DCL01-J. Do not declare more than one variable per declaration

Declaring multiple variables in a single declaration could cause confusion about the types of variables and their initial values. 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. Although it is not required for conformance with this guideline, this practice is also recommended in the Code Conventions for the Java Programming Language, Section 6.1, "Number Per Line" [Conventions 2009].

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.

This guideline applies to

• local variable declaration statements [Java 2005, §14.4]
• field declarations [Java 2005, §8.3]
• field (constant) declarations [Java 2005, §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; i remains uninitialized.

int i, j = 1;

Compliant Solution (Initialization)

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

int i = 1;  // purpose of i...
int j = 1;  // purpose of j...

Compliant Solution (Initialization)

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

int i = 1, j = 1;

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

Noncompliant Code Example (Different Types)

In this noncompliant code example, the programmer declares multiple variables, including an array, on the same line. All instances of the type T have access to methods of the Object class. However, it is easy to forget that arrays require special treatment when some of these methods are overridden.

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;
  }
}

When an Object method, such as toString() is overridden, a programmer could accidentally provide an implementation for type T that fails to consider that c is an array of T, rather than a reference to an object of type T.

public String toString() {
  return a.toString() + b.toString() + c.toString() + d.toString();
}

However, the programmer's actual intent could have been to invoke toString() on each individual element of the 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.

public class Example {
  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

DCL01-EX1: Note that the declaration of a loop counter in a for statement is in violation of this guideline because the declaration is not on its own line with an explanatory comment about the variable's role. However, declaration of loop indices in for statements is not only a common idiom; it also provides the benefit of restricting the scope of the loop index to the for loop itself. This is a specific reason to relax this guideline.

Declarations of loop indices should 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

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

Related Guidelines

C Secure Coding Standard: "DCL04-C. Do not declare more than one variable per declaration"

C++ Secure Coding Standard: "DCL04-CPP. Do not declare more than one variable per declaration"

Bibliography

DCL00-J. Use visually distinct identifiers      01. Declarations and Initialization (DCL)      DCL02-J. Use meaningful symbolic constants to represent literal values in program logic