DCL30-J. Do not attempt to assign to the loop variable in an enhanced for loop

The enhanced for statement introduced in Java 1.5, commonly referred to as the for-each idiom, is primarily used for iterating over collections of objects. While similar to the for statement, assignments to the loop variable do not modify the collection of objects over which the loop iterates. Assignments to the loop variable may not have the effect intended by the developer and should be avoided.

As detailed in the Java Language Specification [[JLS 05]] section 14.14.2, "The enhanced for statement", 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 that the loop iterator refers to. While this modification is not necessarily erroneous, it may obscure the loop functionality or indicate a misunderstanding of the underlying implementation of the enhanced for statement.

It is recommended that all enhanced for statement loop variables be declared final. The final declaration causes Java compilers to flag and reject any assignments made to the loop variable, in the loop body.

Noncompliant Code Example

This noncompliant code example attempts to initialize a Character array using an enhanced for loop. However, because assignments to the loop variable do not modify the array over which the loop iterates, the array is not suitably initialized.

Character[] array = new Character[10];
for(Character c: array) 
  c = 'x'; // initialization attempt

for(int i=0;i<array.length;i++) 
  System.out.print(array[i]);  // prints 10 "null"s

Note that if c is declared final, a compiler error results when an attempt is made to initialize it.

Compliant Solution

This compliant solution correctly initializes the array using a for loop.

Character[] array = new Character[10];
for(int i=0;i<array.length;i++) 
  array[i] = 'x';

for(final Character c: array) 
  System.out.print(c);  // prints 10 "x"s

Risk Assessment

Attempts to assign to the loop variable from within the enhanced for loop (for-each idiom) are futile and may leave the class in a fragile, inconsistent state.

Automated Detection

TODO

Related Vulnerabilities

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

Other Languages

TODO

References

[[JLS 05]] Section 14.14.2 "The enhanced for statement"

DCL08-J. Enforce compile-time type checking of variable argument types      03. Declarations and Initialization (DCL)      DCL31-J. Qualify mathematical constants with the static and final modifiers