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) ?

By "Java release" I mean versions such as:

• JDK 1.0 (January, 1996)
• JDK 1.1 (February, 1997)
• J2SE 1.2 (December, 1998)
• J2SE 1.3 (May, 2000)
• J2SE 1.4 (February, 2002)
• J2SE 5.0 (September, 2004)
• Java SE 6 (December, 2006)

House rules:

• Please include references and code examples where possible.
• Please try to be very specific/concrete in your answer.
• A class that is being marked as @Deprecated does not count as a backwards incompatibility.

Answer 1

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 2

First of all, Sun actually considers all of the releases you mentioned (other than 1.0 of course) to be minor releases, not major ones.

I am unaware of any examples of binary incompatibility in that time. However, there have been some examples of source incompatibility:

• In Java 5, "enum" became a reserved word; it wasn't before. Therefore, there were source files that used enum as an identifier that would compile in java 1.4 that wouldn't compile in java 5.0. However, you could compile with -source 1.4 to get around this.

• Adding methods to an interface can break source compatibility as well. If you implement an interface, and then try to compile that implementation with a JDK that adds new methods to the interface, the source file will no longer compile successfully, because it doesn't implement all of the interface's members. This has frequently happened with java.sql.Statement and the other jdbc interfaces. The compiled forms of these "invalid" implementations will still work unless you actually call one of the methods that doesn't exist; if you do that, a MissingMethodException will be thrown.

These are a few examples I can recall off of the top of my head, there may be others.