ClassCastException

[Hunter McMillen]

Hey everyone,

I'm working on creating a symbol table for a parser I am writing and I am using a hashtable to implement my symbol table. I'm running into some trouble when I try to print out the values in my table. Here's what I'm doing:

    
import java.util.Hashtable;
import java.util.Enumeration;

public abstract class SymbolTable
{
	private Hashtable<String, SymbolTableEntry> symTable;
	
	/**
	 * Default Constructor - symbol tables should start out empty
	 *
	 */
	public SymbolTable()
	{
		this.symTable  = new Hashtable<String, SymbolTableEntry>();
	}
	
	public void add(String key, SymbolTableEntry entry)
	{
		symTable.put(key, entry);
	}
	
	public void printSymTable()
	{
		Enumeration iterator = symTable.elements();
		String element = "";
		
		while(iterator.hasMoreElements())
		{
			element = (String)iterator.nextElement(); //this line is where the ClassCastException occurs
			System.out.println(symTable.get(element));
		}
	}
}

here is my driver:

    
public class symTest
{
	public static void main(String[] args)
	{
		SymbolTable setSymbolTable = new SetSymbolTable();
		SymbolTable stateSymbolTable = new StateSymbolTable();
		
		setSymbolTable.add("M", new SymbolTableEntry(5,2,SymbolStatus.DEFINED, "M"));
		stateSymbolTable.add("X", new SymbolTableEntry(5,2,SymbolStatus.USED, "X"));
		
		setSymbolTable.printSymTable();
		stateSymbolTable.printSymTable();
	}
}

Any help would be great.
Thanks,
Hunter

[Bear Bibeault]

And the problem?

[Seetharaman Venkatasamy]

Hunter McMillen wrote:

	
private Hashtable<String, SymbolTableEntry> symTable;
	
		
	public void printSymTable()
	{
		Enumeration iterator = symTable.elements();
		String element = "";
		
		while(iterator.hasMoreElements())
		{
			element = (String)iterator.nextElement(); //this line is where the ClassCastException occurs
			System.out.println(symTable.get(element));
		}
	}

element = (String)iterator.nextElement(); it gives you a value element of Hashtable which is SymbolTableEntry . SymbolTableEntry is not a String , but you are trying to cast to String, so Obviously you get *ClassCastException*

hth

[Jesper de Jong]

Note that classes Hashtable and Enumeration are legacy collection classes. They have been (more or less) replaced by the collections framework in Java 1.2 (a long time ago!). See Sun's tutorial about collections for more information.

Use HashMap instead of Hashtable and Iterator instead of Enumeration.

[Rob Spoor]

And iterate over the entrySet() instead of iterating over the keySet(), then retrieving the elements using get:

// old style
for (Iterator<Map.Entry<String,SymbolTableEntry>> i = map.entrySet().iterator(); i.hasNext(); )
{
    Map.Entry<String,SymbolTableEntry> entry = i.next();
    String key = entry.getKey();
    SymbolTableEntry tableEntry = entry.getValue();
    ...
}
// new style
for (Map.Entry<String,SymbolTableEntry> entry: map.entrySet())
{
    String key = entry.getKey();
    SymbolTableEntry tableEntry = entry.getValue();
    ...
}