Information about general code smells can be found here.

Be aware of common Java gotchas.
Avoid acronyms and abbreviations that may not be clear to others.
Methods and variables should be named after what they mean. Before giving a name, consider what is the responsibility of that piece of code.¹
Return empty collections instead of returning null elements (reference). It saves the efforts needed for testing on null elements.
Efficiency: Use StringBuilder or StringBuffer for String concatenation
Refactor your code regularly, especially when your classes or methods become too big.²^,³.
Remember to check parameters of public methods for illegal values. These methods shouldn't crash because of a NullPointer.
Order class members by scope from private to public⁴^,⁵ (the reserve order is also fine )
Minimize the accessibility of class members. It enforces information hiding or encapsulation.
Avoid hardcoded values.
Use lazy initialization if performance is critical or use caching if necessary.

Utility and Helper classes¶

Utility class = only static methods and stateless. Don't create an instance of such a class.
Helper class = can be a utility class, or it can be stateful or require an instance be created. It can be any class whose design is to aid another class.
Try to make the name of the utility or helper class more specific (e.g. AdministrationHelper, LoginHelper instead of Helper).

Additional tips¶

If you use functional interfaces, use the standard java ones. If you want to understand the functional style, consult this page.
If you want to use newer Java features, have a look at this page.
Have a look at common sources of complexity.

Last update: November 11, 2021