Performance Difference For-Loop Foreach

Performance Difference For-Loop Foreach - java

I always asked myself "what to use" should I use a for-loop or a foreach.
In my opinion it's both the "same". I know for iterating through a list etc. is a foreach better but what if we have the following case :
for (String zipCode : zipCodes) {
if (zipCode.equals(zip)) {
return true;
}
}
or
for (int i = 0; i < zipCodes.length; i++) {
if (zipCodes[i].equals(zip)) {
return true;
}
}
What would be better? Or is in this case really no difference?

First things first - for-each is nothing but syntactic sugar for Iterator. Read this section of JLS. So, I will address this question as a simple FOR loop vs Iterator.
Now, when you use Iterator to traverse over a collection, at bare minimum you will be using two method - next() and hasNext(), and below are their ArrayList implementations:
public boolean hasNext() {
return cursor != size;
}
#SuppressWarnings("unchecked")
public E next() {
checkForComodification();
int i = cursor;
if (i >= size)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = I]; // hagrawal: this is what simple FOR loop does
}
Now, we all know the basic computing that there will be performance difference if on the processor I have to just execute myArray[i] v/s complete implementation of next() method. So, there has to be a difference in performance.
It is likely that some folk might come back strongly on this, citing performance benchmarks and excerpts from Effective Java, but the only other way I can try to explain is that this is even written in Oracle's official documentation - please read below from RandomAccess interface docs over here.
It is very clearly mentioned that there will be differences. So, if you can convince me that what is written in official docs is wrong and will be changed, I will be ready to accept the argument that there is no performance difference between simple FOR loop and Iterator or for-each.
So IMHO, correct way to put this whole argument is this:
If the collection implements RandomAccess interface then simple FOR loop will perform (at least theoretically) better than Iterator or for-each. (this is what is also written in RandomAccess docs)
If the collection doesn't implement RandomAccess interface then Iterator or for-each will perform (for sure) better than simple FOR loop.
However, for all practical purposes, for-each is the best choice in general.

If zipCodes[i] is not O(1), then the performance of your second case will be much worse. (That said, I don't think there yet exists a container in Java where [] is not O(1)). Put another way, the short form for loop cannot be slower.
Plus the short form for loop is clearer, which really ought to be the primary consideration unless speed matters.

It is less about optimisation nowadays, as any difference will be unnoticeable, unless you need to process a very large amount of data. Also, if you used a Collection, the performance would depend on the chosen implementation.
What you should really think about is the quality of the code. The rule is that you should use as few elements as possible to present the logic as clearly as possible. The second solution introduces a new element, the i index, which is not actually needed and only makes the code this little bit more complicated. Only use the fori loop if you actually need to know the index in each iteration.
So, from code quality perspective, you should use the first solution :-)

Note that there is no performance penalty for using the for-each loop,
even for arrays. In fact, it may offer a slight performance advantage
over an ordinary for loop in some circumstances, as it computes the
limit of the array index only once.
Item 46 in Effective Java by Joshua Bloch

Related

Why should we prefer forEach in Java 8? [duplicate]

Which of the following is better practice in Java 8?
Java 8:
joins.forEach(join -> mIrc.join(mSession, join));
Java 7:
for (String join : joins) {
mIrc.join(mSession, join);
}
I have lots of for loops that could be "simplified" with lambdas, but is there really any advantage of using them? Would it improve their performance and readability?
EDIT
I'll also extend this question to longer methods. I know that you can't return or break the parent function from a lambda and this should also be taken into consideration when comparing them, but is there anything else to be considered?

