Getting operator list based on field display type

public List getOperatorList(Schema.DisplayType fieldType) {
 List operatorList = new List();
 operatorList.add(new SelectOption('', '--None--'));
 
 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) {
  operatorList.add(new SelectOption(ReadConstantClass.CONTAINS, ReadConstantClass.CONTAINS));
 } else if(stringTypes.contains(fieldType)) {
  operatorList.add(new SelectOption(ReadConstantClass.EQUALS, ReadConstantClass.EQUALS));
  operatorList.add(new SelectOption(ReadConstantClass.CONTAINS, ReadConstantClass.CONTAINS));
  operatorList.add(new SelectOption(ReadConstantClass.STARTS_WITH, ReadConstantClass.STARTS_WITH));
  operatorList.add(new SelectOption(ReadConstantClass.ENDS_WITH, ReadConstantClass.ENDS_WITH));
 } else if(numberTypes.contains(fieldType)) {
  operatorList.add(new SelectOption(ReadConstantClass.EQUALS, ReadConstantClass.EQUALS));
  operatorList.add(new SelectOption(ReadConstantClass.NOT_EQUALS, ReadConstantClass.NOT_EQUALS));
  operatorList.add(new SelectOption(ReadConstantClass.LESS_THAN, ReadConstantClass.LESS_THAN));
  operatorList.add(new SelectOption(ReadConstantClass.GREATER_THAN, ReadConstantClass.GREATER_THAN));
  operatorList.add(new SelectOption(ReadConstantClass.LESS_OR_EQUALS, ReadConstantClass.LESS_OR_EQUALS));
  operatorList.add(new SelectOption(ReadConstantClass.GREATER_OR_EQUALS, ReadConstantClass.GREATER_OR_EQUALS));
 } else if(fieldType == Schema.DisplayType.Boolean){
  operatorList.add(new SelectOption(ReadConstantClass.EQUALS, ReadConstantClass.EQUALS));
 } else if(fieldType == Schema.DisplayType.Date || fieldType == Schema.DisplayType.DateTime){
  operatorList.add(new SelectOption(ReadConstantClass.EQUALS, ReadConstantClass.EQUALS));
  operatorList.add(new SelectOption(ReadConstantClass.NOT_EQUALS, ReadConstantClass.NOT_EQUALS));
  operatorList.add(new SelectOption(ReadConstantClass.LESS_THAN, ReadConstantClass.LESS_THAN));
  operatorList.add(new SelectOption(ReadConstantClass.GREATER_THAN, ReadConstantClass.GREATER_THAN));
  operatorList.add(new SelectOption(ReadConstantClass.LESS_OR_EQUALS, ReadConstantClass.LESS_OR_EQUALS));
  operatorList.add(new SelectOption(ReadConstantClass.GREATER_OR_EQUALS, ReadConstantClass.GREATER_OR_EQUALS));
 } 
 return operatorList;
}

ReadConstantClass class

public class ReadConstantClass{
    public static final string EQUALS = 'equals';
    public static final string NOT_EQUALS= 'not equal to';
    public static final string LESS_THAN = 'less than';
    public static final string GREATER_THAN = 'greater than';
    public static final string LESS_OR_EQUALS= 'less or equal';
    public static final string GREATER_OR_EQUALS = 'greater or equal';
    public static final string CONTAINS = 'contains';
    public static final string STARTS_WITH = 'starts with';
    public static final string ENDS_WITH = 'ends with'; 
    public static final string ORDER_BY = ' order by ';  
    public static final string WHERE_CLAUSE = ' where ';   
    public static final string IN_CLAUSE = ' in ';  
    public static final string NOT_OPERATOR = ' not '; 
    public static final string AND_OPERATOR = ' and '; 
    public static final string LIMIT_ROWS = ' not ';  
}