Java Notes

For-each Loop

Purpose

The basic for loop was extended in Java 5 to make iteration over arrays and other collections more convenient. This newer for statement is called the enhanced for or for-each (because it is called this in other programming languages). I've also heard it called the for-in loop.

Use it in preference to the standard for loop if applicable (see last section below) because it's much more readable.

Series of values. The for-each loop is used to access each successive value in a collection of values.

Arrays and Collections. It's commonly used to iterate over an array or a Collections class (eg, ArrayList).

Iterable<E>. It can also iterate over anything that implements the Iterable<E> interface (must define iterator() method). Many of the Collections classes (eg, ArrayList) implement Iterable<E>, which makes the for-each loop very useful. You can also implement Iterable<E> for your own data structures.

General Form

The for-each and equivalent for statements have these forms. The two basic equivalent forms are given, depending one whether it is an array or an Iterable that is being traversed. In both cases an extra variable is required, an index for the array and an iterator for the collection.

for (type var : arr) {
    body-of-loop
}

for (int i = 0; i < arr.length; i++) { 
    type var = arr[i];
    body-of-loop
}

for (type var : coll) {
    body-of-loop
}

for (Iterator<type> iter = coll.iterator(); iter.hasNext(); ) {
    type var = iter.next();
    body-of-loop
}

Example - Adding all elements of an array

Here is a loop written as both a for-each loop and a basic for loop.

double[] ar = {1.2, 3.0, 0.8};
int sum = 0;
for (double d : ar) {  // d gets successively each value in ar.
    sum += d;
}

And here is the same loop using the basic for. It requires an extra iteration variable.

double[] ar = {1.2, 3.0, 0.8};
int sum = 0;
for (int i = 0; i < ar.length; i++) {  // i indexes each element successively.
    sum += ar[i];
}

Where the for-each is not appropriate

Altho the enhanced for loop can make code much clearer, it can't be used in some common situations.

• Access only. Elements can not be assigned to, eg, not to increment each element in a collection.
• Single structure. It's not possible to traverse two structures at once, eg, to compare two arrays.
• Single element. Use only for single element access, eg, not to compare successive elements.
• Forward only. It's only possible to iterate forward by single steps.
• Java 5. Don't use it if you need compatibility with earlier versions before Java 5.