The better practice is to use for-each. Besides violating the Keep It Simple, Stupid principle, the new-fangled forEach() has at least the following deficiencies:
Can't use non-final variables. So, code like the following can't be turned into a forEach lambda:
Object prev = null;
for(Object curr : list)
{
if( prev != null )
foo(prev, curr);
prev = curr;
}
Can't handle checked exceptions. Lambdas aren't actually forbidden from throwing checked exceptions, but common functional interfaces like Consumer don't declare any. Therefore, any code that throws checked exceptions must wrap them in try-catch or Throwables.propagate(). But even if you do that, it's not always clear what happens to the thrown exception. It could get swallowed somewhere in the guts of forEach()
Limited flow-control. A return in a lambda equals a continue in a for-each, but there is no equivalent to a break. It's also difficult to do things like return values, short circuit, or set flags (which would have alleviated things a bit, if it wasn't a violation of the no non-final variables rule). "This is not just an optimization, but critical when you consider that some sequences (like reading the lines in a file) may have side-effects, or you may have an infinite sequence."
Might execute in parallel, which is a horrible, horrible thing for all but the 0.1% of your code that needs to be optimized. Any parallel code has to be thought through (even if it doesn't use locks, volatiles, and other particularly nasty aspects of traditional multi-threaded execution). Any bug will be tough to find.
Might hurt performance, because the JIT can't optimize forEach()+lambda to the same extent as plain loops, especially now that lambdas are new. By "optimization" I do not mean the overhead of calling lambdas (which is small), but to the sophisticated analysis and transformation that the modern JIT compiler performs on running code.
If you do need parallelism, it is probably much faster and not much more difficult to use an ExecutorService. Streams are both automagical (read: don't know much about your problem) and use a specialized (read: inefficient for the general case) parallelization strategy (fork-join recursive decomposition).
Makes debugging more confusing, because of the nested call hierarchy and, god forbid, parallel execution. The debugger may have issues displaying variables from the surrounding code, and things like step-through may not work as expected.
Streams in general are more difficult to code, read, and debug. Actually, this is true of complex "fluent" APIs in general. The combination of complex single statements, heavy use of generics, and lack of intermediate variables conspire to produce confusing error messages and frustrate debugging. Instead of "this method doesn't have an overload for type X" you get an error message closer to "somewhere you messed up the types, but we don't know where or how." Similarly, you can't step through and examine things in a debugger as easily as when the code is broken into multiple statements, and intermediate values are saved to variables. Finally, reading the code and understanding the types and behavior at each stage of execution may be non-trivial.
Sticks out like a sore thumb. The Java language already has the for-each statement. Why replace it with a function call? Why encourage hiding side-effects somewhere in expressions? Why encourage unwieldy one-liners? Mixing regular for-each and new forEach willy-nilly is bad style. Code should speak in idioms (patterns that are quick to comprehend due to their repetition), and the fewer idioms are used the clearer the code is and less time is spent deciding which idiom to use (a big time-drain for perfectionists like myself!).
As you can see, I'm not a big fan of the forEach() except in cases when it makes sense.
Particularly offensive to me is the fact that Stream does not implement Iterable (despite actually having method iterator) and cannot be used in a for-each, only with a forEach(). I recommend casting Streams into Iterables with (Iterable<T>)stream::iterator. A better alternative is to use StreamEx which fixes a number of Stream API problems, including implementing Iterable.
That said, forEach() is useful for the following:
Atomically iterating over a synchronized list. Prior to this, a list generated with Collections.synchronizedList() was atomic with respect to things like get or set, but was not thread-safe when iterating.
Parallel execution (using an appropriate parallel stream). This saves you a few lines of code vs using an ExecutorService, if your problem matches the performance assumptions built into Streams and Spliterators.
Specific containers which, like the synchronized list, benefit from being in control of iteration (although this is largely theoretical unless people can bring up more examples)
Calling a single function more cleanly by using forEach() and a method reference argument (ie, list.forEach (obj::someMethod)). However, keep in mind the points on checked exceptions, more difficult debugging, and reducing the number of idioms you use when writing code.
Articles I used for reference:
Everything about Java 8
Iteration Inside and Out (as pointed out by another poster)
EDIT: Looks like some of the original proposals for lambdas (such as http://www.javac.info/closures-v06a.html Google Cache) solved some of the issues I mentioned (while adding their own complications, of course).

The advantage comes into account when the operations can be executed in parallel. (See http://java.dzone.com/articles/devoxx-2012-java-8-lambda-and - the section about internal and external iteration)
The main advantage from my point of view is that the implementation of what is to be done within the loop can be defined without having to decide if it will be executed in parallel or sequential
If you want your loop to be executed in parallel you could simply write
joins.parallelStream().forEach(join -> mIrc.join(mSession, join));
You will have to write some extra code for thread handling etc.
Note: For my answer I assumed joins implementing the java.util.Stream interface. If joins implements only the java.util.Iterable interface this is no longer true.

