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The enhanced for statement is designed for iteration through Collections and arrays. 

The Java The Java Language Specification (JLS) provides the following example of the enhanced for statement in §14.14.2, "The Enhanced for Statement" [JLS 2014]:

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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.

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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:

List<Integer> list = Arrays.asList(new Integer[] {13, 14, 15});
boolean first = true;

System.out.println("Processing list...");
for (Integer i: list) {
  if (first) {
    first = false;
    i = new Integer(99);
  }
  System.out.println(" New item: " + i);
  // processProcess i
}

System.out.println("Modified list?");
for (Integer i: list) {
  System.out.println("List item: " + i);
}

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Compliant Solution

Declaring i to be final mitigates this problem by causing the compiler to fail to permit i to be assigned a new value.:

// ...
for (final Integer i: list) {

// ...

...

// ...
 
for (final Integer i: list) {
  Integer item = i;
  if (first) {
    first = false;
    item = new Integer(99);
  }
  System.out.println(" New item: " + item);
  // processProcess item
}

// ...

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

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Tool	Version	Checker	Description
Parasoft Jtest	Include Page Parasoft_V Parasoft_V	CERT.DCL02.ITMOD	Do not modify collection while iterating over it

Bibliography

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