Getting customfieldtype based on field display type

public String getCustomFieldType(Schema.DisplayType fieldType) {
 String customFieldType;
 
 Set stringTypes = new Set();
 stringTypes.add(Schema.DisplayType.Reference);
 stringTypes.add(Schema.DisplayType.ID);
 stringTypes.add(Schema.DisplayType.String);
 stringTypes.add(Schema.DisplayType.Email);
 stringTypes.add(Schema.DisplayType.URL);
 stringTypes.add(Schema.DisplayType.Picklist);
 stringTypes.add(Schema.DisplayType.Phone);

 Set numberTypes = new Set();
 numberTypes.add(Schema.DisplayType.Double);
 numberTypes.add(Schema.DisplayType.Percent);
 numberTypes.add(Schema.DisplayType.Integer);
 numberTypes.add(Schema.DisplayType.Currency);
  
 if(fieldType == Schema.DisplayType.MultiPicklist) {
  customFieldType = 'str';
 } else if(stringTypes.contains(fieldType)) {
  customFieldType = 'str'; 
 } else if(numberTypes.contains(fieldType)) {
  customFieldType = 'num';  
 } else if(fieldType == Schema.DisplayType.Boolean){
  customFieldType = 'bool';
 } else if(fieldType == Schema.DisplayType.Date || fieldType == Schema.DisplayType.DateTime){
  if(fieldType == Schema.DisplayType.DateTime) {
   customFieldType = 'dateTimeType';
  } else {
   customFieldType = 'dateType';
  }
 } 
 return customFieldType;
}