When reading this question one can get the impression, that Iterable#forEach in combination with lambda expressions is a shortcut/replacement for writing a traditional for-each loop. This is simply not true. This code from the OP:
joins.forEach(join -> mIrc.join(mSession, join));
is not intended as a shortcut for writing
for (String join : joins) {
mIrc.join(mSession, join);
}
and should certainly not be used in this way. Instead it is intended as a shortcut (although it is not exactly the same) for writing
joins.forEach(new Consumer<T>() {
#Override
public void accept(T join) {
mIrc.join(mSession, join);
}
});
And it is as a replacement for the following Java 7 code:
final Consumer<T> c = new Consumer<T>() {
#Override
public void accept(T join) {
mIrc.join(mSession, join);
}
};
for (T t : joins) {
c.accept(t);
}
Replacing the body of a loop with a functional interface, as in the examples above, makes your code more explicit: You are saying that (1) the body of the loop does not affect the surrounding code and control flow, and (2) the body of the loop may be replaced with a different implementation of the function, without affecting the surrounding code. Not being able to access non final variables of the outer scope is not a deficit of functions/lambdas, it is a feature that distinguishes the semantics of Iterable#forEach from the semantics of a traditional for-each loop. Once one gets used to the syntax of Iterable#forEach, it makes the code more readable, because you immediately get this additional information about the code.
Traditional for-each loops will certainly stay good practice (to avoid the overused term "best practice") in Java. But this doesn't mean, that Iterable#forEach should be considered bad practice or bad style. It is always good practice, to use the right tool for doing the job, and this includes mixing traditional for-each loops with Iterable#forEach, where it makes sense.
Since the downsides of Iterable#forEach have already been discussed in this thread, here are some reasons, why you might probably want to use Iterable#forEach:
To make your code more explicit: As described above, Iterable#forEach can make your code more explicit and readable in some situations.
To make your code more extensible and maintainable: Using a function as the body of a loop allows you to replace this function with different implementations (see Strategy Pattern). You could e.g. easily replace the lambda expression with a method call, that may be overwritten by sub-classes:
joins.forEach(getJoinStrategy());
Then you could provide default strategies using an enum, that implements the functional interface. This not only makes your code more extensible, it also increases maintainability because it decouples the loop implementation from the loop declaration.
To make your code more debuggable: Seperating the loop implementation from the declaration can also make debugging more easy, because you could have a specialized debug implementation, that prints out debug messages, without the need to clutter your main code with if(DEBUG)System.out.println(). The debug implementation could e.g. be a delegate, that decorates the actual function implementation.
To optimize performance-critical code: Contrary to some of the assertions in this thread, Iterable#forEach does already provide better performance than a traditional for-each loop, at least when using ArrayList and running Hotspot in "-client" mode. While this performance boost is small and negligible for most use cases, there are situations, where this extra performance can make a difference. E.g. library maintainers will certainly want to evaluate, if some of their existing loop implementations should be replaced with Iterable#forEach.
To back this statement up with facts, I have done some micro-benchmarks with Caliper. Here is the test code (latest Caliper from git is needed):
#VmOptions("-server")
public class Java8IterationBenchmarks {
public static class TestObject {
public int result;
}
public #Param({"100", "10000"}) int elementCount;
ArrayList<TestObject> list;
TestObject[] array;
#BeforeExperiment
public void setup(){
list = new ArrayList<>(elementCount);
for (int i = 0; i < elementCount; i++) {
list.add(new TestObject());
}
array = list.toArray(new TestObject[list.size()]);
}
#Benchmark
public void timeTraditionalForEach(int reps){
for (int i = 0; i < reps; i++) {
for (TestObject t : list) {
t.result++;
}
}
return;
}
#Benchmark
public void timeForEachAnonymousClass(int reps){
for (int i = 0; i < reps; i++) {
list.forEach(new Consumer<TestObject>() {
#Override
public void accept(TestObject t) {
t.result++;
}
});
}
return;
}
#Benchmark
public void timeForEachLambda(int reps){
for (int i = 0; i < reps; i++) {
list.forEach(t -> t.result++);
}
return;
}
#Benchmark
public void timeForEachOverArray(int reps){
for (int i = 0; i < reps; i++) {
for (TestObject t : array) {
t.result++;
}
}
}
}
And here are the results:
Results for -client
Results for -server
When running with "-client", Iterable#forEach outperforms the traditional for loop over an ArrayList, but is still slower than directly iterating over an array. When running with "-server", the performance of all approaches is about the same.
To provide optional support for parallel execution: It has already been said here, that the possibility to execute the functional interface of Iterable#forEach in parallel using streams, is certainly an important aspect. Since Collection#parallelStream() does not guarantee, that the loop is actually executed in parallel, one must consider this an optional feature. By iterating over your list with list.parallelStream().forEach(...);, you explicitly say: This loop supports parallel execution, but it does not depend on it. Again, this is a feature and not a deficit!
By moving the decision for parallel execution away from your actual loop implementation, you allow optional optimization of your code, without affecting the code itself, which is a good thing. Also, if the default parallel stream implementation does not fit your needs, no one is preventing you from providing your own implementation. You could e.g. provide an optimized collection depending on the underlying operating system, on the size of the collection, on the number of cores, and on some preference settings:
public abstract class MyOptimizedCollection<E> implements Collection<E>{
private enum OperatingSystem{
LINUX, WINDOWS, ANDROID
}
private OperatingSystem operatingSystem = OperatingSystem.WINDOWS;
private int numberOfCores = Runtime.getRuntime().availableProcessors();
private Collection<E> delegate;
#Override
public Stream<E> parallelStream() {
if (!System.getProperty("parallelSupport").equals("true")) {
return this.delegate.stream();
}
switch (operatingSystem) {
case WINDOWS:
if (numberOfCores > 3 && delegate.size() > 10000) {
return this.delegate.parallelStream();
}else{
return this.delegate.stream();
}
case LINUX:
return SomeVerySpecialStreamImplementation.stream(this.delegate.spliterator());
case ANDROID:
default:
return this.delegate.stream();
}
}
}
The nice thing here is, that your loop implementation doesn't need to know or care about these details.

forEach() can be implemented to be faster than for-each loop, because the iterable knows the best way to iterate its elements, as opposed to the standard iterator way. So the difference is loop internally or loop externally.
For example ArrayList.forEach(action) may be simply implemented as
for(int i=0; i<size; i++)
action.accept(elements[i])
as opposed to the for-each loop which requires a lot of scaffolding
Iterator iter = list.iterator();
while(iter.hasNext())
Object next = iter.next();
do something with `next`
However, we also need to account for two overhead costs by using forEach(), one is making the lambda object, the other is invoking the lambda method. They are probably not significant.
see also http://journal.stuffwithstuff.com/2013/01/13/iteration-inside-and-out/ for comparing internal/external iterations for different use cases.

