[java] Hashmap does not work with int, char

Possible Duplicate:
Storing primitive values in a Java collection?

In java when I use the following :-

public HashMap<char, int> buildMap(String letters)
{
    HashMap<char, int> checkSum = new HashMap<char, int>();

    for ( int i = 0; i < letters.length(); ++i )
    {
        checkSum.put(letters.charAt(i), primes[i]);
    }

    return checkSum;
}

I get errors related to inappropriate types. I solved my problem by using Character and Integer instead of char and int respectively. However, I'm having trouble figuring out why HashMap fails to be able to deal with primitive data types.

Generic Collection classes cant be used with primitives. Use the Character and Integer wrapper classes instead.

Map<Character , Integer > checkSum = new HashMap<Character, Integer>();

Generics can't use primitive types in the form of keywords.

Use

public HashMap<Character, Integer> buildMap(String letters)
{
    HashMap<Character, Integer> checkSum = new HashMap<Character, Integer>();

    for ( int i = 0; i < letters.length(); ++i )
    {
        checkSum.put(letters.charAt(i), primes[i]);
    }

    return checkSum;
}

Updated: With Java 7 and later, you can use the diamond operator.

HashMap<Character, Integer> checkSum = new HashMap<>();

Generics only support object types, not primitives. Unlike C++ templates, generics don't involve code generatation and there is only one HashMap code regardless of the number of generic types of it you use.

Trove4J gets around this by pre-generating selected collections to use primitives and supports TCharIntHashMap which to can wrap to support the Map<Character, Integer> if you need to.

TCharIntHashMap: An open addressed Map implementation for char keys and int values.

You cannot put primitive types into collections. However, you can declare them using their corresponding object wrappers and still add the primitive values, as long as the boxing allows you.

Hashmaps can only use classes, not primitives. This page from programmerinterview.com might be of use in guiding you to finding the answer. To be honest, I haven't figured out the answer to this problem in detail myself.

Generics can be defined using Wrapper classes only. If you don't want to define using Wrapper types, you may use the Raw definition as below

@SuppressWarnings("rawtypes")
public HashMap buildMap(String letters)
{
    HashMap checkSum = new HashMap();

    for ( int i = 0; i < letters.length(); ++i )
    {
       checkSum.put(letters.charAt(i), primes[i]);
    }
    return checkSum;
}

Or define the HashMap using wrapper types, and store the primitive types. The primitive values will be promoted to their wrapper types.

public HashMap<Character, Integer> buildMap(String letters)
{
  HashMap<Character, Integer> checkSum = new HashMap<Character, Integer>();

  for ( int i = 0; i < letters.length(); ++i )
  {
    checkSum.put(letters.charAt(i), primes[i]);
  }
  return checkSum;
}