DCL14-J. Use as minimal scope as possible for all variables

Scope minimization helps developers to avoid common programming errors, improves code readability by tying together the declaration and actual use of a variable, and improves maintainability because unused variables are more easily detected and removed.

Noncompliant Code Example

This noncompliant code example shows a variable that is declared outside the for loop. This reduces reusability because the value of the loop index i will have changed after the for statement. Consider, for instance, the case when this code snippet is copied and pasted with the intent to use a different index j. If the index variable change were omitted, the new loop would then attempt to iterate over index i. Unexpected behavior can follow because i remains in scope.

public class Scope {
  public static void main(String[] args) {
    int i = 0;
    for (i = 0; i < 10; i++) {
      // Do operations
    }
  }
}

It should be noted that this code is noncompliant because i is not used outside the for loop. If, for instance, the loop contained a break statement, and the value of i when the loop exits prematurely is inspected, that would be a valid reason for i to be declared local to the method.

Compliant Solution

Minimize the scope of variables where possible, for example, by declaring loop indexes within the for statement.

public class Scope {
  public static void main(String[] args) {
    for (int i = 0; i < 10; i++) { //contains declaration
      // Do operations
    }
  }
}

Noncompliant Code Example

This noncompliant code example shows a variable that is declared outside the counter method. This reduces reusability because the variable is not actually used anywhere outside the counter method.

public class Foo {
  private int count;
  static private final int MAX_COUNT;

  public void counter() {
    count = 0;
    while (condition()) {
      /* ... */
      if (count++ > MAX_COUNT) return;
    }
  }

  /* No other method references count */
}

Compliant Solution

In this case, the count field is only accessible within the counter method.

public class Foo {
  static private final int MAX_COUNT;

  public void counter() {
    int count = 0;
    while (condition()) {
      /* ... */
      if (count++ > MAX_COUNT) return;
    }
  }

  /* No other method references count */
}

Risk Assessment

Using a larger scope than is necessary results in less reliable code.

Guideline	Severity	Likelihood	Remediation Cost	Priority	Level
SCP00-J	low	unlikely	medium	P2	L3

Automated Detection

Detecting local variables that are declared in a larger scope than is required by the as-written code is straightforward and can avoid any possibility of false positives.

Detecting multiple for statements that use the same index variable is straightforward; it will produce false positives in the unusual case where this was intended by the programmer.