How does the Java 'for each' loop work?

Question

Consider:

List<String> someList = new ArrayList<String>();
// add "monkey", "donkey", "skeleton key" to someList

for (String item : someList) {
    System.out.println(item);
}

What would the equivalent for loop look like without using the for each syntax?

Answer 1

It would look something like this. Very crufty.

for (Iterator<String> i = someList.iterator(); i.hasNext(); )
        System.out.println(i.next());

There is a good writeup on for each in the Sun documentation.

Answer 2

As defined in JLS for-each loop can have two forms:

1. If the type of Expression is a subtype of Iterable then translation is as:

   List<String> someList = new ArrayList<String>();
   someList.add("Apple");
   someList.add("Ball");
   for (String item : someList) {
       System.out.println(item);
   }
   
   // IS TRANSLATED TO:
   
   for(Iterator<String> stringIterator = someList.iterator(); stringIterator.hasNext(); ) {
       String item = stringIterator.next();
       System.out.println(item);
   }
   

2. If the Expression necessarily has an array type T[] then:

   String[] someArray = new String[2];
   someArray[0] = "Apple";
   someArray[1] = "Ball";
   
   for(String item2 : someArray) {
       System.out.println(item2);
   }
   
   // IS TRANSLATED TO:
   for (int i = 0; i < someArray.length; i++) {
       String item2 = someArray[i];
       System.out.println(item2);
   }
   

Java 8 has introduced streams which perform generally better. We can use them as:

someList.stream().forEach(System.out::println);
Arrays.stream(someArray).forEach(System.out::println);

Answer 3

It's implied by nsayer's answer, but it's worth noting that the OP's for(..) syntax will work when "someList" is anything that implements java.lang.Iterable -- it doesn't have to be a list, or some collection from java.util. Even your own types, therefore, can be used with this syntax.

Answer 4

List<Item> Items = obj.getItems();
for(Item item:Items)
             {
                System.out.println(item); 
             }

Iterates over all the objects in Items table.

Answer 5

This looks crazy but hey it works

List<String> someList = new ArrayList<>(); //has content
someList.forEach(System.out::println);

This works. Magic

Answer 6

In Java 8 features you can use this:

List<String> messages = Arrays.asList("First", "Second", "Third");

void forTest(){
    messages.forEach(System.out::println);
}

Output

First
Second
Third