Java Method invocation vs using a variable

Don't forget that by assigning the value of getSomething() to a variable rather than calling it twice, you are assuming that getSomething() would have returned the same thing the second time you called it. Perhaps that's a valid assumption in the scenario you are talking about, but there are times when it isn't.

It depends. If you would like to make it clear that you use the same value again and again, I'd assign it to a temp variable. I'd do so if the call of the getter is somewhat lengthy, like myCustomObject.getASpecificValue().

You will get much fewer errors in your code if it is readable. So this is the main point.

The performance differences are very small or not existent.

Never code for performance, always code for readability. Let the compiler do the work.

They can improve the compiler/runtime to run good code faster and suddenly your "Fast" code is actually slowing the system down.

Java compiler & runtime optimizations seem to address more common/readable code first, so your "Optimized" code is more likely to be de-optimized at a later time than code that was just written cleanly.

Note:

This answer is referring to Java code "Tricks" like the question referenced, not bad programming that might raise the level of loops from an O(N) to an O(N^2). Generally write clean, DRY code and wait for an operation to take noticeably too long before fixing it. You will almost never reach this point unless you are a game designer.

Your lead is correct. In modern versions of the VM, simple getters that return a private field are inlined, meaning the performance overhead of a method call doesn't exist.