Uses of 'for' in Java

Question

I am fairly new to Java and in another Stack Overflow question about for loops an answer said that there was two uses of for in Java:

for (int i = 0; i < N; i++) { }   for (String a : anyIterable) { } 

I know the first use of for and have used it a lot, but I have never seen the second one. What is it used to do and when would I use it?

Answer 1

The first of the two you specify is a classic C for loop. This gives the programmer control over the iteration criteria and allows for three operations: the initialization; the loop test ; the increment expression. Though it is used often to incrementally repeat for a set number of attempts, as in yor example:

for (int i=0; i < N : i++) 

There are many more instances in code where the for was use to iterate over collections:

for (Iterator iter = myList.iterator(); iter.hasNext();) 

To aleviate the boilerplating of the second type (where the third clause was often unused), and to compliment the Generics introduced in Java 1.5, the second of your two examples - the enhanced for loop, or the for-each loop - was introduced.

The second is used with arrays and Generic collections. See this documentation. It allows you to iterate over a generic collection, where you know the type of the Collection, without having to cast the result of the Iterator.next() to a known type.

Compare:

for(Iterator iter = myList.iterator; iter.hasNext() ; ) {    String myStr = (String) iter.next();    //...do something with myStr } 

with

for (String myStr : myList) {    //...do something with myStr  } 

This 'new style' for loop can be used with arrays as well:

String[] strArray= ... for (String myStr : strArray) {    //...do something with myStr } 

Answer 2

The "for in" loop is mostly eye-candy that makes code easier to read. (If you pronounce the : as "in").

I use the second type almost exclusively in my own code whenever I'm iterating over a list or array.

I still use the regular for loop when dealing with mostly mathematical code, or when I need an "i" to see how many times I've looped.