Grab a segment of an array in Java without creating a new array on heap

Question

I m looking for a method in Java that will return a segment of an array  An example would be to get the byte array containing the 4th and 5th bytes of a byte array  I don t want to have to create a new byte array in the heap memory just to do that   Right now I have the following code   doSomethingWithTwoBytes byte   twoByteArray    void someMethod byte   bigArray          byte   x    bigArray 4   bigArray 5          doSomethingWithTwoBytes x       I d like to know if there was a way to just do doSomething bigArray getSubArray 4  2   where 4 is the offset and 2 is the length  for example

User · Answer

You could use the ArrayUtils subarray in apache commons   Not perfect but a bit more intuitive than System arraycopy   The downside is that it does introduce another dependency into your code

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One option would be to pass the whole array and the start and end indices  and iterate between those instead of iterating over the whole array passed   void method1 byte   array        method2 array 4 5     void method2 byte   smallarray int start int end        for   int i   start  i  lt   end  i

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Use java nio Buffer s  It s a lightweight wrapper for buffers of various primitive types and helps manage slicing  position  conversion  byte ordering  etc   If your bytes originate from a Stream  the NIO Buffers can use  direct mode  which creates a buffer backed by native resources  This can improve performance in a lot of cases

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How about a thin List wrapper   List lt Byte gt  getSubArrayList byte   array  int offset  int size       return new AbstractList lt Byte gt            Byte get int index             if  index  lt  0    index  gt   size              throw new IndexOutOfBoundsException             return array offset index                 int size              return size                     Untested

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I see the subList answer is already here  but here s code that demonstrates that it s a true sublist  not a copy   public class SubListTest extends TestCase       public void testSubarray   throws Exception           Integer   array    1  2  3  4  5           List lt Integer gt  list   Arrays asList array           List lt Integer gt  subList   list subList 2  4           assertEquals 2  subList size             assertEquals  Integer  3  subList get 0            list set 2  7           assertEquals  Integer  7  subList get 0              I don t believe there s a good way to do this directly with arrays  however

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One way is to wrap the array in java nio ByteBuffer  use the absolute put get functions  and slice the buffer to work on a subarray   For instance   doSomething ByteBuffer twoBytes        byte b1   twoBytes get 0       byte b2   twoBytes get 1              void someMethod byte   bigArray          int offset   4        int length   2        doSomething ByteBuffer wrap bigArray  offset  length  slice         Note that you have to call both wrap   and slice    since wrap   by itself only affects the relative put get functions  not the absolute ones   ByteBuffer can be a bit tricky to understand  but is most likely efficiently implemented  and well worth learning

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List subList int startIndex  int endIndex

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Arrays asList myArray  delegates to new ArrayList myArray   which doesn t copy the array but just stores the reference  Using List subList start  end  after that makes a SubList which just references the original list  which still just references the array   No copying of the array or its contents  just wrapper creation  and all lists involved are backed by the original array   I thought it d be heavier

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Java references always point to an object  The object has a header that amongst other things identifies the concrete type  so casts can fail with ClassCastException   For arrays  the start of the object also includes the length  the data then follows immediately after in memory  technically an implementation is free to do what it pleases  but it would be daft to do anything else   So  you can t have a reference that points somewhere into an array   In C pointers point anywhere and to anything  and you can point to the middle of an array  But you can t safely cast or find out how long the array is  In D the pointer contains an offset into the memory block and length  or equivalently a pointer to the end  I can t remember what the implementation actually does   This allows D to slice arrays  In C   you would have two iterators pointing to the start and end  but C   is a bit odd like that   So getting back to Java  no you can t  As mentioned  NIO ByteBuffer allows you to wrap an array and then slice it  but gives an awkward interface  You can of course copy  which is probably very much faster than you would think  You could introduce your own String-like abstraction that allows you to slice an array  the current Sun implementation of String has a char   reference plus a start offset and length  higher performance implementation just have the char     byte   is low level  but any class-based abstraction you put on that is going to make an awful mess of the syntax  until JDK7  perhaps

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The Lists allow you to use and work with subList of something transparently  Primitive arrays would require you to keep track of some kind of offset - limit  ByteBuffers have similar options as I heard   Edit  If you are in charge of the useful method  you could just define it with bounds  as done in many array related methods in java itself   doUseful byte   arr  int start  int len           implementation here   doUseful byte   arr        doUseful arr  0  arr length       It s not clear  however  if you work on the array elements themselves  e g  you compute something and write back the result

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This is a little more lightweight than Arrays copyOfRange - no range or negative   public static final byte   copy byte   data  int pos  int length         byte   transplant   new byte length        System arraycopy data  pos  transplant  0  length        return transplant

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I needed to iterate through the end of an array and didn t want to copy the array  My approach was to make an Iterable over the array   public static Iterable lt String gt  sliceArray final String   array                                             final int start      return new Iterable lt String gt          String   values   array      int posn   start        Override     public Iterator lt String gt  iterator           return new Iterator lt String gt               Override         public boolean hasNext               return posn  lt  values length                      Override         public String next               return values posn                         Override         public void remove               throw new UnsupportedOperationException  No remove

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unique72 answer as a simple function or line  you may need to replace Object  with the respective class type you wish to  slice   Two variants are given to suit various needs       Extract out array from starting position onwards public static Object   sliceArray  Object   inArr  int startPos         return Arrays asList inArr  subList startPos  inArr length  toArray           Extract out array from starting position to ending position public static Object   sliceArray  Object   inArr  int startPos  int endPos         return Arrays asList inArr  subList startPos  endPos  toArray

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If you re seeking a pointer style aliasing approach  so that you don t even need to allocate space and copy the data then I believe you re out of luck   System arraycopy    will copy from your source to destination  and efficiency is claimed for this utility  You do need to allocate the destination array

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Disclaimer  This answer does not conform to the constraints of the question      I don t want to have to create a new byte array in the heap memory just to do that     Honestly  I feel my answer is worthy of deletion  The answer by  unique72 is correct  Imma let this edit sit for a bit and then I shall delete this answer       I don t know of a way to do this directly with arrays without additional heap allocation  but the other answers using a sub-list wrapper have additional allocation for the wrapper only     but not the array      which would be useful in the case of a large array     That said  if one is looking for brevity  the utility method Arrays copyOfRange   was introduced in Java 6  late 2006     byte    a   new byte     0  1  2  3  4  5  6  7       get a 4   a 5   byte    subArray   Arrays copyOfRange a  4  6

[java] Grab a segment of an array in Java without creating a new array on heap

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