How to update the Custom field value(select option) in post-function?

[zhang san]

1.  Create a custom field "abc" and its selection options are "aa","bb"
2. In the post-function, I added the Custom script post-function. How to achieve the custom field "abc" equal to "aa" or "bb" and update it on the issue in groovy script?
3. If I run the following code in JIRA, I got the following error: java.util.String cannot be cast to com.atlassian.jira.issue.customfields.option.Option.  What should I do to achieve this feature? thanks a lot.

log = Category.getInstance("com.onresolve.jira.groovy.AutoCloseChildIssues");
def ComponentManager compoentManager = ComponentManager.getInstance();
def CustomFieldManager customFieldManager = compoentManager.getCustomFieldManager();
def CustomField deliverCycleField = customFieldManager.getCustomFieldObjectByName("customeTime");
def deliverCycleValue = issue.getCustomFieldValue(deliverCycleField);
def CustomField deliverCycleCountField = customFieldManager.getCustomFieldObjectByName("abc");

if (deliverCycleValue <= 16) {

issue.setCustomFieldValue(deliverCycleCountField, "aa");

} else {
    issue.setCustomFieldValue(deliverCycleCountField, "bb");
    log.error("-------------------------------------------5" + "deliverCycleValue=" + deliverCycleValue);
}
def FieldLayoutItem fieldLayoutItem = compoentManager.getFieldLayoutManager().getFieldLayout(issue).getFieldLayoutItem(deliverCycleCountField);
DefaultIssueChangeHolder defaultIssueChangeHolder = new DefaultIssueChangeHolder();
def modifiedFields = issue.getModifiedFields();
final ModifiedValue modifiedValue = (ModifiedValue) modifiedFields.get(deliverCycleCountField.getId());
deliverCycleCountField.updateValue(fieldLayoutItem, issue, modifiedValue, defaultIssueChangeHolder);

[RambanamP]

issue.getCustomFieldValue(deliverCycleField) method will return Option object so you need use condition something like, verfy this api before using 

if (deliverCycleValue <= deliverCycleValue .name)

You need to pass Option object to the following method instead of 'aa'

issue.setCustomFieldValue(deliverCycleCountField, "aa");

check the following post to know how to get the option object

https://answers.atlassian.com/questions/18840