TL;DR: List.stream().forEach() was the fastest.
I felt I should add my results from benchmarking iteration.
I took a very simple approach (no benchmarking frameworks) and benchmarked 5 different methods:
classic for
classic foreach
List.forEach()
List.stream().forEach()
List.parallelStream().forEach
the testing procedure and parameters
private List<Integer> list;
private final int size = 1_000_000;
public MyClass(){
list = new ArrayList<>();
Random rand = new Random();
for (int i = 0; i < size; ++i) {
list.add(rand.nextInt(size * 50));
}
}
private void doIt(Integer i) {
i *= 2; //so it won't get JITed out
}
The list in this class shall be iterated over and have some doIt(Integer i) applied to all it's members, each time via a different method.
in the Main class I run the tested method three times to warm up the JVM. I then run the test method 1000 times summing the time it takes for each iteration method (using System.nanoTime()). After that's done i divide that sum by 1000 and that's the result, average time.
example:
myClass.fored();
myClass.fored();
myClass.fored();
for (int i = 0; i < reps; ++i) {
begin = System.nanoTime();
myClass.fored();
end = System.nanoTime();
nanoSum += end - begin;
}
System.out.println(nanoSum / reps);
I ran this on a i5 4 core CPU, with java version 1.8.0_05
classic for
for(int i = 0, l = list.size(); i < l; ++i) {
doIt(list.get(i));
}
execution time: 4.21 ms
classic foreach
for(Integer i : list) {
doIt(i);
}
execution time: 5.95 ms
List.forEach()
list.forEach((i) -> doIt(i));
execution time: 3.11 ms
List.stream().forEach()
list.stream().forEach((i) -> doIt(i));
execution time: 2.79 ms
List.parallelStream().forEach
list.parallelStream().forEach((i) -> doIt(i));
execution time: 3.6 ms

I feel that I need to extend my comment a bit...
About paradigm\style
That's probably the most notable aspect. FP became popular due to what you can get avoiding side-effects. I won't delve deep into what pros\cons you can get from this, since this is not related to the question.
However, I will say that the iteration using Iterable.forEach is inspired by FP and rather result of bringing more FP to Java (ironically, I'd say that there is no much use for forEach in pure FP, since it does nothing except introducing side-effects).
In the end I would say that it is rather a matter of taste\style\paradigm you are currently writing in.
About parallelism.
From performance point of view there is no promised notable benefits from using Iterable.forEach over foreach(...).
According to official docs on Iterable.forEach :
Performs the given action on the contents of the Iterable, in the
order elements occur when iterating, until all elements have been
processed or the action throws an exception.
... i.e. docs pretty much clear that there will be no implicit parallelism. Adding one would be LSP violation.
Now, there are "parallell collections" that are promised in Java 8, but to work with those you need to me more explicit and put some extra care to use them (see mschenk74's answer for example).
BTW: in this case Stream.forEach will be used, and it doesn't guarantee that actual work will be done in parallell (depends on underlying collection).
UPDATE: might be not that obvious and a little stretched at a glance but there is another facet of style and readability perspective.
First of all - plain old forloops are plain and old. Everybody already knows them.
Second, and more important - you probably want to use Iterable.forEach only with one-liner lambdas. If "body" gets heavier - they tend to be not-that readable.
You have 2 options from here - use inner classes (yuck) or use plain old forloop.
People often gets annoyed when they see the same things (iteratins over collections) being done various vays/styles in the same codebase, and this seems to be the case.
Again, this might or might not be an issue. Depends on people working on code.

One of most upleasing functional forEach's limitations is lack of checked exceptions support.
One possible workaround is to replace terminal forEach with plain old foreach loop:
Stream<String> stream = Stream.of("", "1", "2", "3").filter(s -> !s.isEmpty());
Iterable<String> iterable = stream::iterator;
for (String s : iterable) {
fileWriter.append(s);
}
Here is list of most popular questions with other workarounds on checked exception handling within lambdas and streams:
Java 8 Lambda function that throws exception?
Java 8: Lambda-Streams, Filter by Method with Exception
How can I throw CHECKED exceptions from inside Java 8 streams?
Java 8: Mandatory checked exceptions handling in lambda expressions. Why mandatory, not optional?

The advantage of Java 1.8 forEach method over 1.7 Enhanced for loop is that while writing code you can focus on business logic only.
forEach method takes java.util.function.Consumer object as an argument, so It helps in having our business logic at a separate location that you can reuse it anytime.
Have look at below snippet,
Here I have created new Class that will override accept class method from Consumer Class,
where you can add additional functionility, More than Iteration..!!!!!!
class MyConsumer implements Consumer<Integer>{
#Override
public void accept(Integer o) {
System.out.println("Here you can also add your business logic that will work with Iteration and you can reuse it."+o);
}
}
public class ForEachConsumer {
public static void main(String[] args) {
// Creating simple ArrayList.
ArrayList<Integer> aList = new ArrayList<>();
for(int i=1;i<=10;i++) aList.add(i);
//Calling forEach with customized Iterator.
MyConsumer consumer = new MyConsumer();
aList.forEach(consumer);
// Using Lambda Expression for Consumer. (Functional Interface)
Consumer<Integer> lambda = (Integer o) ->{
System.out.println("Using Lambda Expression to iterate and do something else(BI).. "+o);
};
aList.forEach(lambda);
// Using Anonymous Inner Class.
aList.forEach(new Consumer<Integer>(){
#Override
public void accept(Integer o) {
System.out.println("Calling with Anonymous Inner Class "+o);
}
});
}
}

Java 8 Iterable.forEach() vs foreach loop

Which of the following is better practice in Java 8?
Java 8:
joins.forEach(join -> mIrc.join(mSession, join));
Java 7:
for (String join : joins) {
mIrc.join(mSession, join);
}
I have lots of for loops that could be "simplified" with lambdas, but is there really any advantage of using them? Would it improve their performance and readability?
EDIT
I'll also extend this question to longer methods. I know that you can't return or break the parent function from a lambda and this should also be taken into consideration when comparing them, but is there anything else to be considered?

