Item 57: Minimize the scope of local variables

[gbaswath]

Minimize the accessibility of classes and members.” By minimizing the scope of local variables, you increase the readability and maintainability of your code and reduce the likelihood of error.

Highlights

• Powerful Technique
• Local Variable Initialization
• Loop Variable Declaration
• Final Technique

[gbaswath]

Powerful Technique

Older programming languages like C mandated local variables to be declared at the head of the block and some programmers continue to do that. As a gentle reminder, Java lets you to declare variables anywhere the statement is legal as does C. since C99

The most powerful technique for minimizing the scope of a local variable is to declare it where it is first used.

Local Variable Declaration

• If a variable is declared before it is used, it’s just clutter

One more thing to distract the reader who is trying to figure out what the program does. By the time the variable is used, the reader might not remember the variable’s type or initial value.

• Declaring a local variable prematurely can cause its scope not only to begin too early but also to end too late.

The scope of a local variable extends from the point where it is declared to the end of the enclosing block. If a variable is used accidentally before or after its region of intended use, the consequences can be disastrous.

[gbaswath]

Local Variable Initialization

Nearly every local variable declaration should contain an initializer.

If you don’t yet have enough information to initialize a variable sensibly, you should postpone the declaration until you do.

Exceptional Case

One exception to this rule concerns try-catch statements.

If a variable is initialized to an expression whose evaluation can throw a checked exception, the variable must be initialized inside a try block (unless the enclosing method can propagate the exception). If the value must be used outside of the try block, then it must be declared before the try block, where it cannot yet be “sensibly initialized.”

[gbaswath]

Loop Variable Declaration

Loops present a special opportunity to minimize the scope of variables. The for loop, in both its traditional and for-each forms, allows you to declare loop variables, limiting their scope to the exact region where they’re needed.

Prefer for loops to while loops

The region consists of the body of the loop and the code in parentheses between the for keyword and the body. Assuming the contents of the loop variable aren’t needed after the loop terminates. As while loop requires the variable to be declared prior before checking the condition of the loop to run.

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[gbaswath]

Final Technique

To minimize the scope of local variables is to keep methods small and focused.

If we 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.

The important thing to notice about this idiom is that 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 in every iteration.

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