The enhanced for statement is equivalent to a basic for statement of the form:
#i is an automatically generated identifier that is distinct from any other identifiers (automatically generated or otherwise) that are in scope...at the point where the enhanced for statement occurs.
Unlike the basic
for statement, assignments to the loop variable fail to affect the loop's iteration order over the underlying set of objects. 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 can obscure the loop functionality or indicate a misunderstanding of the underlying implementation of the enhanced
Declare all enhanced
for statement loop variables 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 integers using an enhanced
for loop. It further intends to modify one item in the collection for processing:
However, this code does not actually modify the list, as shown by the program's output:
New item: 99
New item: 14
New item: 15
List item: 13
List item: 14
List item: 15
i to be final mitigates this problem by causing the compiler to fail to permit
i to be assigned a new value:
This compliant solution processes the "modified" list but leaves the actual list unchanged:
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.