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Every declaration should be for a single variable, Declare each variable on its own line , with an explanatory comment about the role of the variable. Declaring multiple variables in a single statement declaration may cause confusion regarding the types of the variables and their initial values. When more than one variable is declared in a single declaration, ensure that the type and initial value of the variable are self evident.

This guideline applies to:

1. local variable declaration statements (JLS §14.4)
2. field declarations (JLS §8.3)
3. field (constant) declarations (JLS §9.3)

Noncompliant Code Example

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

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...

Noncompliant Code Example

In this noncompliant code example, a programmer or code reviewer could mistakenly believe that the variables {{src}} and {{c}} are both declared to be type {{int}}. In fact, {{src}} is of type {{int\[\]}}, while {{c}} has a type of {{int}}.

int src[], c;

This Note: this example declares the array in an antiquated and unpopular style, with the brackets appearing after the variable name. Arrays should be declared type[] name for improved clarity.

Compliant Solution

In this compliant solution, each variable is declared on a separate line, using the preferred style for declaring arrays.

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Although this change has no effect on compilation, it clarifies the programmer's intent.

Noncompliant Code Example

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

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...

Noncompliant Code Example

In this noncompliant code example, the programmer declared multiple variables, including an array, on the same line. All instances of the type T have access to methods of the class Object. 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 a method of Object like such as toString() is overridden, a programmer might accidentally provide an implementation for type T that fails to account for the fact consider that c is an array of T, rather than a reference to an object of type T.

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

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

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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

DCL04DCL01-01EX1: 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, declaration of loop indices in for statements is not only a very common idiom; it also provides the benefit of restricting the scope of the loop index to that of the for loop itself. These are sufficient reasons to relax this guideline in this specific case.

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Risk Assessment

Declaration of more than one variable multiple variables per line could can reduce code readability and lead to programmer confusion.

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