What is difference between Collection.stream().forEach() and Collection.forEach()?

Question

I understand that with .stream(), I can use chain operations like .filter() or use parallel stream. But what is difference between them if I need to execute small operations (for example, printing the elements of the list)?

collection.stream().forEach(System.out::println);
collection.forEach(System.out::println);

Answer 1

For simple cases such as the one illustrated, they are mostly the same. However, there are a number of subtle differences that might be significant.

One issue is with ordering. With Stream.forEach, the order is undefined. It's unlikely to occur with sequential streams, still, it's within the specification for Stream.forEach to execute in some arbitrary order. This does occur frequently in parallel streams. By contrast, Iterable.forEach is always executed in the iteration order of the Iterable, if one is specified.

Another issue is with side effects. The action specified in Stream.forEach is required to be non-interfering. (See the java.util.stream package doc.) Iterable.forEach potentially has fewer restrictions. For the collections in java.util, Iterable.forEach will generally use that collection's Iterator, most of which are designed to be fail-fast and which will throw ConcurrentModificationException if the collection is structurally modified during the iteration. However, modifications that aren't structural are allowed during iteration. For example, the ArrayList class documentation says "merely setting the value of an element is not a structural modification." Thus, the action for ArrayList.forEach is allowed to set values in the underlying ArrayList without problems.

The concurrent collections are yet again different. Instead of fail-fast, they are designed to be weakly consistent. The full definition is at that link. Briefly, though, consider ConcurrentLinkedDeque. The action passed to its forEach method is allowed to modify the underlying deque, even structurally, and ConcurrentModificationException is never thrown. However, the modification that occurs might or might not be visible in this iteration. (Hence the "weak" consistency.)

Still another difference is visible if Iterable.forEach is iterating over a synchronized collection. On such a collection, Iterable.forEach takes the collection's lock once and holds it across all the calls to the action method. The Stream.forEach call uses the collection's spliterator, which does not lock, and which relies on the prevailing rule of non-interference. The collection backing the stream could be modified during iteration, and if it is, a ConcurrentModificationException or inconsistent behavior could result.

Answer 2

This answer concerns itself with the performance of the various implementations of the loops. Its only marginally relevant for loops that are called VERY OFTEN (like millions of calls). In most cases the content of the loop will be by far the most expensive element. For situations where you loop really often, this might still be of interest.

You should repeat this tests under the target system as this is implementation specific, (full source code).

I run openjdk version 1.8.0_111 on a fast Linux machine.

I wrote a test that loops 10^6 times over a List using this code with varying sizes for integers (10^0 -> 10^5 entries).

The results are below, the fastest method varies depending on the amount of entries in the list.

But still under worst situations, looping over 10^5 entries 10^6 times took 100 seconds for the worst performer, so other considerations are more important in virtually all situations.

public int outside = 0;

private void iteratorForEach(List<Integer> integers) {
    integers.forEach((ii) -> {
        outside = ii*ii;
    });
}

private void forEach(List<Integer> integers) {
    for(Integer next : integers) {
        outside = next * next;
    }
}

private void forCounter(List<Integer> integers) {
    for(int ii = 0; ii < integers.size(); ii++) {
        Integer next = integers.get(ii);
        outside = next*next;
    }
}

private void iteratorStream(List<Integer> integers) {
    integers.stream().forEach((ii) -> {
        outside = ii*ii;
    });
}

Here are my timings: milliseconds / function / number of entries in list. Each run is 10^6 loops.

                           1    10    100    1000    10000
       iterator.forEach   27   116    959    8832    88958
               for:each   53   171   1262   11164   111005
         for with index   39   112    920    8577    89212
iterable.stream.forEach  255   324   1030    8519    88419

If you repeat the experiment, I posted the full source code. Please do edit this answer and add you results with a notation of the tested system.

---

Using a MacBook Pro, 2.5 GHz Intel Core i7, 16 GB, macOS 10.12.6:

                           1    10    100    1000    10000
       iterator.forEach   27   106   1047    8516    88044
               for:each   46   143   1182   10548   101925
         for with index   49   145    887    7614    81130
iterable.stream.forEach  393   397   1108    8908    88361

---

Java 8 Hotspot VM - 3.4GHz Intel Xeon, 8 GB, Windows 10 Pro

                            1    10    100    1000    10000
        iterator.forEach   30   115    928    8384    85911
                for:each   40   125   1166   10804   108006
          for with index   30   120    956    8247    81116
 iterable.stream.forEach  260   237   1020    8401    84883

---

Java 11 Hotspot VM - 3.4GHz Intel Xeon, 8 GB, Windows 10 Pro
(same machine as above, different JDK version)

                            1    10    100    1000    10000
        iterator.forEach   20   104    940    8350    88918
                for:each   50   140    991    8497    89873
          for with index   37   140    945    8646    90402
 iterable.stream.forEach  200   270   1054    8558    87449

---

Java 11 OpenJ9 VM - 3.4GHz Intel Xeon, 8 GB, Windows 10 Pro
(same machine and JDK version as above, different VM)

                            1    10    100    1000    10000
        iterator.forEach  211   475   3499   33631   336108
                for:each  200   375   2793   27249   272590
          for with index  384   467   2718   26036   261408
 iterable.stream.forEach  515   714   3096   26320   262786

---

Java 8 Hotspot VM - 2.8GHz AMD, 64 GB, Windows Server 2016

                            1    10    100    1000    10000
        iterator.forEach   95   192   2076   19269   198519
                for:each  157   224   2492   25466   248494
          for with index  140   368   2084   22294   207092
 iterable.stream.forEach  946   687   2206   21697   238457

---

Java 11 Hotspot VM - 2.8GHz AMD, 64 GB, Windows Server 2016
(same machine as above, different JDK version)

                            1    10    100    1000    10000
        iterator.forEach   72   269   1972   23157   229445
                for:each  192   376   2114   24389   233544
          for with index  165   424   2123   20853   220356
 iterable.stream.forEach  921   660   2194   23840   204817

---

Java 11 OpenJ9 VM - 2.8GHz AMD, 64 GB, Windows Server 2016
(same machine and JDK version as above, different VM)

                            1    10    100    1000    10000
        iterator.forEach  592   914   7232   59062   529497
                for:each  477  1576  14706  129724  1190001
          for with index  893   838   7265   74045   842927
 iterable.stream.forEach 1359  1782  11869  104427   958584

---

The VM implementation you choose also makes a difference Hotspot/OpenJ9/etc.

Answer 3

There is no difference between the two you have mentioned, atleast conceptually, the Collection.forEach() is just a shorthand.

Internally the stream() version has somewhat more overhead due to object creation, but looking at the running time it neither has an overhead there.

Both implementations end up iterating over the collection contents once, and during the iteration print out the element.