Find first element by predicate

Question

I ve just started playing with Java 8 lambdas and I m trying to implement some of the things that I m used to in functional languages   For example  most functional languages have some kind of find function that operates on sequences  or lists that returns the first element  for which the predicate is true  The only way I can see to achieve this in Java 8 is   lst stream        filter x - gt  x  gt  5       findFirst     However this seems inefficient to me  as the filter will scan the whole list  at least to my understanding  which could be wrong   Is there a better way

User · Answer

Improved One-Liner answer  If you are looking for a boolean return value  we can do it better by adding isPresent       return dataSource getParkingLots   stream   filter parkingLot - gt  Objects equals parkingLot getId    id   findFirst   isPresent

User · Answer

return dataSource getParkingLots                     stream                     filter parkingLot - gt  Objects equals parkingLot getId    id                     findFirst                     orElse null     I had to filter out only one object from a list of objects  So i used this  hope it helps

User · Answer

No  filter does not scan the whole stream  It s an intermediate operation  which returns a lazy stream  actually all intermediate operations return a lazy stream   To convince you  you can simply do the following test   List lt Integer gt  list   Arrays asList 1  10  3  7  5   int a   list stream                peek num - gt  System out println  will filter     num                filter x - gt  x  gt  5               findFirst                get    System out println a     Which outputs   will filter 1 will filter 10 10   You see that only the two first elements of the stream are actually processed   So you can go with your approach which is perfectly fine

User · Answer

Already answered by  AjaxLeung  but in comments and hard to find  For check only lst stream        filter x - gt  x  gt  5       findFirst        isPresent    is simplified to lst stream        anyMatch x - gt  x  gt  5

User · Answer

In addition to Alexis C s answer  If you are working with an array list  in which you are not sure whether the element you are searching for exists  use this   Integer a   list stream                    peek num - gt  System out println  will filter     num                    filter x - gt  x  gt  5                   findFirst                    orElse null     Then you could simply check whether a is null

User · Answer

However this seems inefficient to me  as the filter will scan the whole list   No it won t - it will  break  as soon as the first element satisfying the predicate is found  You can read more about laziness in the stream package javadoc  in particular  emphasis mine       Many stream operations  such as filtering  mapping  or duplicate removal  can be implemented lazily  exposing opportunities for optimization  For example   find the first String with three consecutive vowels  need not examine all the input strings  Stream operations are divided into intermediate  Stream-producing  operations and terminal  value- or side-effect-producing  operations  Intermediate operations are always lazy

User · Answer

import org junit Test   import java util Arrays  import java util List  import java util Optional      Stream is  30 times slower for same operation    public class StreamPerfTest        int iterations   100      List lt Integer gt  list   Arrays asList 1  10  3  7  5            55 ms      Test     public void stream              for  int i   0  i  lt  iterations  i                  Optional lt Integer gt  result   list stream                        filter x - gt  x  gt  5                       findFirst                 System out println result orElse null                            2 ms      Test     public void loop              for  int i   0  i  lt  iterations  i                  Integer result   null              for  Integer walk   list                    if  walk  gt  5                        result   walk                      break                                              System out println result

[java] Find first element by predicate

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