Immutable array in Java

Question

Is there an immutable alternative to the primitive arrays in Java  Making a primitive array final doesn t actually prevent one from doing something like  final int   array   new int    0  1  2  3   array 0    42    I want the elements of the array to be unchangeable

User · Answer

No  this is not possible  However  one could do something like this   List lt Integer gt  temp   new ArrayList lt Integer gt     temp add Integer valueOf 0    temp add Integer valueOf 2    temp add Integer valueOf 3    temp add Integer valueOf 4    List lt Integer gt  immutable   Collections unmodifiableList temp     This requires using wrappers  and is a List  not an array  but is the closest you will get

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If you want to avoid both mutability and boxing  there is no way out of the box  But you can create a class which holds primitive array inside and provides read-only access to elements via method s

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Not with primitive arrays  You ll need to use a List or some other data structure   List lt Integer gt  items   Collections unmodifiableList Arrays asList 0 1 2 3

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Another one answer  static class ImmutableArray lt T gt        private final T   array       private ImmutableArray T   a           array   Arrays copyOf a  a length              public static  lt T gt  ImmutableArray lt T gt  from T   a           return new ImmutableArray lt T gt  a              public T get int index           return array index                  final ImmutableArray lt String gt  sample   ImmutableArray from new String    a    b    c

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Since Guava 22  from package com google common primitives you can use three new classes  which have a lower memory footprint compared to ImmutableList    ImmutableIntArray  ImmutableLongArray  ImmutableDoubleArray   They also have a builder  Example   int size   2  ImmutableLongArray longArray   ImmutableLongArray builder size     add 1L     add 2L     build      or  if the size is known at compile-time   ImmutableLongArray longArray   ImmutableLongArray of 1L  2L     This is another way of getting an immutable view of an array for Java primitives

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As of Java 9 you can use List of       JavaDoc   This method returns an immutable List and is very efficient

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As others have noted  you can t have immutable arrays in Java   If you absolutely need a method that returns an array that doesn t influence the original array  then you d need to clone the array each time   public int   getFooArray       return fooArray    null   null   fooArray clone        Obviously this is rather expensive  as you ll create a full copy each time you call the getter   but if you can t change the interface  to use a List for example  and can t risk the client changing your internals  then it may be necessary   This technique is called making a defensive copy

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My recommendation is to not use an array or an unmodifiableList but to use Guava s ImmutableList  which exists for this purpose   ImmutableList lt Integer gt  values   ImmutableList of 0  1  2  3

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Well   arrays are useful to pass as constants  if they were  as variants parameters

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While it s true that Collections unmodifiableList   works  sometimes you may have a large library having methods already defined to return arrays  e g  String     To prevent breaking them  you can actually define auxiliary arrays that will store the values   public class Test       private final String   original      private final String   auxiliary          constructor        public Test String    values            original   new String  values length              Pre-allocated array          auxiliary   new String  values length           System arraycopy  values  0  original  0   values length                 Get array values         public String   getValues                No need to call clone   - we pre-allocated auxiliary          System arraycopy original  0  auxiliary  0  original length           return auxiliary            To test       Test test   new Test new String    a    b    C         System out println Arrays asList test getValues          String   values   test getValues        values 0     foobar          At this point   foobar  exist in  auxiliary  but since we are         copying  original  to  auxiliary  for each call  the next line        will print the original values  a    b    c       System out println Arrays asList test getValues        Not perfect  but at least you have  pseudo immutable arrays   from the class perspective  and this will not break related code

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There is one way to make an immutable array in Java   final String   IMMUTABLE   new String 0     Arrays with 0 elements  obviously  cannot be mutated   This can actually come in handy if you are using the List toArray method to convert a List to an array  Since even an empty array takes up some memory  you can save that memory allocation by creating a constant empty array  and always passing it to the toArray method  That method will allocate a new array if the array you pass doesn t have enough space  but if it does  the list is empty   it will return the array you passed  allowing you to reuse that array any time you call toArray on an empty List   final static String   EMPTY STRING ARRAY   new String 0    List lt String gt  emptyList   new ArrayList lt String gt     return emptyList toArray EMPTY STRING ARRAY      returns EMPTY STRING ARRAY

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In some situations  it will be lighter weight to use this static method from Google Guava library  List lt Integer gt  Ints asList int    backingArray   Examples    List lt Integer gt  x1   Ints asList 0  1  2  3  List lt Integer gt  x1   Ints asList new int     0  1  2  3

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The of E    elements  method in Java9 can be used to create immutable list using just a line   List lt Integer gt  items   List of 1 2 3 4 5     The above method returns an immutable list containing an arbitrary number of elements  And adding any integer to this list would result in java lang UnsupportedOperationExceptionexception  This method also accepts a single array as an argument   String   array         List lt String   gt  list   List  lt String   gt of array

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If you need  for performance reason or to save memory  native  int  instead of  java lang Integer   then you would probably need to write your own wrapper class  There are various IntArray implementations on the net  but none  I found  was immutable  Koders IntArray  Lucene IntArray  There are probably others

[java] Immutable array in Java

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