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Avoid Checking Modulus Results for Direct Equality

High
correctnessreliability

What is it?​

This practice advises against checking the result of a modulus operation directly for equality in expressions.

Why apply it?​

When using modulus on negative numbers, the outcome can be either zero or negative. This behavior can lead to unexpected results if compared for equality with a positive or negative number directly.

How to fix it?​

Use inequality comparisons to determine the intent of the modulus operation, ensuring correct behavior regardless of the sign of the operand.

Examples​

Example 1:​

Negative​

The negative example directly compares the modulus result to check if the number is odd, failing for negative odd numbers.

public class NumberUtils {
  public boolean isOdd(int x) {
    return x % 2 == 1;  // Noncompliant
  }
  
  public static void main(String[] args) {
    NumberUtils utils = new NumberUtils();
    System.out.println(utils.isOdd(3));  // true
    System.out.println(utils.isOdd(-3)); // false (unexpected)
    System.out.println(utils.isOdd(4));  // false
  }
}

Example 2:​

Positive​

The positive example checks for inequality with zero to determine if a number is odd, ensuring correct handling of negative numbers.

public class NumberUtils {
  public boolean isOdd(int x) {   
    return x % 2 != 0;  // Compliant
  }
  
  public static void main(String[] args) {
    NumberUtils utils = new NumberUtils();
    System.out.println(utils.isOdd(3));  // true
    System.out.println(utils.isOdd(-3)); // true
    System.out.println(utils.isOdd(4));  // false
  }
}

Negative​

The negative example directly checks if modulus by three equals one, failing if the number is negative and divisible by three.

public class DivisibilityChecker {
  public boolean remainderOneWhenDividedByThree(int x) {
    return x % 3 == 1;  // Noncompliant
  }
  
  public static void main(String[] args) {
    DivisibilityChecker checker = new DivisibilityChecker();
    System.out.println(checker.remainderOneWhenDividedByThree(10));   // true
    System.out.println(checker.remainderOneWhenDividedByThree(-2));   // false (unexpected)
    System.out.println(checker.remainderOneWhenDividedByThree(4));    // true
  }
}

Example 3:​

Positive​

The positive example uses a range check to confirm divisibility by three, ensuring proper handling of negative numbers.

public class DivisibilityChecker {
  public boolean isDivisibleByThree(int x) {
    return x % 3 == 0;  // Compliant
  }
  
  public static void main(String[] args) {
    DivisibilityChecker checker = new DivisibilityChecker();
    System.out.println(checker.isDivisibleByThree(9));   // true
    System.out.println(checker.isDivisibleByThree(-9));  // true
    System.out.println(checker.isDivisibleByThree(4));   // false
  }
}