The advantage comes into account when the operations can be executed in parallel. (See http://java.dzone.com/articles/devoxx-2012-java-8-lambda-and - the section about internal and external iteration)
The main advantage from my point of view is that the implementation of what is to be done within the loop can be defined without having to decide if it will be executed in parallel or sequential
If you want your loop to be executed in parallel you could simply write
joins.parallelStream().forEach(join -> mIrc.join(mSession, join));
You will have to write some extra code for thread handling etc.
Note: For my answer I assumed joins implementing the java.util.Stream interface. If joins implements only the java.util.Iterable interface this is no longer true.

When reading this question one can get the impression, that Iterable#forEach in combination with lambda expressions is a shortcut/replacement for writing a traditional for-each loop. This is simply not true. This code from the OP:
joins.forEach(join -> mIrc.join(mSession, join));
is not intended as a shortcut for writing
for (String join : joins) {
mIrc.join(mSession, join);
}
and should certainly not be used in this way. Instead it is intended as a shortcut (although it is not exactly the same) for writing
joins.forEach(new Consumer<T>() {
#Override
public void accept(T join) {
mIrc.join(mSession, join);
}
});
And it is as a replacement for the following Java 7 code:
final Consumer<T> c = new Consumer<T>() {
#Override
public void accept(T join) {
mIrc.join(mSession, join);
}
};
for (T t : joins) {
c.accept(t);
}
Replacing the body of a loop with a functional interface, as in the examples above, makes your code more explicit: You are saying that (1) the body of the loop does not affect the surrounding code and control flow, and (2) the body of the loop may be replaced with a different implementation of the function, without affecting the surrounding code. Not being able to access non final variables of the outer scope is not a deficit of functions/lambdas, it is a feature that distinguishes the semantics of Iterable#forEach from the semantics of a traditional for-each loop. Once one gets used to the syntax of Iterable#forEach, it makes the code more readable, because you immediately get this additional information about the code.
Traditional for-each loops will certainly stay good practice (to avoid the overused term "best practice") in Java. But this doesn't mean, that Iterable#forEach should be considered bad practice or bad style. It is always good practice, to use the right tool for doing the job, and this includes mixing traditional for-each loops with Iterable#forEach, where it makes sense.
Since the downsides of Iterable#forEach have already been discussed in this thread, here are some reasons, why you might probably want to use Iterable#forEach:
To make your code more explicit: As described above, Iterable#forEach can make your code more explicit and readable in some situations.
To make your code more extensible and maintainable: Using a function as the body of a loop allows you to replace this function with different implementations (see Strategy Pattern). You could e.g. easily replace the lambda expression with a method call, that may be overwritten by sub-classes:
joins.forEach(getJoinStrategy());
Then you could provide default strategies using an enum, that implements the functional interface. This not only makes your code more extensible, it also increases maintainability because it decouples the loop implementation from the loop declaration.
To make your code more debuggable: Seperating the loop implementation from the declaration can also make debugging more easy, because you could have a specialized debug implementation, that prints out debug messages, without the need to clutter your main code with if(DEBUG)System.out.println(). The debug implementation could e.g. be a delegate, that decorates the actual function implementation.
To optimize performance-critical code: Contrary to some of the assertions in this thread, Iterable#forEach does already provide better performance than a traditional for-each loop, at least when using ArrayList and running Hotspot in "-client" mode. While this performance boost is small and negligible for most use cases, there are situations, where this extra performance can make a difference. E.g. library maintainers will certainly want to evaluate, if some of their existing loop implementations should be replaced with Iterable#forEach.
To back this statement up with facts, I have done some micro-benchmarks with Caliper. Here is the test code (latest Caliper from git is needed):
#VmOptions("-server")
public class Java8IterationBenchmarks {
public static class TestObject {
public int result;
}
public #Param({"100", "10000"}) int elementCount;
ArrayList<TestObject> list;
TestObject[] array;
#BeforeExperiment
public void setup(){
list = new ArrayList<>(elementCount);
for (int i = 0; i < elementCount; i++) {
list.add(new TestObject());
}
array = list.toArray(new TestObject[list.size()]);
}
#Benchmark
public void timeTraditionalForEach(int reps){
for (int i = 0; i < reps; i++) {
for (TestObject t : list) {
t.result++;
}
}
return;
}
#Benchmark
public void timeForEachAnonymousClass(int reps){
for (int i = 0; i < reps; i++) {
list.forEach(new Consumer<TestObject>() {
#Override
public void accept(TestObject t) {
t.result++;
}
});
}
return;
}
#Benchmark
public void timeForEachLambda(int reps){
for (int i = 0; i < reps; i++) {
list.forEach(t -> t.result++);
}
return;
}
#Benchmark
public void timeForEachOverArray(int reps){
for (int i = 0; i < reps; i++) {
for (TestObject t : array) {
t.result++;
}
}
}
}
And here are the results:
Results for -client
Results for -server
When running with "-client", Iterable#forEach outperforms the traditional for loop over an ArrayList, but is still slower than directly iterating over an array. When running with "-server", the performance of all approaches is about the same.
To provide optional support for parallel execution: It has already been said here, that the possibility to execute the functional interface of Iterable#forEach in parallel using streams, is certainly an important aspect. Since Collection#parallelStream() does not guarantee, that the loop is actually executed in parallel, one must consider this an optional feature. By iterating over your list with list.parallelStream().forEach(...);, you explicitly say: This loop supports parallel execution, but it does not depend on it. Again, this is a feature and not a deficit!
By moving the decision for parallel execution away from your actual loop implementation, you allow optional optimization of your code, without affecting the code itself, which is a good thing. Also, if the default parallel stream implementation does not fit your needs, no one is preventing you from providing your own implementation. You could e.g. provide an optimized collection depending on the underlying operating system, on the size of the collection, on the number of cores, and on some preference settings:
public abstract class MyOptimizedCollection<E> implements Collection<E>{
private enum OperatingSystem{
LINUX, WINDOWS, ANDROID
}
private OperatingSystem operatingSystem = OperatingSystem.WINDOWS;
private int numberOfCores = Runtime.getRuntime().availableProcessors();
private Collection<E> delegate;
#Override
public Stream<E> parallelStream() {
if (!System.getProperty("parallelSupport").equals("true")) {
return this.delegate.stream();
}
switch (operatingSystem) {
case WINDOWS:
if (numberOfCores > 3 && delegate.size() > 10000) {
return this.delegate.parallelStream();
}else{
return this.delegate.stream();
}
case LINUX:
return SomeVerySpecialStreamImplementation.stream(this.delegate.spliterator());
case ANDROID:
default:
return this.delegate.stream();
}
}
}
The nice thing here is, that your loop implementation doesn't need to know or care about these details.

