Why is Collections.addAll supposed to be faster than c.addAll

Question

The Java API docs say the following about Collections.addAll

The behavior of this convenience method is identical to that of c.addAll(Arrays.asList(elements)), but this method is likely to run significantly faster under most implementations.

So if I understand correctly, a) is slower than b):

a)

Collection<Integer> col = new ArrayList<Integer>();
col.addAll(Arrays.asList(1, 2, 3, 4, 5));

b)

Collection<Integer> col = new ArrayList<Integer>();
// Collections.addAll(col, Arrays.asList(1, 2, 3, 4, 5)); <-- won't compile
Collections.addAll(col, 1, 2, 3, 4, 5);

Can anyone explain to me, why that is?

edited: corrected code example. thx to polygenelubricants

Answer 1

Let's take a closer look at the two of them:

// a)
col.addAll(Arrays.asList(1, 2, 3, 4, 5));

Here's what happens:

1. varags + autoboxing creates Integer[]
2. Arrays.asList creates a List<Integer> backed by the array
3. addAll iterates over a Collection<Integer> using Iterator<Integer>

// b)
Collections.addAll(col, 1, 2, 3, 4, 5);

Here's what happens:

1. varargs + autoboxing creates Integer[]
2. addAll iterates over an array (instead of an Iterable<Integer>)

We can see now that b) may be faster because:

• Arrays.asList call is skipped, i.e. no intermediary List is created.
• Since the elements are given in an array (thanks to varargs mechanism), iterating over them may be faster than using Iterator.

That said, unless profiling shows otherwise, the difference isn't likely to be "significant". Do not optimize prematurely. While Java Collection Framework classes may be slower than arrays, they perform more than adequately for most applications.

API links

• Collections.addAll(Collection<? super T> c, T... elements) - varargs i.e. array-based
• Collection.addAll(Collection<? extends E> c) - Collection-based

See also

• Java Language Guide/Autoboxing
• Java Language Guide/Varargs
• Effective Java 2nd Edition, Item 25: Prefer lists to arrays

Related questions

• Array or List in Java. Which is faster ?

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Summary

• If you're adding elements from an array, you can use Collections.addAll(col, arr)
  • Remember that varargs are also done using arrays
• If you're adding elements from a Collection, use col.addAll(otherCol)
  • Do NOT e.g. Collections.addAll(col, otherCol.toArray())
    • Such roundabout way is likely to be slower!
• It's not that one is supremely faster than the other
  • It's about skipping unnecessary steps given the current situation

Answer 2

The only reason it might be faster is that it avoids the call to Arrays.asList which should be relatively cheap since it just wraps the array. Some Collection implementations, for example LinkedList convert the passed collection back to an array before adding the elements, causing additional overhead.

On the other hand, ArrayList.addAll allocates the needed space once before adding any elements and so should be much faster when Collections.addAll requires multiple resizing of the backing array.

In summary, Collections.addAll could be faster when repeatedly adding only a few elements to a collection, but I doubt that this case would ever be a performance bottleneck.