Java for loop syntax: “for (T obj : objects)”

问题:

I came across some Java syntax that I haven't seen before. I was wondering if someone could tell me what's going on here.

for (ObjectType objectName : collectionName.getObjects())

回答1:

It's called a for-each or enhanced for statement. See the JLS §14.14.2.

It's syntactic sugar provided by the compiler for iterating over Iterables and arrays. The following are equivalent ways to iterate over a list:

List<Foo> foos = ...;
for (Foo foo : foos)
{
    foo.bar();
}

// equivalent to:
List<Foo> foos = ...;
for (Iterator<Foo> iter = foos.iterator(); iter.hasNext();)
{
    Foo foo = iter.next();
    foo.bar();
}

and these are two equivalent ways to iterate over an array:

int[] nums = ...;
for (int num : nums)
{
    System.out.println(num);
}

// equivalent to:
int[] nums = ...;
for (int i=0; i<nums.length; i++)
{
    int num = nums[i];
    System.out.println(num);
}

Further reading

• How is Java's for loop code generated by the compiler
• The For-Each Loop

回答2:

The variable objectSummary holds the current object of type S3ObjectSummary returned from the objectListing.getObjectSummaries() and iterate over the collection.

Here is an example of this enhanced for loop from Java Tutorials

class EnhancedForDemo {
 public static void main(String[] args){
      int[] numbers = {1,2,3,4,5,6,7,8,9,10};
      for (int item : numbers) {
        System.out.println("Count is: " + item);
      }
 }
}

In this example, the variable item holds the current value from the numbers array.

Output is as follows:

Count is: 1
Count is: 2
Count is: 3
Count is: 4
Count is: 5
Count is: 6
Count is: 7
Count is: 8
Count is: 9
Count is: 10

Hope this helps !

回答3:

yes... This is for each loop in java.

Generally this loop is become useful when you are retrieving data or object from the database.

Syntex :

for(Object obj : Collection obj)
{
     //Code enter code here
}

Example :

for(User user : userList)
{
     System.out.println("USer NAme :" + user.name);
   // etc etc
}

This is for each loop.

it will incremental by automatically. one by one from collection to USer object data has been filled. and working.

回答4:

That's the for each loop syntax. It is looping through each object in the collection returned by objectListing.getObjectSummaries().

回答5:

for each S3ObjecrSummary in objectListing.getObjectSummaries()

it's looping through each item in the collection

回答6:

Yes, It is called the for-each loop. Objects in the collectionName will be assigned one after one from the beginning of that collection, to the created object reference, 'objectName'. So in each iteration of the loop, the 'objectName' will be assigned an object from the 'collectionName' collection. The loop will terminate once when all the items(objects) of the 'collectionName' Collection have finished been assigning or simply the objects to get are over.

for (ObjectType objectName : collectionName.getObjects()){ //loop body> //You can use the 'objectName' here as needed and different objects will be //reepresented by it in each iteration. }

回答7:

public class ForEachLoopExample {

    public static void main(String[] args) {

        System.out.println("For Each Loop Example: ");

        int[] intArray = { 1,2,3,4,5 };

        //Here iteration starts from index 0 to last index
        for(int i : intArray)
            System.out.println(i);
    }

}