Hi, we have a android application that works fine up to android 2.1, but when we try to release it for android 2.2 we get a nullpointerexception. This happens when we're parsing a xml, on the following line of code:
((node) getText.item(0).getNodeValue();
help appreciated!
That's a malformed code. Please correct. Then make sure you do not call any method on a null value.
NullPointerException may be thrown under several different scenarios. From the documentation:
Thrown when an application attempts to use
nullin a case where an object is required. These include:
- Calling the instance method of a
nullobject.- Accessing or modifying the field of a
nullobject.- Taking the length of
nullas if it were an array.- Accessing or modifying the slots of
nullas if it were an array.- Throwing
nullas if it were aThrowablevalue.Applications should throw instances of this class to indicate other illegal uses of the
nullobject.
As with any exception, you should carefully examine the stack trace to see if there's any useful information about why it was thrown. If indeed it is caused by illegally accessing a null reference as if it was a real object, then the general approach to debugging these kinds of problems is to to break up expressions like this:
// chained method invokations, can be hard to debug
var.method1().method2(arg).method3()
Into multiple statements like this:
// broken into multiple assignments
var1 = var.method1();
var2 = var1.method2();
var3 = var2.method3();
Now when a NullPointerException occurs, you'll know the exact part of the chain that throws it, namely which var is null.
Identifying the where part quickly narrows the search for the why. If var itself is null, then trace it back to previous assignments. If varN is null, then look up the documentation for methodN() to see under what circumstances can it return null.
If the null comes from a get-type operation on a container/mutable data structure, then the problem is no longer as neatly localized, and you'll have to figure out where the corresponding set happened. If null is never a legal value for this container, then perhaps it should perform the null-check during set: it's always better to cause failure earlier rather than later, because the source of the bug can be localized more easily.
On debugging NullPointerException:
On application-specific behavior for throwing NullPointerException:
On other NullPointerException scenarios instructive for learning:
String.valueOf(null) throw a NullPointerException?null == 0 throws NullPointerException in Java?int num = Integer.getInteger("123") throw NullPointerException?On null itself: