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copied!<p><code>NullPointerException</code>s 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 <code>NullPointerException</code>. These are the most common, but other ways are listed on the <a href="http://docs.oracle.com/javase/7/docs/api/java/lang/NullPointerException.html" rel="noreferrer"><code>NullPointerException</code></a> javadoc page.</p> <p>Probably the quickest example code I could come up with to illustrate a <code>NullPointerException</code> would be:</p> <pre><code>public class Example { public static void main(String[] args) { Object obj = null; obj.hashCode(); } } </code></pre> <p>On the first line inside <code>main</code>, I'm explicitly setting the <code>Object</code> reference <code>obj</code> equal to <code>null</code>. This means I have a reference, but it isn't pointing to any object. After that, I try to treat the reference as though it points to an object by calling a method on it. This results in a <code>NullPointerException</code> because there is no code to execute in the location that the reference is pointing.</p> <p>(This is a technicality, but I think it bears mentioning: A reference that points to null isn't the same as a C pointer that points to an invalid memory location. A null pointer is literally not pointing <em>anywhere</em>, which is subtly different than pointing to a location that happens to be invalid.)</p>

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