CREATE OR REPLACE FUNCTION "public"."AddNewElement" (name varchar, out newelementid bigint) RETURNS bigint AS
$body$
BEGIN

    INSERT INTO "Elements" ("Name")
    VALUES(name);
    
    SELECT INTO newelementid currval('"public"."elements_elementid_seq"');
    
END;
$body$
LANGUAGE 'plpgsql' VOLATILE CALLED ON NULL INPUT SECURITY INVOKER;

CREATE TABLE "public"."Elements" (
  "ElementID" BIGINT DEFAULT nextval(('"public"."elements_elementid_seq"'::text)::regclass) NOT NULL, 
  "Name" VARCHAR( 50 ), 
  CONSTRAINT "Elements_pkey" PRIMARY KEY("ElementID")
) WITHOUT OIDS;

              cst = localConnection.prepareCall("{ call \"AddNewElement\"(?, ?) }");

              String elName = "newElement";

              cst.setString( 1 ,  elName);
              cst.registerOutParameter( 2 , java.sql.Types.BIGINT);

              cst.execute();
              ret = cst.getLong( 2 );
              cst.close();

org.postgresql.util.PSQLException: A CallableStatement function was executed and the return was of type java.sql.Types=- 5  however type java.sql.Types= 0  was registered.

Query OK ( 0 . 05  sec)
Return Value:  21 

CREATE OR REPLACE FUNCTION .... RETURNS bigint AS
...
    SELECT INTO newelementid currval('"public"."elements_elementid_seq"');    
END;
$body$

if(rs.next()) {
  ret = rs.getLong(...);
}

              ResultSet rs = cst.getResultSet();
              if(rs != null)
              {
                  while(rs.next())
                  {
                     ret = rs.getLong( 1 );
                  }
              }