Backward compatibility

The maintainers of JVM and Java Class Library understand the needs of business developers. The code that is written today should ideally run tomorrow. JVM offers reasonable backward compatibility. Developers familiar with Python 2 and 3 will know that this is not a given in the industry.

Newer JVM versions can run applications that were compiled for older JVM versions, as long as the application's code does not use APIs or technologies that were removed from the JVM version that is running the application. Here's an example: libraries compiled for Java 6 can still be loaded and used in projects that run on a Java 8 JVM instance. But this is not the case the other way around; applications running on a Java 6 JVM instance cannot load classes compiled for later versions.

Of course, like every other platform or language, the JDK and Java Class Library maintainers have to deprecate classes and whole technologies from time to time. While there are issues, backward compatibility on JVM is generally better than many other platforms and languages. Also, APIs are generally only removed if proper and well-documented alternatives exist.