HashMap and int as key

Question

I am trying to build a HashMap which will have integer as keys and objects as values   My syntax is   HashMap lt int  myObject gt  myMap   new HashMap lt int  myObject gt       However  the error returned is - Syntax error on token  int   Dimensions expected after this token - I don t understand why I should add a dimension  ie  making the int into an array  since I only need to store a digit as key   What could I do   Thanks in advance

User · Answer

For everybody who codes Java for Android devices and ends up here: use SparseArray for better performance;

private final SparseArray<myObject> myMap = new SparseArray<myObject>();

with this you can use int instead of Integer like;

int newPos = 3;

myMap.put(newPos, newObject);
myMap.get(newPos);

User · Answer

You may try to use Trove http   trove starlight-systems com  TIntObjectHashMap is probably what you are looking for

User · Answer

Please use  HashMap lt Integer  myObject gt  myMap   new HashMap lt Integer  myObject gt

User · Answer

HashMap does not allow primitive data types as arguments  It can only accept objects so   HashMap lt int  myObject gt  myMap   new HashMap lt int  myObject gt       will not work   You have to change the declaration to   HashMap lt Integer  myObject gt  myMap   new HashMap lt Integer  myObject gt       so even when you do the following  myMap put 2 myObject     The primitive data type is autoboxed to an Integer object    8  int      boxing     gt  8  Integer    You can read more on autoboxing here http   docs oracle com javase tutorial java data autoboxing html

User · Answer

Use Integer instead   HashMap lt Integer  MyObject gt  myMap   new HashMap lt Integer  MyObject gt       Java will automatically autobox your int primitive values to Integer objects   Read more about autoboxing from Oracle Java documentations

User · Answer

For somebody who is interested in a such map because you want to reduce footprint of autoboxing in Java of wrappers over primitives types  I would recommend to use Eclipse collections  Trove isn t supported any more  and I believe it is quite unreliable library in this sense  though it is quite popular anyway  and couldn t be compared with Eclipse collections  import org eclipse collections impl map mutable primitive IntObjectHashMap   public class Check       public static void main String   args            IntObjectHashMap map   new IntObjectHashMap             map put 5  quot It works quot            map put 6  quot without quot            map put 7  quot boxing  quot             System out println map get 5            System out println map get 6            System out println map get 7             In this example above IntObjectHashMap  As you need int- gt object mapping  also consider usage of YourObjectType   array or List lt YourObjectType gt  and access values by index  as map is  by nature  an associative array with int type as index

User · Answer

I don t understand why I should add a dimension  ie  making the int into an array  since I only need to store a digit as key    An array is also an Object  so HashMap lt int    MyObject gt  is a valid construct that uses int arrays as keys   Compiler doesn t know what you want or what you need  it just sees a language construct that is almost correct  and warns what s missing for it to be fully correct

User · Answer

use int as Object not as primitive type  HashMap lt Integer  myObject gt  myMap   new HashMap lt Integer  myObject gt

User · Answer

You can t use a primitive because HashMap use object internally for the key  So you can only use an object that inherits from Object  that is any object    That is the function put   in HashMap and as you can see it uses Object for K   public V put K key  V value        if  key    null          return putForNullKey value       int hash   hash key       int i   indexFor hash  table length       for  Entry lt K V gt  e   table i   e    null  e   e next            Object k          if  e hash    hash  amp  amp    k   e key     key    key equals k                  V oldValue   e value              e value   value              e recordAccess this               return oldValue                       modCount        addEntry hash  key  value  i       return null      The expression  k   e key  should make it clear   I suggest to use a wrapper like Integer and autoboxing

User · Answer

If you code in Android  there is SparseArray  mapping integer to object

User · Answer

If you are using Netty and want to use a map with primitive int type keys  you can use its IntObjectHashMap Some of the reasons to use primitive type collections   improve performance by having specialized code reduce garbage that can put pressure on the GC  The question of specialized vs generalized collections can make or break programs with high throughput requirements

User · Answer

The main reason with HashMap not allowing primitive as keys is that HashMap is designed in such a way that for comparing the keys  it makes use of equals   method  and a method can be called only on an object not on a primitive   Thus when int is autoboxed to Integer  Hashmap can call equals   method on Integer object   That is why  you should use Integer instead of int  I mean hashmap throws an error while putting int as a key  Don t know the meaning of the error that is thrown   And if you think that  you can make Map performance faster by making a primitive as a key  there is a library called FastUtil which contains a Map implementation with int type as a key   Because of this  it is much faster than Hashmap

[java] HashMap and int as key

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