Break or return from Java 8 stream forEach

Question

When using external iteration over an Iterable we use break or return from enhanced for-each loop as   for  SomeObject obj   someObjects       if  some condition met          break     or return obj          How can we break or return using the internal iteration in a Java 8 lambda expression like   someObjects forEach obj - gt         what to do here

User · Answer

You can achieve that using a mix of peek(..) and anyMatch(..).

Using your example:

someObjects.stream().peek(obj -> {
   <your code here>
}).anyMatch(obj -> !<some_condition_met>);

Or just write a generic util method:

public static <T> void streamWhile(Stream<T> stream, Predicate<? super T> predicate, Consumer<? super T> consumer) {
    stream.peek(consumer).anyMatch(predicate.negate());
}

And then use it, like this:

streamWhile(someObjects.stream(), obj -> <some_condition_met>, obj -> {
   <your code here>
});

User · Answer

public static void main String   args        List lt String gt  list   Arrays asList  quot one quot    quot two quot    quot three quot    quot seven quot    quot nine quot        AtomicBoolean yes   new AtomicBoolean true       list stream   takeWhile value - gt  yes get    forEach value - gt            System out println  quot prior cond quot    value           if  value equals  quot two quot                  System out println value               yes set false                            System out println  quot Hello World quot

User · Answer

A return in a lambda equals a continue in a for-each  but there is no equivalent to a break  You can just do a return to continue   someObjects forEach obj - gt       if  some condition met          return

User · Answer

I have achieved by something like this     private void doSomething                 List lt Action gt  actions   actionRepository findAll                boolean actionHasFormFields   actions stream   anyMatch actionHasMyFieldsPredicate                 if  actionHasFormFields                   context addError someError                                      private Predicate lt Action gt  actionHasMyFieldsPredicate            return action - gt  action getMyField1      null

User · Answer

This is possible for Iterable forEach    but not reliably with Stream forEach     The solution is not nice  but it is possible   WARNING  You should not use it for controlling business logic  but purely for handling an exceptional situation which occurs during the execution of the forEach    Such as a resource suddenly stops being accessible  one of the processed objects is violating a contract  e g  contract says that all the elements in the stream must not be null but suddenly and unexpectedly one of them is null  etc   According to the documentation for Iterable forEach        Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception    Exceptions thrown by the action are relayed to the caller    So you throw an exception which will immediately break the internal loop   The code will be something like this - I cannot say I like it but it works  You create your own class BreakException which extends RuntimeException   try       someObjects forEach obj - gt               some useful code here         if some exceptional condition met                throw new BreakException                     catch  BreakException e           here you know that your condition has been met at least once     Notice that the try   catch is not around the lambda expression  but rather around the whole forEach   method  To make it more visible  see the following transcription of the code which shows it more clearly   Consumer lt   super SomeObject gt  action   obj - gt           some useful code here     if some exceptional condition met            throw new BreakException               try       someObjects forEach action     catch  BreakException e           here you know that your condition has been met at least once

User · Answer

int valueToMatch   7  Stream of 1 2 3 4 5 6 7 8  anyMatch val- gt      boolean isMatch   val    valueToMatch     if isMatch            Do whatever you want             System out println val           return isMatch        It will do only operation where it find match  and after find match it stop it s iteration

User · Answer

For maximal performance in parallel operations use findAny   which is similar to findFirst     Optional lt SomeObject gt  result       someObjects stream   filter obj - gt  some condition met  findAny      However If a stable result is desired  use findFirst   instead   Also note that matching patterns  anyMatch   allMatch  will return only boolean  you will not get matched object

User · Answer

Update with Java 9  with takeWhile   MutableBoolean ongoing   MutableBoolean of true   someobjects stream     takeWhile t - gt  ongoing value    forEach t - gt           doing something      if            want to break          ongoing setFalse

User · Answer

Below you find the solution I used in a project  Instead forEach just use allMatch   someObjects allMatch obj - gt        return  some condition met

User · Answer

What about this one   final BooleanWrapper condition   new BooleanWrapper    someObjects forEach obj - gt       if  condition ok              YOUR CODE to control      condition stop               Where BooleanWrapper is a class you must implement to control the flow

User · Answer

If you need this  you shouldn t use forEach  but one of the other methods available on streams  which one  depends on what your goal is   For example  if the goal of this loop is to find the first element which matches some predicate   Optional lt SomeObject gt  result       someObjects stream   filter obj - gt  some condition met  findFirst       Note  This will not iterate the whole collection  because streams are lazily evaluated - it will stop at the first object that matches the condition    If you just want to know if there s an element in the collection for which the condition is true  you could use anyMatch   boolean result   someObjects stream   anyMatch obj - gt  some condition met

User · Answer

You can use java8   rxjava     import java util stream IntStream    import rx Observable       IntStream intStream    IntStream range 1 10000000       Observable from    - gt  intStream iterator                 takeWhile n - gt  n  lt  10               forEach n- gt  System out println n

User · Answer

I would suggest using anyMatch  Example -  return someObjects stream   anyMatch obj - gt       some condition met       You can refer this post for understanding anyMatch - https   beginnersbook com 2017 11 java-8-stream-anymatch-example

User · Answer

Either you need to use a method which uses a predicate indicating whether to keep going  so it has the break instead  or you need to throw an exception - which is a very ugly approach  of course   So you could write a forEachConditional method like this   public static  lt T gt  void forEachConditional Iterable lt T gt  source                                            Predicate lt T gt  action        for  T item   source            if   action test item                 break                      Rather than Predicate lt T gt   you might want to define your own functional interface with the same general method  something taking a T and returning a bool  but with names that indicate the expectation more clearly - Predicate lt T gt  isn t ideal here

[java] Break or return from Java 8 stream forEach?

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