Searching issue by custom field value through the API

We plan to implement Jira in our company and for this we must prove that regular updates from our old issue tracking system are possible. For this I am writing a Jira service that should get all the issues that have changed in the old system and create/update the issues in Jira.

For the update I have to check whether an issue has already been imported or not, thus I need to search a Jira issue by a custom field value. After studying your APIs for some hours the best I could come up with is the following code - which unfortuately tries to instatiate an interface

private Issue jiraSearchByTicketId(Integer ticketId) {
SearchRequest sr = new SearchRequest(true); // search without security checks
CustomFieldParameter cfp;

cfp.setCustomField(cfTicketID);
cfp.addValue("" + ticketId);
sr.addParameter(cfp);
try

{ List issues = issueManager.execute(sr, adminUser); return (Issue) issues.get(0); }

catch (SearchException e)

{ return null; }

}

Any ideas/hints whatsoever to do this?