How to convert int to Integer in Java

Question

I m new to Java and very confused   I have a large dataset of length 4 int   and I want to count the number of times that each particular combination of 4 integers occurs   This is very similar to counting word frequencies in a document   I want to create a Map lt int    double gt  that maps each int   to a running count as the list is iterated over  but Map doesn t take primitive types   so I made Map lt Integer    Double gt   my data is stored as an ArrayList lt int   gt  so my loop should be something like   ArrayList lt int   gt  data          load a dataset   Map lt Integer    Double gt  frequencies   new HashMap lt Integer    Double gt      for int   q   data              DO SOMETHING TO convert q from int   to Integer   so I can put it in the map      if frequencies containsKey q         frequencies put q  tfs get q    p         else           frequencies put q  p             I m not sure what code I need at the comment to make this work to convert an int   to an Integer     Or maybe I m fundamentally confused about the right way to do this

User · Answer

I was wrong in a previous answer  The proper solution is to use this class as a key in the map wrapping the actual int     public class IntArrayWrapper           int   data           public IntArrayWrapper int   data                this data   data                      Override         public boolean equals Object o                if  this    o  return true              if  o    null    getClass      o getClass    return false               IntArrayWrapper that    IntArrayWrapper  o               if   Arrays equals data  that data   return false               return true                      Override         public int hashCode                 return data    null   Arrays hashCode data    0                    and change your code like this      Map lt IntArrayWrapper  Double  gt  freqs   new HashMap lt IntArrayWrapper  Double gt          for  int   data   datas            IntArrayWrapper wrapper   new IntArrayWrapper data            if   freqs containsKey wrapper                 freqs put wrapper  freqs get wrapper    p                      freqs put wrapper  p

User · Answer

you don t need  int   is an object and can be used as a key inside a map   Map lt int    Double gt  frequencies   new HashMap lt int    Double gt       is the proper definition of the frequencies map   This was wrong  -   The proper solution is posted too  -

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This worked like a charm   int   mInt   new int 10   Integer   mInteger   new Integer mInt length    List lt Integer gt  wrapper   new AbstractList lt Integer gt           Override     public int size             return mInt length              Override     public Integer get int i            return mInt i             wrapper toArray mInteger

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Native Java 8  one line   With Java 8  int   can be converted to Integer   easily   int   data    1 2 3 4 5 6 7 8 9 10       To boxed array Integer   what   Arrays stream  data   boxed   toArray  Integer    new    Integer   ever   IntStream of  data   boxed   toArray  Integer    new        To boxed list List lt Integer gt  you    Arrays stream  data   boxed   collect  Collectors toList      List lt Integer gt  like   IntStream of  data   boxed   collect  Collectors toList        As others stated  Integer   is usually not a good map key  But as far as conversion goes  we now have a relatively clean and native code

User · Answer

Rather than write your own code you can use an IntBuffer to wrap the existing int   without having to copy the data into an Integer array  int   a    1 2 3 4   IntBuffer b   IntBuffer wrap a     IntBuffer implements comparable so you are able to use the code you already have written  Formally maps compare keys such that a equals b  is used to say two keys are equal  so two IntBuffers with array 1 2 3 - even if the arrays are in different memory locations - are said to be equal and so will work for your frequency code   ArrayList lt int   gt  data          load a dataset   Map lt IntBuffer  Double gt  frequencies   new HashMap lt IntBuffer  Double gt      for int   a   data         IntBuffer q   IntBuffer wrap a        if frequencies containsKey q             frequencies put q  tfs get q    p         else           frequencies put q  p              Hope that helps

User · Answer

Convert int   to Integer    public static Integer   toConvertInteger int   ids       Integer   newArray   new Integer ids length        for  int i   0  i  lt  ids length  i             newArray i    Integer valueOf ids i              return newArray     Convert Integer   to int    public static int   toint Integer   WrapperArray        int   newArray   new int WrapperArray length         for  int i   0  i  lt  WrapperArray length  i               newArray i    WrapperArray i  intValue               return newArray

User · Answer

Simply use   public static int   intArrayToIntegerArray int   a        Integer   b   new Integer a length       for int i   0  i  lt  array length  i             b i    a i             return g

User · Answer

Using regular for-loop without external libraries   Convert int   to Integer     int   primitiveArray    1  2  3  4  5   Integer   objectArray   new Integer primitiveArray length    for int ctr   0  ctr  lt  primitiveArray length  ctr          objectArray ctr    Integer valueOf primitiveArray ctr       returns Integer value     Convert Integer   to int     Integer   objectArray    1  2  3  4  5   int   primitiveArray   new int objectArray length    for int ctr   0  ctr  lt  objectArray length  ctr          primitiveArray ctr    objectArray ctr  intValue       returns int value

User · Answer

Presumably you want the key to the map to match on the value of the elements instead of the identity of the array  In that case you want some kind of object that defines equals and hashCode as you would expect  Easiest is to convert to a List lt Integer gt   either an ArrayList or better use Arrays asList  Better than that you can introduce a class that represents the data  similar to java awt Rectangle but I recommend making the variables private final  and the class final too

User · Answer

Not sure why you need a Double in your map   In terms of what you re trying to do  you have an int   and you just want counts of how many times each sequence occurs   Why would this required a Double anyway   What I would do is to create a wrapper for the int array with a proper  equals and  hashCode methods to account for the fact that int   object itself doesn t consider the data in it s version of these methods   public class IntArrayWrapper       private int values         public IntArrayWrapper int   values            super            this values   values              Override     public int hashCode             final int prime   31          int result   1          result   prime   result   Arrays hashCode values           return result              Override     public boolean equals Object obj            if  this    obj              return true          if  obj    null              return false          if  getClass      obj getClass                return false          IntArrayWrapper other    IntArrayWrapper  obj          if   Arrays equals values  other values               return false          return true             And then use google guava s multiset  which is meant exactly for the purpose of counting occurances  as long as the element type you put in it has proper  equals and  hashCode methods   List lt int   gt  list        HashMultiset lt IntArrayWrapper gt  multiset   HashMultiset create    for  int values     list        multiset add new IntArrayWrapper values        Then  to get the count for any particular combination   int cnt   multiset count new IntArrayWrapper new int     0  1  2  3

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Update  Though the below compiles  it throws a ArrayStoreException at runtime  Too bad  I ll let it stay for future reference     Converting an int    to an Integer     int   old      Integer   arr   new Integer old length   System arraycopy old  0  arr  0  old length     I must admit I was a bit surprised that this compiles  given System arraycopy being lowlevel and everything  but it does  At least in java7   You can convert the other way just as easily

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If you want to convert an int   to an Integer    there isn t an automated way to do it in the JDK   However  you can do something like this   int   oldArray          Here you would assign and fill oldArray  Integer   newArray   new Integer oldArray length   int i   0  for  int value   oldArray        newArray i      Integer valueOf value       If you have access to the Apache lang library  then you can use the ArrayUtils toObject int    method like this   Integer   newArray   ArrayUtils toObject oldArray

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Convert int   to Integer         import java util Arrays               int   aint    1 2 3 4 5 6 7 8 9 10       Integer   aInt   new Integer aint length        Arrays setAll aInt  i - gt  aint i

[java] How to convert int[] to Integer[] in Java?

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