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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 Variables of different types
• a 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 While not required for conformance with this guideline, this practice is also recommended in the Code Conventions for the Java Programming Language, Section 6§6.1, "Number Per Line" [Conventions 2009].

This guideline applies to

• local Local variable declaration statements [Java 2011JLS 2013, §14.4]
• field Field declarations [Java 2011JLS 2013, §8.3]
• field Field (constant) declarations [Java 2011JLS 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.:

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;  // purposePurpose of i...
int j = 1;  // purposePurpose 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 preferred method. However, multiple variables on one line are acceptable when they are trivial temporary variables such as array indexesindices.

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

...

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

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

Applicability

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

...

Declarations of loop indices should be included within a for statement (although this, technically, violates this guideline because the declaration is not on its own line with an explanatory comment)even when this results in variable declarations that lack a comment about the purpose of the variable:

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

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

  }
}

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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