Multi value fields and Beans in XPages

I had an issue with multi value fields bound to bean fields in an XPages I worked on.

I got this error message on refresh:

java.lang.IllegalArgumentException: argument type mismatch

The issue turned out to be a somewhat inconsistent underlying API. When the field is empty, or has single value, it tries to set a string. When there are multiple values, it tries to set a list.

To work around the issue, have the getter return an Object, and the setter accept an object.
E.g.

public Object getInputMulti() {
 return this.inputMulti;
}
...
@SuppressWarnings( "unchecked" )
public void setInputMulti( Object inputMulti ) {
 if( inputMulti == null ){
  this.inputMulti = null;
 }
 
 if( inputMulti instanceof String ) {
  Vector inputNameList = new Vector();
  inputNameList.add( (String) inputMulti );
  this.inputMulti = inputNameList;  
 }
 if( inputMulti instanceof List ) {
  this.inputMulti = (Vector) inputMulti;
 }
}

You can have the translation for the setter be done in a utility method. E.g.

@SuppressWarnings("unchecked")
public static Vector translateToVector( Object object ){
 if( object instanceof String ){
  Vector list = new Vector();
  list.add( object );
  return list;
 }
 
 if( object instanceof List ){
  return (Vector)object;
 }
 
 return null;
}

Then, for the setter:

@SuppressWarnings( "unchecked" )
public void setInputMulti( Object inputMulti ) {
  this.inputMulti = UtilityClass.translateToVector( inputMulti );
}

Example of multi value fields: Checkbox group, multi value fields (fields with multipleSeparator), list boxes.