forEach() can be implemented to be faster than for-each loop, because the iterable knows the best way to iterate its elements, as opposed to the standard iterator way. So the difference is loop internally or loop externally.
For example ArrayList.forEach(action) may be simply implemented as
for(int i=0; i<size; i++)
action.accept(elements[i])
as opposed to the for-each loop which requires a lot of scaffolding
Iterator iter = list.iterator();
while(iter.hasNext())
Object next = iter.next();
do something with `next`
However, we also need to account for two overhead costs by using forEach(), one is making the lambda object, the other is invoking the lambda method. They are probably not significant.
see also http://journal.stuffwithstuff.com/2013/01/13/iteration-inside-and-out/ for comparing internal/external iterations for different use cases.

TL;DR: List.stream().forEach() was the fastest.
I felt I should add my results from benchmarking iteration.
I took a very simple approach (no benchmarking frameworks) and benchmarked 5 different methods:
classic for
classic foreach
List.forEach()
List.stream().forEach()
List.parallelStream().forEach
the testing procedure and parameters
private List<Integer> list;
private final int size = 1_000_000;
public MyClass(){
list = new ArrayList<>();
Random rand = new Random();
for (int i = 0; i < size; ++i) {
list.add(rand.nextInt(size * 50));
}
}
private void doIt(Integer i) {
i *= 2; //so it won't get JITed out
}
The list in this class shall be iterated over and have some doIt(Integer i) applied to all it's members, each time via a different method.
in the Main class I run the tested method three times to warm up the JVM. I then run the test method 1000 times summing the time it takes for each iteration method (using System.nanoTime()). After that's done i divide that sum by 1000 and that's the result, average time.
example:
myClass.fored();
myClass.fored();
myClass.fored();
for (int i = 0; i < reps; ++i) {
begin = System.nanoTime();
myClass.fored();
end = System.nanoTime();
nanoSum += end - begin;
}
System.out.println(nanoSum / reps);
I ran this on a i5 4 core CPU, with java version 1.8.0_05
classic for
for(int i = 0, l = list.size(); i < l; ++i) {
doIt(list.get(i));
}
execution time: 4.21 ms
classic foreach
for(Integer i : list) {
doIt(i);
}
execution time: 5.95 ms
List.forEach()
list.forEach((i) -> doIt(i));
execution time: 3.11 ms
List.stream().forEach()
list.stream().forEach((i) -> doIt(i));
execution time: 2.79 ms
List.parallelStream().forEach
list.parallelStream().forEach((i) -> doIt(i));
execution time: 3.6 ms

One of most upleasing functional forEach's limitations is lack of checked exceptions support.
One possible workaround is to replace terminal forEach with plain old foreach loop:
Stream<String> stream = Stream.of("", "1", "2", "3").filter(s -> !s.isEmpty());
Iterable<String> iterable = stream::iterator;
for (String s : iterable) {
fileWriter.append(s);
}
Here is list of most popular questions with other workarounds on checked exception handling within lambdas and streams:
Java 8 Lambda function that throws exception?
Java 8: Lambda-Streams, Filter by Method with Exception
How can I throw CHECKED exceptions from inside Java 8 streams?
Java 8: Mandatory checked exceptions handling in lambda expressions. Why mandatory, not optional?

