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Every declaration should be for a single variable, on its own line, with an explanatory comment about the role of the variable. Declaring multiple variables in a single statement can declaration could cause confusion regarding about the types of the variables and their initial values. If more than one variable is declared in a declaration, care must be taken to ensure that the type and initialized value of the variable is self evident.

Noncompliant Code Example

In particular, do not declare any of the following in a single declaration:

• Variables of different types
• A mixture of initialized and uninitialized variables

In general, you should declare each variable on its own line with an explanatory comment regarding its role. While not required for conformance with this guideline, this practice is also recommended in the Code Conventions for the Java Programming Language, §6.1, "Number Per Line" [Conventions 2009].

This guideline applies to

• Local variable declaration statements [JLS 2013, §14.4]
• Field declarations [JLS 2013, §8.3]
• Field (constant) declarations [JLS 2013, §9.3]

Noncompliant Code Example (Initialization)

This noncompliant code example might lead a programmer or reviewer to mistakenly believe that both i and j are initialized to 1. In fact, only j is initialized, while i remains uninitialized: Wiki MarkupIn this noncompliant code example, a programmer or code reviewer might mistakenly conceive that the two variables {{src}} and {{c}} are declared as {{int}}. In fact, {{src}} is of type {{int\[\]}}, while {{c}} has a type of {{int}}.

int src[], c;

Another fallout of this example is that it declares the array in a largely antiquated and unpopular style, with the brackets appearing after the variable name as in type name[]. In practice, arrays are typically declared as type[] name.

i, j = 1;

Compliant Solution (Initialization)

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In this compliant solution, each variable is declared on a separate line. It also uses the preferable style for declaring arrays.it is readily apparent that both i and j are initialized to 1:

Code Block
bgColor #ccccff
int[] srcint i = 1; //* sourcePurpose array */of i... int c;j = 1; //* maxPurpose value */

Although this change has no effect on compilation, the programmer's intent is clearer.

Noncompliant Example

A programmer or code reviewer might mistakenly believe that both i and j are initialized to 1 while reviewing this noncompliant code example. In fact, only j is initialized, while i remains uninitialized.

int i, j = 1;

of j...

Compliant Solution (Initialization)

...

In this compliant solution, it is readily apparent that both i and j are initialized to 1.:

Code Block
bgColor #ccccff
int i = 1; int, j = 1;

...

Declaring each variable on a separate line is the preferred method. However, multiple variables on one line are acceptable when they are trivial temporary variables such as array indices.

Noncompliant Code Example (Different Types)

In this noncompliant code example, the programmer declared declares multiple variables, including an array, on the same line. All instances of the type T have access to methods of the Object class Object. However, it is easy to miss forget that arrays need require special treatment when some of these methods are overridden. Oversights of this genre typically go undetected by compilers and IDEs, alike.

public class Example<T> {
  private T a, b, c[], d;

  public Example(T in) {
    a = in;
    b = in;
    c = (T[]) new Object[10];
    d = in;
  }
}

As a result, when a method of Object like When an Object method, such as toString(), is overridden, a programmer might could accidentally provide a general an implementation for type T without realizing that fails to consider that c is an array of T rather than a reference to an object of type T.

// The oversight error leads to an incorrect implementation
public String toString() {
  return a.toString() + b.toString() + 
         c.toString() + d.toString();
}

However, the real programmer's intent might could have been to invoke toString() on each individual member element of the type T, in array c.

// Correct functional implementation
public String toString(){
  String s = a.toString() + b.toString();
  for (int i = 0; i < c.length; i++){
    s += c[i].toString();
  }
  s += d.toString();
  return s;
}

Compliant Solution

...

(Different Types)

This compliant solution places each declaration on its own line and uses the preferred notation for array declaration:To be compliant, move each declaration to a different line. Furthermore, declare arrays by placing the brackets adjacent to the type, as opposed to using the postfix notation.

public class ExampleExample<T> {
  private T a;   // Purpose of a...
  private T b;   // Purpose of b...
  private T[] c; // Purpose of c[]...
  private T d;   // Purpose of d...

  public Example(T in){
    a = in;
    b = in;
    c = (T[]) new Object[10];
    d = in;
  }
}

Exceptions

DCL04-01: Note that the declaration of a loop counter in a for statement is in violation of this recommendation because the declaration is not on its own line with an explanatory comment about the role of the variable. However, the intent of the loop counter is clear enough from the context in which it is declared to typically not require a comment about the role of the variable.

Applicability

Declaration of multiple variables per line can reduce code readability and lead to programmer confusion.

When more than one variable is declared in a single declaration, ensure that both the type and the initial value of each variable are self-evident.

Declarations of loop indices should be included within a for statement even when this results in variable declarations that lack a comment about the purpose of the variableTrivial declarations for loop counters can reasonably be included within a for statement:

public class Example {
  void function() {
    int mx = 100; // Some max value

    for (int i = 0; i < mx; ++i ) {
      /* ... */
    }

  }
}

Risk Assessment

Failing to declare no more than one variable per declaration can affect code readability and cause misinterpretations.

Other Languages

This rule appears in the C Secure Coding Standard as DCL04-C. Do not declare more than one variable per declaration.

This rule appears in the C++ Secure Coding Standard as DCL04-CPP. Do not declare more than one variable per declaration.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[JLS 2005|AA. Java References#JLS 05]\] Section 6.1, "Declarations", Section 4.3.2, "The class Object"
\[[ESA 2005|AA. Java References#ESA 05]\] Rule 9: Put single variable definitions in separate lines.
\[[Conventions 2009|AA. Java References#Conventions 09]\] 6.1 Number Per Line

Such declarations are not required to be in a separate line, and the explanatory comment may be omitted.

Automated Detection

Bibliography

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Image Added Image Added Image AddedDCL00-J. Use visually distinct identifiers      03. Declarations and Initialization (DCL)      DCL02-J. Use meaningful symbolic constants to represent literal values in program logic