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Use visually distinct identifiers that are unlikely to be misread during development and review of code. Depending on the fonts used, certain characters are visually similar or even identical and can be misinterpreted. Consider the examples in the following table.

Misleading characters

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Do not use multiple identifiers that vary by only one or more visually similar characters. Also, make the initial portions of long identifiers distinct to aid recognition.

Noncompliant Code Example

This noncompliant code example has two variables, stem and stern, within the same scope that can be easily confused and accidentally interchanged:

int stem;  // Position near the front of the boat
/* ... */
int stern; // Position near the back of the boat

Compliant Solution

This compliant solution eliminates the confusion by assigning visually distinct identifiers to the variables:

int bow;   // Position near the front of the boat
/* ... */
int stern; // Position near the back of the boat

Noncompliant Code Example

This noncompliant example prints the result of adding an int and a long value even though it appears that two integers 11111 are being added. According to the JLS, §3.10.1, "Integer Literals" [JLS 2013],

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public class Visual {
  public static void main(String[] args) {
    System.out.println(11111 + 1111l);
  }
}

Compliant Solution

This compliant solution uses an uppercase L (long) instead of lowercase l to disambiguate the visual appearance of the second integer. Its behavior is the same as that of the noncompliant code example, but the programmer's intent is clear:

public class Visual {
  public static void main(String[] args) {
    System.out.println(11111 + 1111L);
  }
}

Noncompliant Code Example

This noncompliant example mixes decimal values and octal values while storing them in an array. Integer literals with leading zeros denote octal values–not decimal values. According to §3.10.1, "Integer Literals" of the JLS [JLS 2013],

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The third element in array was likely intended to hold the decimal value 42. However, the decimal value 34 (corresponding to the octal value 42) is assigned.

Compliant Solution

When integer literals are intended to represent a decimal value, avoid padding with leading zeros. Use another technique instead, such as padding with whitespace, to preserve digit alignment.

int[] array = new int[3];

void exampleFunction() {
  array[0] = 2719;
  array[1] = 4435;
  array[2] =   42;
  // ...
}

Applicability

Failing to use visually distinct identifiers could result in the use of the wrong identifier and lead to unexpected program behavior.

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Detection of integer literals that have a leading zero is trivial. However, determining whether the programmer intended to use an octal literal or a decimal literal is infeasible. Accordingly, sound automated detection is also infeasible. Heuristic checks may be useful.

Bibliography

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