How to cast from List Double to double in Java

Question

I have a variable like that   List lt Double gt  frameList    new ArrayList lt Double gt         Double elements has added to frameList      How can I have a new variable has a type of double   from that variable in Java with high performance

User · Answer

You can use primitive collections from Eclipse Collections and avoid boxing altogether.

DoubleList frameList = DoubleLists.mutable.empty();
double[] arr = frameList.toArray();

If you can't or don't want to initialize a DoubleList:

List<Double> frames = new ArrayList<>();
double[] arr = ListAdapter.adapt(frames).asLazy().collectDouble(each -> each).toArray();

Note: I am a contributor to Eclipse Collections.

User · Answer

You can use the ArrayUtils class from commons-lang to obtain a double   from a Double     Double   ds   frameList toArray new Double frameList size          double   d   ArrayUtils toPrimitive ds

User · Answer

High performance - every Double object wraps a single double value  If you want to store all these values into a double   array  then you have to iterate over the collection of Double instances  A O 1  mapping is not possible  this should be the fastest you can get    double   target   new double doubles size      for  int i   0  i  lt  target length  i          target i    doubles get i  doubleValue        java 1 4 style        or      target i    doubles get i                     java 1 5  style  outboxing         Thanks for the additional question in the comments    Here s the sourcecode of the fitting ArrayUtils toPrimitive method   public static double   toPrimitive Double   array      if  array    null        return null      else if  array length    0        return EMPTY DOUBLE ARRAY        final double   result   new double array length     for  int i   0  i  lt  array length  i          result i    array i  doubleValue          return result       And trust me  I didn t use it for my first answer - even though it looks     pretty similiar  -D    By the way  the complexity of Marcelos answer is O 2n   because it iterates twice  behind the scenes   first to make a Double   from the list  then to unwrap the double values

User · Answer

You can convert to a Double   by calling frameList toArray new Double frameList size      but you ll need to iterate the list array to convert to double

User · Answer

Guava has a method to do this for you  double   Doubles toArray Collection lt Double gt    This isn t necessarily going to be any faster than just looping through the Collection and adding each Double object to the array  but it s a lot less for you to write

User · Answer

As per your question   List lt Double gt  frameList    new ArrayList lt Double gt        First you have to convert List lt Double gt  to Double   by using   Double   array   frameList toArray new Double frameList size       Next you can convert Double   to double   using  double   doubleArray   ArrayUtils toPrimitive array      You can directly use it in one line   double   array   ArrayUtils toPrimitive frameList toArray new Double frameList size

User · Answer

With java-8  you can do it this way   double   arr   frameList stream   mapToDouble Double  doubleValue  toArray      via method reference double   arr   frameList stream   mapToDouble d - gt  d  toArray      identity function  Java unboxes automatically to get the double value   What it does is     get the Stream lt Double gt  from the list map each double instance to its primitive value  resulting in a DoubleStream call toArray   to get the array

[java] How to cast from List<Double> to double[] in Java?

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