NullPointerExceptions are exceptions that occur when you try to use a reference that points to no location in memory (null) as though it were referencing an object. Calling a method on a null reference or trying to access a field of a null reference will trigger a NullPointerException.
For example,
String s = null;
int len = s.length(); // NullPointerException because s is null
So you should check if the variable is null before calling any method on it, for example:
int len;
if (s == null) {
len = 0;
}
else {
len = s.length(); // safe, s is never null when you get here
}
How to avoid NullPointerExceptions?
Use the final modifier to enforce good initialization.
Avoid returning null in methods, for example returning empty collections when applicable.
Use annotations @NotNull and @Nullable
Fail fast and use asserts to avoid propagation of null objects through the whole application when they shouldn't be null.
Use equals with a known object first: if("knownObject".equals(unknownObject)
Prefer valueOf() over toString().
Use null safe StringUtils methods StringUtils.isEmpty(null).
