Question

Which would you do ?

 doThings(folder.getInstructions()) ;
 for (Instruction instruction : folder.getInstructions()) {
    // do things
 }
functionCall(folder.getInstructions()) ;

Or this :

instructions = folder.getInstructions() ;
doThings(instructions)
for (Instruction instruction : instructions) {
  // do things
}
functionCall(instructions) ;

Above all, I would like to know when it is more efficient to store a value in a local variable, and when it is better to make function calls.

Answer 1

More readable is more efficient. Temporary expressions and local variables need the same space and from CPU/JVM perspective it doesn’t make much difference. JVM will do a better job optimizing/inling it.

However if getInstructions() method call is expensive, cache it in local variable. If it’s just a plain getter, it will be inlined anyway. Also IMHO in your particular case local variable is more readable and maybe even more correct if getInstructions() may have different results over time.