Reasons for a Java Lang Incompatible Class Change Error

The incompatible class change error comes from only recompiling one project file.

Not all errors Java programmers encounter are results of the code they write. Some errors, such as the incompatible class change error, come from the way that the Java compiler created bytecode files for the Java Virtual Machine (JVM) to execute. This error happens when programmers rebuild one class file without rebuilding the other files in the application.

Multiple Class Files

While simple Java programs can fit in a single source code file, good programming style requires developers to spread more sophisticated functions out across multiple classes. Good object-oriented programming style, in turn, requires that programmers use a separate source code file for each class they create. The presence of multiple class files or the fact that all the classes are stuck inside a single file make no difference to the JVM. Spreading classes out across multiple files does make it much easier for developers to develop and maintain a program, however.

Code Compilation

When developers initially compile a Java program, the program consults the source file for every class the application's code references. Based on the current state of those files, it will build bytecode class files with the necessary links between each other and the individual methods within those class files. As long as the programmer bundles these all together in a single package, the program will execute normally.

Error Definition

If the links a compiler creates between class files break, then the program will not work. The "Java Lang Incompatible Class Change Error" means that the bytecode code for one of the program's class files has changed in such as to break these links. This can happen when developers make changes to, and recompile, the source code for one of a program's classes. A significant change to the source code's content will change the bytecode file's structure so that other class files will be unable to communicate with it.

Resolution

This error happens when programmers edit and recompile a class file in isolation from the project's other files. Fixing the issue is a simple matter. Programmers have to go back and recompile all the program's source code files at the same time. This allows the Java compiler to update the other bytecode files to incorporate the changes the programmer made, re-establishing the links between the files and restoring their compatibility.

References

• "Core Java Volume I: Fundamentals": Cay S. Horstmann and Gary Cornell
• Oracle Java Platform: Class IncompatibleClassChangeError