Android文本字符串比较

Question

In a custom SimpleCursorAdapter, I'm trying to compare a status String, with confusing results.

My string is initialised from the cursor like this (and I've checked with toast that it contains the expected values).

String visitStatus = cursor.getString(cursor.getColumnIndex(CallData.COLUMN_VisitStatus));

visitStatus can be null, Open, Cancelled or Complete.

If I try to compare visitStatus to "any string in quotes", the app crashes with a NullPointerException. Only if I compare to null do I get anything at all - and that is no use to me

if(visitStatus.equals(null)) // the app crashes with a NullPointerException
if(visitStatus == null) // doesn't crash
if(visitStatus != null) // doesn't crash
if(visitStatus == "Complete") // doesn't crash or do anything
if(visitStatus.equals("Complete")) // the app crashes with a NullPointerException.

Basically, I can compare to null, but only in the way that isn't supposed to work. I can't compare to actual strings such as "Open" or "Complete".

I'm going slightly nuts with this, and am badly missing my C# comfort zone. This particular activity is a nightmare of listfragments, contentproviders, customadapters, viewpagers, pagertitlestrips and list row xml templates!

halp!

Answer 1

This is because visitStatus is null . Whenever you try to access its methods, it crashes. ( That is: visitString.equals() , visitString.length() , etc., all will crash. )

However, the equality operator ( == ) supports null parameters on either side of it. (So, if (null == null) is a valid check.)

You should check like this:

if (visitStatus != null && visitStatus.equals("Complete")) {
    // ...
}

Or, you can do "Yoda syntax" (backwards checking), which supports null parameters:

if ("Complete".equals(visitStatus)) {
    // ...
}

Also, a final note: You cannot compare string contents using == (as in, you cannot do "a" == new String("a") , nor visitString == "Complete" ). For a detailed explanation on that, see this Q&A thread .

Answer 2

String should be compared using .equals()

The NullPointerException caused because the visitStatus is null