Java many to many association map

Question

I have two classes, ClassA and ClassB, as well as a \"many to many\" AssociationClass. I want a structure that holds the associations

Answer 1

Thanks for your suggestions.

I finally reinvented the wheel ... I have written a generic class for holding associations. I use two maps of maps, synchronized.

The associations holder provides the following methods

void setAssociation(LeftClass left, RightClass right, AssociationClass assoc);
AssociationClass getAssociation(LeftClass left, RightClass right);
Map getAssocationsLeft(LeftClass left);
Map getAssocationsRight(RightClass right); 
void removeAssociation(LeftClass left, RightClass right);

Here is the code:

import java.util.HashMap;

/** This class holds many to many associations between two classes. */
public class AssociationHolder {

    // -------------------------------------------------------
    // Attributes
    // -------------------------------------------------------

    private HashMap> associationsLeft = 
        new HashMap>();
    private HashMap> associationsRight = 
        new HashMap>();     

    // -------------------------------------------------------
    // Methods
    // -------------------------------------------------------

    /** 
     *  Set an association between two instance.
     *  Any prior association is overwritten.
     */
    public void setAssociation(LeftClass left, RightClass right, AssociationClass association) {

        // Get the map for the left 
        HashMap leftMap = this.associationsLeft.get(left);

        // No association defined yet for this left key ? => Create new map
        if (leftMap == null) {
            leftMap = new HashMap();
            this.associationsLeft.put(left, leftMap);
        }

        // Get the map for the right 
        HashMap rightMap = this.associationsRight.get(right);

        // No association defined yet for this right key ? => Create new map
        if (rightMap == null) {
            rightMap = new HashMap();
            this.associationsRight.put(right, rightMap);
        }

        // Set the assoication on both maps
        leftMap.put(right, association);
        rightMap.put(left, association);        

    } 

    /** @return null if no association found. */
    public AssociationClass getAssociation(LeftClass left, RightClass right) {

        // Use left maps (could have used the right one as well)
        HashMap leftMap = this.associationsLeft.get(left);
        if (leftMap == null) return null;
        return leftMap.get(right);
    }

    /** Get all associations defined for a given Left instance.  */
    public HashMap getAssociationsLeft(LeftClass left) {

        HashMap leftMap = this.associationsLeft.get(left);

        // No map defined ? return empty one instead of null
        if (leftMap == null) {
            return new HashMap();
        } else {
            return leftMap;
        }   
    }

    /** Get all associations defined for a given Right instance.  */
    public HashMap getAssociationsRight(RightClass right) {

        HashMap rightMap = this.associationsRight.get(right);

        // No map defined ? return empty one instead of null
        if (rightMap == null) {
            return new HashMap();
        } else {
            return rightMap;
        }   
    }

    /** 
     *  Remove an association between two instances.
     */
    public void removeAssociation(LeftClass left, RightClass right) {
        HashMap leftMap = this.getAssociationsLeft(left);
        HashMap rightMap = this.getAssociationsRight(right);
        leftMap.remove(right);      
        rightMap.remove(left);  
    }
}

I hope this can help someone in the future.