The advantage of Java 1.8 forEach method over 1.7 Enhanced for loop is that while writing code you can focus on business logic only.
forEach method takes java.util.function.Consumer object as an argument, so It helps in having our business logic at a separate location that you can reuse it anytime.
Have look at below snippet,
Here I have created new Class that will override accept class method from Consumer Class,
where you can add additional functionility, More than Iteration..!!!!!!
class MyConsumer implements Consumer<Integer>{
#Override
public void accept(Integer o) {
System.out.println("Here you can also add your business logic that will work with Iteration and you can reuse it."+o);
}
}
public class ForEachConsumer {
public static void main(String[] args) {
// Creating simple ArrayList.
ArrayList<Integer> aList = new ArrayList<>();
for(int i=1;i<=10;i++) aList.add(i);
//Calling forEach with customized Iterator.
MyConsumer consumer = new MyConsumer();
aList.forEach(consumer);
// Using Lambda Expression for Consumer. (Functional Interface)
Consumer<Integer> lambda = (Integer o) ->{
System.out.println("Using Lambda Expression to iterate and do something else(BI).. "+o);
};
aList.forEach(lambda);
// Using Anonymous Inner Class.
aList.forEach(new Consumer<Integer>(){
#Override
public void accept(Integer o) {
System.out.println("Calling with Anonymous Inner Class "+o);
}
});
}
}

Recursion or Looping [duplicate]

This question already has answers here:
Recursion or iteration?
(14 answers)
Closed 2 years ago.
I have this method that calculates some statistics:
public void calculateAverage(int hour){
if (hour != 20) {
int data =0;
int times = 0;
for (CallQueue cq : queues) {
data += cq.getCallsByTime().get(hour);
times++;
}
averageData.add((double)data/times);
calculateAverage(hour + 1);
}
}
Now I am very proud that I have created a recursive method but I know that this could have been solved with a loop.
My question is: is it better to solve these kind of problems recursive or with a loop?

Recursion in general
In general, a recursion would be more expensive, because the stack has to be modified with copies of variables for each time the function recurses.
A set of addresses & states need to be saved, so that the recursive procedure can return to the right state after that particular run.
Iteration would be better if possible. Recursion, when iteration just won't cut it, or will result in a lot more complicated code.
Code Maintenance
From a maintenance perspective, debugging iterative code is a lot easier than recursive procedures as it is relatively easier to understand what the state is at any particular iteration, as compared to thinking about a particular recursion.
Your code
The procedure calls itself, but each run has nothing to do with the results of the previous run. Each run being independent, is usually the biggest give-away, that recursion there might not be necessary.
In my opinion, calculateAverage(hour + 1); should be moved outside the function, as it would also be clearer to someone reading your code. that each call is independent.

In Java, C, and Python, recursion is fairly expensive compared to iteration (in general) because it requires the allocation of a new stack frame. In some C compilers, one can use a compiler flag to eliminate this overhead, which transforms certain types of recursion (actually, certain types of tail calls) into jumps instead of function calls. (source)

For this particular problem there isn't too much of a runtime difference. I personally would rather use iteration, I think it would be more simple and easier to understand, but to each his own I suppose.
now some recursive functions(like recursive Fibonacci numbers for example) should be done by iteration instead, simply because they can have exponential growth.
generally, I don't use recursion unless It would make my problem actually easier to understand.

You should investigate the perimeter circumstances. For big recursions stack might get overflow, thats +1 for loops.
I'm not sure which one runs faster but that is relatively easy to measure, taking JIT and other stuff into considerations.
Code maintenance aspect: it is much easier for the most of us to understand and fix loops than recursion. Developers time is usually more important than minor performance differences.

It depends on the context. For example if I have a tree of Composite objects (in SWT) and you wish to traverse them the easiest way is to use recursion like this:
private boolean checkControlParent(Composite comp) {
boolean ret = false;
if (comp != null) {
if (this.equals(comp)) {
ret = true;
} else {
ret = checkControlParent(comp.getParent());
}
}
return ret;
}
otherwise if performance is important be advised that recursive calls are slower in most cases than simple loops because of the function/method call overhead.
So the main thing is that if you need to iterate through objects where recursion is a natural solution and you don't risk a StackOverflowError go ahead and use recursion. Otherwise you'll probably better off with a loop.
One more thing: recursive methods are sometimes tend to be harder to read, understand and debug.

Best Practice in `For` loop in java [duplicate]

This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
for loop optimization
In java i have a block of code:
List e = {element1, element2, ...., elementn};
for(int i = 0; i < e.size(); i++){//Do something in here
};
and another block:
List e = {element1, element2, ...., elementn};
int listSize = e.size();
for(int i = 0; i < listSize; i++){//Do something in here
};
I think that the second block is better, because in the first block, if i++, we have to calculate e.size() one more times to compare the condition in the for loop. Is it right or wrong?
And comparing the two block above, what is the best practice for writing for? And why?Explain clearly and try this loop yourself

Personally I'd use the enhanced for statement instead:
for (Object element : e) {
// Use element
}
Unless you need the index, of course.
If I had to use one of the two forms, I'd use the first as it's tidier (it doesn't introduce another local variable which is only used in that loop), until I had concrete evidence that it was causing a problem. (In most list implementations, e.size() is a simple variable access which can be inlined by the JIT anyway.)

