## Limitations

When using ProGuard, you should be aware of a few technical issues, all of which are easily avoided or resolved:

- For efficiency, ProGuard always ignores any private or package visible library classes while reading library jars. If any of them are extended by public library classes, and then extended again by input classes, ProGuard will complain it can't find them. In that case, you'll have to use the `-dontskipnonpubliclibraryclasses` option, and maybe even the `-dontskipnonpubliclibraryclassmembers` option. The graphical user interface has checkboxes for these settings.

- For best results, ProGuard's optimization algorithms assume that the processed code never intentionally throws NullPointerExceptions or ArrayIndexOutOfBoundsExceptions, or even OutOfMemoryErrors or StackOverflowErrors, in order to achieve something useful. For instance, it may remove a method call `myObject.myMethod()` if that call wouldn't have any effect. It ignores the possibility that `myObject` might be null, causing a NullPointerException. In some way this is a good thing: optimized code may throw fewer exceptions. Should this entire assumption be false, you'll have to switch off optimization using the `-dontoptimize` option.

- If an input jar and a library jar contain classes in the same package, the obfuscated output jar may contain class names that overlap with class names in the library jar. This is most likely if the library jar has been obfuscated before, as it will then probably contain classes named 'a', 'b', etc. Packages should therefore never be split across input jars and library jars.

- When obfuscating, ProGuard will write out class files named "`a.class`", "`b.class`", etc. If there is a large numbers of classes in the same package, it may also write out "`aux.class`". Windows doesn't allow creating files with this reserved name (among a few other names), so it's generally better to write the output to a jar, in order to avoid such problems.