_10\bin\java Sort8 this is a test unsorted: [this, is, a, test] sorted  : [this, test, is, a]  > c:\tools\jdk1.8.0_05\bin\java Sort8 this is a test unsorted: [this, is, a, test] sorted  : [this, test, is, a]  > del Sort8*.class  > c:\tools\jdk1.8.0_05\bin\javac Sort8.java Sort8.java:46: error: sort(Comparator<? super V>) in SortableList cannot implement sort(Comparator<? super E>) in List                 public SortableList<V> sort(Comparator<? super V> cmp) {                                        ^   return type SortableList<V> is not compatible with void   where V,E are type-variables:     V extends Object declared in class SortableList     E extends Object declared in interface List 1 error 

Answer 1

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":

10. 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.