why are there two different kinds of for loops in java

Question

I have a for loop  and I am not sure how it works  I am familiar with   for int i   0  i  lt   9  i             implementation        I am confused about a for loop in the following form   String   myString   new String     one    two    three    some other stuff     String str1       str2          for  String s    myString           implementation        How do these types of for loops work  what do they do differently then regular for loops

User · Answer

Using the first for-loop you manually enumerate through the array by increasing an index to the length of the array  then getting the value at the current index manually   The latter syntax is added in Java 5 and enumerates an array by using an Iterator instance under the hoods  You then have only access to the object  not the index  and you won t be able to adjust the array while enumerating   It s convenient when you just want to perform some actions on all objects in an array

User · Answer

The first is the original for loop  You initialize a variable  set a terminating condition  and provide a state incrementing decrementing counter  There are exceptions  but this is the classic   For that   for  int i 0 i lt myString length i         System out println myString i         is correct   For Java 5 an alternative was proposed  Any thing that implements iterable can be supported  This is particularly nice in Collections  For example you can iterate the list like this  List lt String gt  list       load up with stuff  for  String string   list      System out println string       instead of  for  int i 0  i lt list size   i        System out println list get i        So it s just an alternative notation really  Any item that implements Iterable  i e  can return an iterator  can be written that way    What s happening behind the scenes is somethig like this    more efficient  but I m writing it explicitly   Iterator lt String gt  it   list iterator    while  it hasNext        String string it next      System out println string       In the end it s just syntactic sugar  but rather convenient

User · Answer

Something none of the other answers touch on is that your first loop is indexing though the list  Whereas the for-each loop is using an Iterator  Some lists like LinkedList will iterate faster with an Iterator versus get i   This is because because link list s iterator keeps track of the current pointer  Whereas each get in your for i 0 to 9 has to recompute the offset into the linked list  In general  its better to use for-each or an Iterator because it will be using Collections iterator  which in theory is optimized for the collection type

User · Answer

The second for loop is any easy way to iterate over the contents of an array  without having to manually specify the number of items in the array manual enumeration    It is much more convenient than the first when dealing with arrays

User · Answer

The For-each loop  as it is called  is a type of for loop that is used with collections to guarantee that all items in a collection are iterated over  For example  for   Object o   objects         System out println o toString         Will call the toString   method on each object in the collection  objects   One nice thing about this is that you cannot get an out of bounds exception

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The for-each loop was introduced in Java 1 5 and is used with collections  and to be pedantic  arrays  and anything implementing the Iterable lt E gt  interface     which the article notes    http   download oracle com javase 1 5 0 docs guide language foreach html

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There is an excellent summary of this feature in the article The For-Each Loop  It shows by example how using the for-each style can produce clearer code that is easier to read and write

User · Answer

The new for-each loop is just a short hand for the older loop   The new loop is easier to use for arrays and some iterators  but not as flexible  so the old loop has to be using in some situations  like counting from 0 to 9

[java] why are there two different kinds of for loops in java?

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