public function toJSON(){
        $outcomeValue = null;
        if($this->defaultValue != ''){
            $outcomeValue = Array($this->defaultValue);
        }else if($this->getAttributeValue('baseType') == 'integer' || $this->getAttributeValue('baseType') == 'float'){
            $outcomeValue = Array(0);
        }else{
            $outcomeValue = null;
        }

        $returnValue = taoQTI_models_classes_Matching_VariableFactory::createJSONVariableFromQTIData(
                        $this->getIdentifier()
                        , $this->getAttributeValue('cardinality')
                        , $this->getAttributeValue('baseType')
                        , $outcomeValue
        );

        return $returnValue;
    }