Effective Java 45 Minimize the scope of local variables

Principle

1. The most powerful technique for minimizing the scope of a local variable is to declare it where it is first used.
2. Nearly every local variable declaration should contain an initializer.

   Note

   If a variable is initialized by a method that throws a checked exception, it must be initialized inside a try block. If the value must be used outside of the try block, then it must be declared before the try block.

3. prefer for loops to while loops

   Advantage

   1. The loops are completely independent, so there's no harm in reusing the element (or iterator) variable name.
   2. It's much less likely that you'll make the cut-and-paste error, as there's no incentive to use different variable names in the two loops.

         

   // Preferred idiom for iterating over a collection

   for (Element e : c) {

   doSomething(e);

   }

      

   // No for-each loop or generics before release 1.5

   for (Iterator i = c.iterator(); i.hasNext(); ) {

   doSomething((Element) i.next());

   }

      

   Use this idiom below if the loop test involves a method invocation that is guaranteed to return the same result on each iteration.

      

   for (int i = 0, n = expensiveComputation(); i < n; i++) {

   doSomething(i);

   }

      

   It has two loop variables, i and n, both of which have exactly the right scope. The second variable, n, is used

   to store the limit of the first, thus avoiding the cost of a redundant computation on every iteration.

      

4. Keep methods small and focused .

   If you combine two activities in the same method, local variables relevant to one activity may be in the scope of the code performing the other activity. To prevent this from happening, simply separate the method into two: one

   for each activity.

Summary

By minimizing the scope of local variables, you increase the read-ability and maintainability of your code and reduce the likelihood of error.