What should I look for when improving performance in Java?

I work in the maintenance team of a big project (around 7k+ classes) and my daily work is mainly to fix bugs. Sometimes, though, I have no bugs to work in. When this happens, I spent most of the time looking for performance gaps in the code. The fact that I have more than 7 thousand classes to look in means it’s not obvious to find these gaps.

So I’d like to know what simple things should I look for when trying to improve the performance of the system?

I’m not asking about specific code techniques, but general ones. For example:

  • I already looked for all occurrences of code like String a = new String("") and changed to StringBuilder a = new StringBuilder();
  • I already changed all the access to the database, where applicable, to use PreparedStatement
  • All of the Debug logging were removed, and the Finest ones were removed when possible

As you can see, those changes could easily be made because they do not require measuring the system performance — the only thing I needed to do was using the search tool inside Eclipse.

What should I look for when improving performance in Java?