Usually, the most brief and readable code is the best choice, all things being equal. In the case of Java, the enhanced for loop (which works with any class that implements Iterable) is the way to go.
for (Object object : someCollection) { // do something }
In terms solely of the two you posted, I think the first is the better option. It's more readable, and you have to remember that, under the hood, JIT will attempt to optimize a great deal of the code you write anyway.
EDIT: Have you heard the phrase "premature optimisation is the root of all evil"? Your second block is an example of premature optimisation.

If you check the size() implementation on a LinkedList class, you will find that the size is incemented or decremented when an element is added or removed from the list.
Calling size() just returns the value of this property and does not involve any calculation.
So directly calling size() method should be better as you will save on the save for another integer.

I would always use (if you need an index variable):
List e = {element1, element2, ...., elementn};
for(int i = 0, size = e.size(); i < size; i++){
// Do something in here
};
Since e.size() could be an expensive operation.
Your 2nd option is not good, since it introduces a new variable outside of the for loop. I recommend to keep variable visibility as limited as possible.
Otherwise a
for (MyClass myObj : list) {
// Do something here
}
is even cleaner, but might introduce a small performance hit (the index approach doesn't require to instantiate an Iterator).

Yes, the second form is marginally more efficient as you don't repeated perform the size() method invocation. Compilers are good are doing this sort of optimisation themselves.
However, it's unlikely that this would be the performance bottleneck of your application. Avoid premature optimisation. Make your code clean and readable foremost.

HotSpot will move e.size() from cycle in most cases. So it will calculate size of List only once.
As for me I prefer the following notation:
for (Object elem: e) {
//Do something
}

i think this should be much more better..
may be initializing the int variable every time can be escaped from this..
List e = {element1, element2, ...., elementn};
int listSize = e.size();
int i=0;
for(i = 0; i < listSize; i++){//Do something in here
};

Second one is better approach because in the first block, you are calling the e.size() is a method which is an operation in a loop that is a extra burden to JVM.

Im not so sure but i think the optimizer of java will replace the value with a static value, so in the end it will be the same.

To avoid all this numbering and iterators and checkings in writing the code use the following simple most readable code that has its performance to maximum.
Why this has maximum performance (details are coming up)
for (Object object : aCollection) {
// Do something here
}
If the index is needed then:
To choose between the above two forms:
The second is the better as you said because it only calculated the size once.

I think now we have a tendency to write short and understandable code, so the first option is better.

the second is better , cos in the firt loop in the body of it maybe u will do this statment
e.remove, and then the size of e will be changed , so it is better to save the size in a parameter before the looop

foreach loop a java creation?

Why was this loop introduced in java?Is it a java creation? What is its purpose(increases memory/cpu utilisation efficiency)?

Why was this loop introduced in java?
It's just to ease looping over generic collections and arrays. Instead of
for (int i = 0; i < strings.length; i++) {
String string = strings[i];
// ...
}
you can just do
for (String string : strings) {
// ...
}
which makes the code more readable and better maintainable.
Is it a java creation?
No, it existed in other languages long before Java. Java was relatively late in implementing it.
What is its purpose?
See the first answer.
To learn more about it, checkout the Sun guide on the subject.
Update: this does not mean that it makes the other kinds of loops superfluous. the for loop using index is still useful if you'd like to maintain a loop counter for other purposes than getting the item by index. The for loop using an iterator is still useful if you'd like to remove or change elements of the collection itself inside a loop.

It masks the use of iterators, which are heavy and clumsy to use. There are many, many instances where you just want to iterate over a collection without working about its index. The java foreach structure makes this possible.

Please see Foreach:
For each (or foreach) is a computer
language idiom for traversing items in
a collection. Foreach is usually used
in place of a standard for statement.
Unlike other for loop constructs,
however, foreach loops 1 usually
maintain no explicit counter: they
essentially say "do this to everything
in this set", rather than "do this x
times". This avoids potential
off-by-one errors and makes code
simpler to read. In object-oriented
languages an iterator, even if
implicit, is often used as the means
of traversal.
Several languages, including Python,
have only a foreach loop, requiring
explicit counting to achieve
"standard" for behavior.
And specifically the section on Java:
A foreach-construct was introduced in JDK 5.0. Official sources use several names for the construct. It is referred to as the "Enhanced for Loop" the "For-Each Loop" and the "foreach statement".

It's really just Java's imitation of a functional construct that's been around much longer, it's called map. The reason for implementing it is that it is common to make a loop that simply performs an action to every element of a container without regard to it's index. Java's for(element : container) { doSomethingWith(element); } syntax is just a cleaner way to do it than the alternatives, which are either to make a for loop with an index
for(int i=0; i<container.size(); ++i) { doSomethingWith(container.at(i)); }
which is longer and creates a needless index variable, or to do a loop with an iterator
Iterator it = container.iterator();
while(it.hasNext()) { doSomethingWith(it.next()); }
which is also longer. This loop is essentially what the for( : ) {} loop gets compiled as, although there may be some slight differences (I haven't actually seen the bytecode).

It is plain "Syntactic sugar"
Dont think there is any efficiency improvement.
Java community wanted the language to be a bit modernized, competing with C# and Ruby..

Develop Reference

Java is a programming language and computing platform first released by Sun Microsystems in 1995.