What are the syntax of the for loop in java?

Question

Hello!

Everyone programming in Java should be tired of using loops for, that almost always has a syntax similar to this:

for (int x = 0; x < y; x++) {
    ...
}

I was analyzing some codes and came across a different syntax:

for (String novaString : arrayDeStrings) {
    ...
{ 

From what I understand, the loop will repeat X times, where X is equal to the size of the array of strings (arrayDeStrings.length) and with each repetition, novaString will receive the contents of one of the strings of arrayDeStrings.

Doubts

Is this really how this second syntax presented works? Are there other syntaxes besides those mentioned in this question? If so, how do they work?

Answer 1

This is a loop mode equivalent to for ... each of other languages.

As you suspected, it iterates the array arrayDeStrings, sequentially picking up each of its items and returning in novaString, running the inside of the loop with the value of each item respectively.

Imagine the following array of Strings:

String[] arrayDeStrings = { "Banana", "Maçã", "Pera" };

With this array, doing so much:

// Sintaxe foreach
for (String novaString : arrayDeStrings) {
   System.out.println( novaString );
}

how much so:

// Sintaxe for com iterador
for (Iterator<String> i = arrayDeStrings.iterator(); i.hasNext(); ) {
   String novaString = i.next();
   System.out.println( novaString );
}

or so:

// Sintaxe for convencional
for(int i = 0; i < arrayDeStrings.length; i++) {
   System.out.println( arrayDeStrings[i] );
}

You will get the expected result:

Banana
Maçã
Pera

Note that the foreach is just one syntax-sugar, as appropriate comment from @mgibsonbr, it will be transformed into one of the two codes that were exposed next, depending on whether it is a Iterable or Array in the parameter.

Answer 2

This type of construction started with Java 5. It is called for-each and does just this: for every element given in loop of for, executes one or more commands.

With it the reading of the code becomes easier, and internally it is compiled for the same bytecodes that the for or the while correspondents would make.

It can be used in arrays, Collections and any other construction that implements an interface Iterable.

There are situations where you will not be able to use this for:

• to update values of Array: it only reads the array values, you cannot use this for to update its values. For example, the code below is invalid:

for (String novaString : arrayDeStrings) {     
 novaString = "banana";     
}

• to traverse two structures at the same time: the for runs through only one structure. There is no construction of this for that allows you to traverse two arrays comparing their values, for example.