Java Language Tutorial => Java Pitfalls

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The value null is the default value for an uninitialized value of a field whose type is a reference type.

NullPointerException (or NPE) is the exception that is thrown when you attempt to perform an inappropriate operation on the null object reference. Such operations include:

calling an instance method on a null target object,
accessing a field of a null target object,
attempting to index a null array object or access its length,
using a null object reference as the mutex in a synchronized block,
casting a null object reference,
unboxing a null object reference, and
throwing a null object reference.

The most common root causes for NPEs:

forgetting to initialize a field with a reference type,
forgetting to initialize elements of an array of a reference type, or
not testing the results of certain API methods that are specified as returning null in certain circumstances.

Examples of commonly used methods that return null include:

The get(key) method in the Map API will return a null if you call it with a key that doesn't have a mapping.
The getResource(path) and getResourceAsStream(path) methods in the ClassLoader and Class APIs will return null if the resource cannot be found.
The get() method in the Reference API will return null if the garbage collector has cleared the reference.
Various getXxxx methods in the Java EE servlet APIs will return null if you attempt fetch a non-existent request parameter, session or session attribute and so on.

There are strategies for avoiding unwanted NPEs, such as explicitly testing for null or using "Yoda Notation", but these strategies often have the undesirable result of hiding problems in your code that really ought to be fixed.

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