Converting Array to List

Question

In order to convert an Integer array to a list of integer I have tried the following approaches    Initialize a list Integer type   iterate over the array and insert into the                    list By using Java 8 Streams   int   ints    1  2  3   List lt Integer gt  list   new ArrayList lt Integer gt     Collections addAll list  Arrays stream ints  boxed   toArray Integer    new       Which is better in terms of performance

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where stateb is  List   bucket is a two dimensional array  statesb     IntStream of bucket j-1   boxed   collect Collectors toList      with  import java util stream IntStream   see https   examples javacodegeeks com core-java java8-convert-array-list-example

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If you don t want to alter the list   List lt Integer gt  list   Arrays asList array    But if you want to modify it then you can use this   List lt Integer gt  list   new ArrayList lt Integer gt  Arrays asList ints      Or just use java8 like the following   List lt Integer gt  list   Arrays stream ints  collect Collectors toList       Java9 has introduced this method   List lt Integer gt  list   List of ints     However  this will return an immutable list that you can t add to   You need to do the following to make it mutable   List lt Integer gt  list   new ArrayList lt Integer gt  List of ints

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Arrays stream ints  forEach list  add     This basically does 1  iterate over the array  with 2  using Java 8    with 1 and 2 referring to your original question

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Old way but still working    int   values    1 2 3 4    List lt Integer gt  list   new ArrayList lt  gt  values length     for int valor   values         list add valor

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If you don t mind a third-party dependency  you could use a library which natively supports primitive collections like Eclipse Collections and avoid the boxing altogether  You can also use primitive collections to create boxed regular collections if you need to   int   ints    1  2  3   MutableIntList intList   IntLists mutable with ints   List lt Integer gt  list   intList collect Integer  valueOf     If you want the boxed collection in the end  this is what the code for collect on IntArrayList is doing under the covers   public  lt V gt  MutableList lt V gt  collect IntToObjectFunction lt   extends V gt  function        return this collect function  FastList newList this size       public  lt V  R extends Collection lt V gt  gt  R collect IntToObjectFunction lt   extends V gt  function                                                 R target        for  int i   0  i  lt  this size  i                  target add function valueOf this items i               return target      Since the question was specifically about performance  I wrote some JMH benchmarks using your solutions  the most voted answer and the primitive and boxed versions of Eclipse Collections    import org eclipse collections api list primitive IntList  import org eclipse collections impl factory primitive IntLists  import org openjdk jmh annotations Benchmark  import org openjdk jmh annotations BenchmarkMode  import org openjdk jmh annotations Fork  import org openjdk jmh annotations Mode  import org openjdk jmh annotations OutputTimeUnit  import org openjdk jmh annotations Scope  import org openjdk jmh annotations State  import org openjdk jmh runner Runner  import org openjdk jmh runner RunnerException  import org openjdk jmh runner options Options  import org openjdk jmh runner options OptionsBuilder   import java util ArrayList  import java util Arrays  import java util Collections  import java util List  import java util concurrent TimeUnit  import java util stream Collectors  import java util stream IntStream    State Scope Thread   BenchmarkMode Mode Throughput   OutputTimeUnit TimeUnit SECONDS   Fork 2  public class IntegerArrayListFromIntArray       private int   source   IntStream range 0  1000  toArray         public static void main String   args  throws RunnerException               Options options   new OptionsBuilder   include                         IntegerArrayListFromIntArray class getSimpleName                            forks 2                   mode Mode Throughput                   timeUnit TimeUnit SECONDS                   build            new Runner options  run                Benchmark     public List lt Integer gt  jdkClassic                 List lt Integer gt  list   new ArrayList lt  gt  source length           for  int each   source                        list add each                     return list              Benchmark     public List lt Integer gt  jdkStreams1                 List lt Integer gt  list   new ArrayList lt  gt  source length           Collections addAll list                  Arrays stream source  boxed   toArray Integer    new            return list              Benchmark     public List lt Integer gt  jdkStreams2                 return Arrays stream source  boxed   collect Collectors toList                 Benchmark     public IntList ecPrimitive                 return IntLists immutable with source               Benchmark     public List lt Integer gt  ecBoxed                 return IntLists mutable with source  collect Integer  valueOf             These are the results from these tests on my Mac Book Pro  The units are operations per second  so the bigger the number  the better  I used an ImmutableIntList for the ecPrimitive benchmark  because the MutableIntList in Eclipse Collections doesn t copy the array by default  It merely adapts the array you give it  This was reporting even larger numbers for ecPrimitive  with a very large margin of error because it was essentially measuring the cost of a single object creation      Run complete  Total time  00 06 52  Benchmark                                  Mode  Cnt        Score      Error  Units IntegerArrayListFromIntArray ecBoxed      thrpt   40   191671 859    2107 723  ops s IntegerArrayListFromIntArray ecPrimitive  thrpt   40  2311575 358    9194 262  ops s IntegerArrayListFromIntArray jdkClassic   thrpt   40   138231 703    1817 613  ops s IntegerArrayListFromIntArray jdkStreams1  thrpt   40    87421 892    1425 735  ops s IntegerArrayListFromIntArray jdkStreams2  thrpt   40   103034 520    1669 947  ops s   If anyone spots any issues with the benchmarks  I ll be happy to make corrections and run them again     Note  I am a committer for Eclipse Collections

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the first way is better  the second way cost more time on creating a new array and converting to a list

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If you are dealing with String   instead of int    We can use  ArrayList lt String gt  list   new ArrayList lt  gt     list addAll Arrays asList StringArray

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Simply try something like   Arrays asList array

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The second one creates a new array of Integers  first pass   and then adds all the elements of this new array to the list  second pass   It will thus be less efficient than the first one  which makes a single pass and doesn t create an unnecessary array of Integers   A better way to use streams would be  List lt Integer gt  list   Arrays stream ints  boxed   collect Collectors toList       Which should have roughly the same performance as the first one   Note that for such a small array  there won t be any significant difference  You should try to write correct  readable  maintainable code instead of focusing on performance

[arrays] Converting Array to List

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