# Null pointer exceptions
Of the things which can go wrong at run time in Java programs, null pointer exceptions are by far the most common. Recall that all values in Java programs are either scalars or references to objects, and that all variables have default values, which in the case of object references is null.Cause:
Thrown when an application attempts to use
null
in a
case where an object is required. These include:
- Calling the instance method of a
null
object. - Accessing or modifying the field of a
null
object. - Taking the length of
null
as if it were an array. - Accessing or modifying the slots of
null
as if it were an array. - Throwing
null
as if it were aThrowable
value.
Solutions:
1. Avoid the null pointer exception by checking if the variable is null prior to using it.
2. Always declare variables just before where they are going to be used.
3. Religiously initialize (fill) arrays immediately after declaration. Arrays mostly causes NullPointerException.
4.Return Iterators instead of Lists. This allows to return empty Iterator and the consumer need not do if-null-check. NullPointerException will not creep into this pattern.
5. Erroneously don’t declare a variable inside a constructor, it will hide the instance variable.
Rferences:
1.Reference(a)