Question

I have a hashtable looking like this:

Hashtable<Mapping, Integer> mappingCount = new Hashtable<Mapping, Integer>();

I want to use this code:

if (mappingCount.get(currentMapping) != null)
    mappingCount.put(currentMapping, mappingCount.get(currentMapping) + 1);
else
    mappingCount.put(currentMapping, 1);

In order to be able to get the value from the hashtable, for the class Mapping I did the following:

@Override
public boolean equals(Object obj) {
    return ((Mapping)obj).mappingXML.equals(this.mappingXML);
}

However, this doesn’t do the trick since mappingCount.get(currentMapping) always results in null. To be sure that something’s not wrong, I did the following:

if (aaa.contains(currentMapping.getMappingXML()))
    System.out.println("found it!");
else
    aaa.add(currentMapping.getMappingXML());

where aaa is List<String> aaa = new ArrayList<String>(). Of course, found it is printed many times. What am I doing wrong?

Answer 1

You also need to override the hashCode() method.

From the JavaDocs:

To successfully store and retrieve
objects from a hashtable, the objects
used as keys must implement the
hashCode method and the equals method.

The reason for this is that Hashtable uses hashCode as a preliminary test to see if two objects are equals. If the hashCode matches, then it uses equals to check for collissions.

The default implementation of hashCode() returns the memory address of the object, and for two objects that are equal, their hashcodes must also be equal.

Also look at the general contract for hashCode().