With the file extension class, CLASS files of the Java programming language are identified. (Classes Are Programming Constructs) Source code files (file extension java) are compiled into so-called (virtual) machine-readable CLASS files and contain bytecode. This bytecode is only executable in a Java virtual machine (JVM). Since there are JVMs for many different systems, Java is considered a platform-independent programming language.
Each CLASS present in a source code is compiled into a separate CLASS file, which is usually summarized in a jar file. If a JVM is installed, java files can be compiled with the javac command. Alternatively one can also use a Java IDE (integrated development environment), for example Eclipse for Java.
Since there are different versions of the JVMs (1.4, 1.5 and 1.6), it may happen that a CLASS file on certain JVMs may not work. Note that JVMs CLASS can only execute files but not generate them.
The developer of Java - Sun Microsystems - has since been bought by Oracle. A common name for the JVMs is the Java Runtime Environment (JRE).
Most often resolving problems with opening CLASS files is very simple. Just install an appropriate program that supports such files. All of the listed programs support CLASS files, but may vary in offered function and purpose. Some programs may be capable only of viewing contents of CLASS files or offer file conversion options, but may not be capable of editing such files.
It is possible that although a compatible program has been installed on user’s system, it is not used by the system to open it. This may happen because no default application was associated with this type of files. To associate CLASS files with given application, select "Open with" from drop-down menu (accessed by right-clicking on the file). Then select from the list the program or application you want to be used to open this type of file. If the Apple Jar Launcher applications is not on the list, select "Browse" option in order to manually locate the directory where Apple Jar Launcher has been installed.
Sometimes problems with opening CLASS files may not lie with the application itself, but can arise due to other causes, such as: