HashMap for Huffman Tree

Question

I have an assignment dealing with Huffman trees. My professor mentioned that we should use a HashMap but didn't explain anything about them. I've looked things up about them but I am running into problems with them in my code. These are the error messages that I am getting.

n00832607.java:542: error: incompatible types
for(Map.Entry e : charCounts.entrySet())
                                    ^
required: Entry
found:    Object
n00832607.java:610: error: incompatible types
for(Map.Entry entry : codeTable.entrySet())
                                       ^
required: Entry
found:    Object

So I can tell that it wants it to return an Entry but I am not exactly sure how to approach this. Am I supposed to be casting it as something?

This is my code.

//Beging Huffman Class
class Huffman{

Tree tree;

TreeMap codeTable;


protected HashMap calculateFrequency(String message)
{

int messageLength = message.length();
char ch;
int count;
HashMap charCounts = new HashMap();

for(int i = 0; i< messageLength; i++)
{

ch = message.charAt(i); 

if(charCounts.containsKey(ch))
{
count = (int) charCounts.get(ch);
++count;
charCounts.put(ch, count);   
}

else
{
charCounts.put(ch, 1);
}
}
//end for
return charCounts;
}
//end calculateFrequency

protected void createHuffmanTree(String message)
{
HashMap charCounts = calculateFrequency(message);
PriorityQueue trees = new PriorityQueue();
Tree temp;

for(Map.Entry e : charCounts.entrySet())
  {

  temp = new Tree((char)e.getKey(), (int)e.getValue());
  trees.add(temp);

  }


while(trees.size() > 1)
  {
  temp = new Tree((Tree)trees.remove(), (Tree)trees.remove());
  trees.add(temp);   
  }
tree = (Tree)trees.remove();      

}
//end createHuffmanTree

//Begin displayCodeTable
public void displayCodeTable()
{

System.out.println("Character Code");

for(Map.Entry entry : codeTable.entrySet())
  {
  char key = (char)entry.getKey();
  System.out.println(key + "" + entry.getValue());
  }
}
//End displayCodeTable 

Answer 1

It's just that the fancy new Java syntax (available as of Java 5 or 6, i forget) is not very casting friendly.

In the old days, you'd have to do this:

    TreeMap map = new TreeMap();
    Iterator itr = map.entrySet().iterator();
    while (itr.hasNext())
    {
        E thing = (E)itr.next();            
    }

You can get around this by typing your TreeMap with <,>, and using the keySet() instead of the entrySet().
That is:

    TreeMap map = new TreeMap<String, String>();  (or whatever types you're mapping)

If you want a quick fix, use this:

    for (Object o : map.entrySet()) {
       Map.Entry me = (Map.Entry)o;  
    }

Answer 2

Use generics to specify the types of key and value used in the collections. For instance

HashMap < Character, Integer >

is a hashmap mapping chars to ints. You cannot use the primitive types (int, char, long, short, byte) in generics, so use the class wrappers (Integer, Character, Long, Short, Byte) instead.