Querying the Google ads API
SQL statement construction

Your third-party library do support bindings, so instead of .where("id = " + lineItemId.trim()), you should consider:

//  .where("id = " + lineItemId.trim())
    .where("id = :lineItem")
    .withBindVariableValue("lineItem", lineItemId.trim());


Early variable assignment

LineItem lineItem = null;
// ...
if (/* ... */) {
    lineItem = /* ... */;
}
return lineItem;


You usually should not need to declare the return result right at the start, perform the optional assignment, and then return from the method. You should just return below, without needing the lineItem variable:

// ...
if (/* ... */ ) {
    return /* ... */;
}
return null;