Why Doesn't Assigning to the Iteration Variable Change the Original Array in a Java For-Each Loop?

Why the Iteration Variable Assignment Fails

In Java's foreach loop, it's a common misconception that assigning to the iteration variable modifies the underlying data. Consider the following code:

String boss = "boss";
char[] array = boss.toCharArray();

for (char c : array) {
    if (c == 'o') {
        c = 'a';
    }
}
System.out.println(new String(array)); // Prints "boss", not "bass"

Unexpectedly, the output remains "boss" despite the apparent change of 'o' to 'a' in the loop. This behavior stems from the nature of foreach loops.

When using foreach, the iteration variable is a copy of the array element. Assigning to the iteration variable only modifies the copy, not the array element itself. To update the underlying array, explicit modifications are necessary:

for (int i = 0; i < array.length; i++) {
    if (array[i] == 'o') {
        array[i] = 'a';
    }
}

This revised code assigns to the actual array elements, ensuring the intended modification of 'o' to 'a'.

In summary, foreach loops provide convenience for iterating over collections, but assignments to the iteration variable only affect the local copy. For persistent modifications, explicit array indexing is required.

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