Are there any specific examples of backward incompatibilities between Java versions?

Question

Have there been incompatibilities between Java releases where Java source code/Java class files targeting Java version X won\'t compile/run under version Y (where Y > X) ?

Answer 1

The semantics of the memory model changed from 1.4 to 1.5. It was changed to allow besides other things double checked locking again. (I think volatile semantics were fixed.) It was broken.

Answer 2

Every release of Swing broke something for us, from 1.3 through 1.6.

The JDBC issue has already been mentioned, but existing code worked.

From 1.5 to 1.6 there was a change in the behavior of Socket which broke the Cisco client.

Of course new reserved keywords were introduced.

The big one which I think was truely unforgivable on Sun's part was System.getenv(). It worked in 1.0, and then was deprecated and changed to throw an error on all platforms under the rather dubious justification that the Mac didn't have system environment variables. Then the Mac got system environment variables, so in 1.5 it was undeprecated and works. There is no reasonable justification for doing that. Return an empty set on a Mac (Swing has much bigger cross-platform issues if you want to care about that level of cross platform consistency) or even on all platforms.

I never agreed with them turning off the feature, but to change it to throw an error was just a pure breaking change that if they were going to do, they should have just removed the method entirely.

But, really from 1.0 to 1.1 they were less concerned about backwards compatability. For example, they dropped "private protected" as a modifier.

So the upshot is that every version changes enough to require close evaluation, that is why you still see many 1.4 questions here on SO.

Answer 3

Obviously the naming convention of release names is not backwards-compatible.

• JDK 1.0 (January 23, 1996)
• JDK 1.1 (February 19, 1997)
• J2SE 1.2 (December 8, 1998)
• J2SE 1.3 (May 8, 2000)
• J2SE 1.4 (February 6, 2002)
• J2SE 5.0 (September 30, 2004)
• Java SE 6 (December 11, 2006)
• Java SE 6 Update 10, Update 12, Update 14, Update 16
• Java SE 7 ??? JDK7?

(The list is from Wikipedia.)

Answer 4

Compatibility notes for various versions:

• Java 1.4
• Java 5
• Java 6
• Java 7
• Java 8

The first major hiccup I remember was the introduction of assert in Java 1.4. It affected a lot of JUnit code.

Answer 5

The interface java.sql.Connection was extended from Java 1.5 to Java 1.6 making compilation of all classes that implemented this interface fail.

Answer 6

The main one that I can think of is the introduction of new reserved words:

Java 1.3: strictfp
Java 1.4: assert
Java 5.0: enum

Any code that previously used these values as identifiers would not compile under a later version.

One other issue that I remember causing problems on a project that I worked on was that there was a change in the default visibility of JInternalFrames between 1.2 and 1.3. They were visible by default, but when we upgraded to 1.3 they all seemed to have disappeared.