It has previously 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. 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