Obfuscate the code with a tool or library.
For more information on this topic, see: https://www.owasp.org/index.php/Bytecode_obfuscation#How_to_prevent_Java_code_from_being_Reverse-engineered_.3F which describes:
How to prevent Java code from being Reverse-engineered ?
Several actions can be taken for preventing reverse-engineering :
Code Obfuscation.
This is done mainly through variable renaming; see next paragraph for more precisions,
Suppression of End Of Line Characters. This makes the code difficult to parse,
Use of anonymous classes for handling events. This seems not to be handled by many Decompiler; however, JAD copes pretty well with this.
Class file encryption.
This implies some overhead for uncyphering at runtime. Several tools are available:: Canner, by Cinnabar Systems, or JLock by JSoft. They are available for evaluation, and the first is proposed currently for Windows Platforms only.
Replacing the method names with certain characters e.g '/' or '.' in the class header causes the popular decompilation tools such as JAD to dump the source code which is incomprehensible (you cannot determine the control flow from the source).
Note: Beware of 100% Java solutions using encryption to protect class files as these are more than likely snake oil. Since the JVM has to read unencrypted class files at some point, even if the class files are encrypted on the disk, they will have to be decrypted before being passed to the JVM. An attacker could modify the local JVM to simply write the class files to disk in their unencrypted form at this point. (See: Javaworld article).
Conjecture: It's is very easy to circumvent these methods to reveal bytecode using a Java profiler.
What obfuscation tools are available ?
A lot of tools exist for Java code Obfuscation. You can find extensive lists under following URLs, or simply type 'obfuscator' in your favorite search engine :http://directory.google.com/Top/Computers/Programming/Languages/Java/Development_Tools/Obfuscators/
http://proguard.sourceforge.net/alternatives.html
KlassMaster, shrinks and obfuscates both code and string constants. It can also translate stack traces back to readable form if you save the obfuscation log.
Proguard is a shrinker (make code more compact), and optimizer and obfuscator.
Jode is a decompiler, an optimizer and an obfuscator. It contains facilities for cleaning logging statements,,
Jarg,
Javaguard, which is a simple obfuscator, without many documentation,
CafeBabe, which allows precise view of Bytecode files and single file obfuscation; a good tool for teaching ByteCode Structure, more than a production tool.
If you want to prevent 'code tampering', then I recommend you sign your jar files. This would make the jar unusable if tampered with. For more information see: https://docs.oracle.com/javase/tutorial/deployment/jar/signindex.html