Declaring variables inside or outside of a loop

by

The scope of local variables should always be the smallest possible.

In your example I presume str is not used outside of the while loop, otherwise you would not be asking the question, because declaring it inside the while loop would not be an option, since it would not compile.

So, since str is not used outside the loop, the smallest possible scope for str is within the while loop.

So, the answer is emphatically that str absolutely ought to be declared within the while loop. No ifs, no ands, no buts.

The only case where this rule might be violated is if for some reason it is of vital importance that every clock cycle must be squeezed out of the code, in which case you might want to consider instantiating something in an outer scope and reusing it instead of re-instantiating it on every iteration of an inner scope. However, this does not apply to your example, due to the immutability of strings in java: a new instance of str will always be created in the beginning of your loop and it will have to be thrown away at the end of it, so there is no possibility to optimize there.

EDIT: (injecting my comment below in the answer)

In any case, the right way to do things is to write all your code properly, establish a performance requirement for your product, measure your final product against this requirement, and if it does not satisfy it, then go optimize things. And what usually ends up happening is that you find ways to provide some nice and formal algorithmic optimizations in just a couple of places which make our program meet its performance requirements instead of having to go all over your entire code base and tweak and hack things in order to squeeze clock cycles here and there.

More Related Contents:

  1. Java – What would cause a while-loop to execute, even when false?
  2. What is a NumberFormatException and how can I fix it?
  3. Integers caching in Java [duplicate]
  4. How to efficiently remove duplicates from an array without using Set
  5. Most efficient way to increment a Map value in Java
  6. Why does the JVM still not support tail-call optimization?
  7. How to get out of while loop in java with Scanner method “hasNext” as condition?
  8. Are “while(true)” loops so bad? [closed]
  9. Timer & TimerTask versus Thread + sleep in Java
  10. What is better: multiple “if” statements or one “if” with multiple conditions?
  11. Print All JVM Flags
  12. Efficiently compute Intersection of two Sets in Java?
  13. How do I exit a while loop in Java?
  14. Is the ternary operator faster than an “if” condition in Java [duplicate]
  15. Would Java inline method(s) during optimization?
  16. ResultSet: Retrieving column values by index versus retrieving by label
  17. Fast transcendent / trigonometric functions for Java
  18. What’s wrong with using associativity by compilers?
  19. Scanner input validation in while loop
  20. Whats wrong with this while loop? [duplicate]
  21. Most efficient way to see if an ArrayList contains an object in Java
  22. when to use while loop rather than for loop
  23. What are the cases in which it is better to use unconditional AND (& instead of &&)
  24. Why defining class as final improves JVM performance?
  25. How to remove large if-else-if chain [duplicate]
  26. Fastest way to strip all non-printable characters from a Java String
  27. Does Java recognize infinite loops?
  28. Do java finals help the compiler create more efficient bytecode? [duplicate]
  29. Is there a memory-efficient replacement of java.lang.String?
  30. What’s the fastest way to read from System.in in Java?

Leave a Comment