DCL05-J. Declare all enhanced for statement loop variables to be final

The enhanced for statement introduced in Java V1.5 (also known as the for-each idiom) is primarily used for iterating over collections of objects. Unlike the original for statement, assignments to the loop variable fail to affect the loop's iteration order over the underlying set of objects. Consequently, assignments to the loop variable can have an effect other than what is intended by the developer. This provides yet another reason to avoid assigning to the loop variable in a for loop.

As detailed in §14.14.2, "The Enhanced For Statement" of the Java Language Specification [[JLS 2005]]

an enhanced for statement of the form

for (ObjType obj : someIterableItem) { 
  // ...
}

is equivalent to a standard for loop of the form

for (Iterator myIterator = someIterableItem.iterator(); iterator.hasNext();) {
  ObjType obj = myIterator.next();
  // ...
}

Consequently, an assignment to the loop variable is equivalent to modifying a variable local to the loop body whose initial value is the object referenced by the loop iterator. This modification is not necessarily erroneous, but it can obscure the loop functionality or indicate a misunderstanding of the underlying implementation of the enhanced for statement.

Declare all enhanced for statement loop variables to be final. The final declaration causes Java compilers to flag and reject any assignments made to the loop variable.

Noncompliant Code Example

This noncompliant code example attempts to process a collection of objects using an enhanced for loop. It further intends to skip processing one item in the collection.

Collection<ProcessObj> processThese = // ... 

for (ProcessObj processMe: processThese) {
  if (someCondition) { // found the item to skip
    someCondition = false;
    processMe = processMe.getNext(); // attempt to skip to next item
  }
  processMe.doTheProcessing(); // process the object
}

The attempt to skip to the next item is "successful" because the assignment is successful, and the value of processMe is updated. Unlike an original for loop, however, the assignment leaves the overall iteration order of the loop unchanged. As a result, the object following the skipped object is processed twice; this is unlikely to be what the programmer intended.

Note that if processMe were declared final, a compiler error would result at the attempted assignment.

Compliant Solution

This compliant solution correctly processes the objects in the collection not more than one time.

Collection<ProcessObj> processThese = // ... 

for (final ProcessObj processMe: processThese) {
  if (someCondition) { // found the item to skip
    someCondition = false;
    continue; // skip by continuing to next iteration
  }
  processMe.doTheProcessing(); // process the object
}

Risk Assessment

Assignments to the loop variable of an enhanced for loop (for-each idiom) fail to affect the overall iteration order, lead to programmer confusion, and can leave data in a fragile or inconsistent state.

Automated Detection

This rule is easily enforced with static analysis.

Related Guidelines

Bibliography

DCL04-J. Do not derive a value associated with an enum from its ordinal      01. Declarations and Initialization (DCL)      02. Expressions (EXP)