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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 \[[JLS 05|AA. Java References#JLS 05]\] 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();
  // ...
}

ThereforeConsequently, an assignment to the loop variable is equivalent to modifying a variable local to the loop body, whose initial value is the collection item referred to by 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 will cause the causes Java compiler 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 to do not modify the collection or array over which the loop is iteratingiterates, 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 was is declared final in the noncompliant code example the Java compiler would issue a compilation error regarding the "c = 'x'; statement, 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

...