Convert Task ActivityDate into String

Question

I need in my wrapper class the activity date from the task object as string to display at a visual force page. My two version below does n't work.

If I use this version I get an error (Attempt to de-reference a null object).

public class Wrapper implements Comparable {
    
    private final Id id;
    private final DateTime createDateSort;
    private final String createDate;
       
    public wrapper(Task t) { 
        id                    = t.Id;
        DateTime activitydate = t.ActivityDate;
        createDate            = activitydate.format('dd.MM.yy', UserInfo.getTimeZone().toString());
        createDateSort        = t.ActivityDate;           
    }

And if I use this version I get the error (Method does not exist or incorrect signature: void format(String) from the type Date

public class Wrapper implements Comparable {
    
    private final Id id;
    private final DateTime createDateSort;
    private final String createDate;
       
    public wrapper(Task t) { 
        id                  = t.Id;
        createDate          = t.activitydate.format('dd.MM.yy');
        createDateSort      = t.ActivityDate;           
    }

Answer 1

Try this

createDate = string.valueOfGmt(createDateSort);

Answer 2

I found the problem. It was because there were tasks with no activitydate. I add an If-Clause to solve the problem.

    If (t.ActivityDate == Null) { 
        createDateSort  = t.CreatedDate;
        createDate      = t.CreatedDate.format('dd.MM.yy', UserInfo.getTimeZone().toString());
    } ELSE {
        DateTime ActivityDate = t.ActivityDate;
        createDateSort        = t.ActivityDate;
        createDate            = ActivityDate.format('dd.MM.yy', UserInfo.getTimeZone().toString());
    }