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    <p>The issue is that <code>String.valueOf</code> method is <em>overloaded</em>:</p> <ul> <li><a href="http://java.sun.com/javase/6/docs/api/java/lang/String.html#valueOf%28java.lang.Object%29" rel="noreferrer"><code>String.valueOf(Object)</code></a></li> <li><a href="http://java.sun.com/javase/6/docs/api/java/lang/String.html#valueOf%28char%5B%5D%29" rel="noreferrer"><code>String.valueOf(char[])</code></a></li> </ul> <p>Java Specification Language mandates that in these kind of cases, <em>the most specific overload</em> is chosen:</p> <h3><a href="http://docs.oracle.com/javase/specs/jls/se5.0/html/expressions.html#15.12.2.5" rel="noreferrer">JLS 15.12.2.5 Choosing the Most Specific Method</a></h3> <blockquote> <p>If more than one member method is both accessible and applicable to a method invocation, it is necessary to choose one to provide the descriptor for the run-time method dispatch. The Java programming language uses the rule that the <em>most specific</em> method is chosen.</p> </blockquote> <p>A <code>char[]</code> <em>is-an</em> <code>Object</code>, but not all <code>Object</code> <em>is-a</em> <code>char[]</code>. Therefore, <code>char[]</code> is <em>more specific</em> than <code>Object</code>, and as specified by the Java language, the <code>String.valueOf(char[])</code> overload is chosen in this case.</p> <p><code>String.valueOf(char[])</code> expects the array to be non-<code>null</code>, and since <code>null</code> is given in this case, it then throws <code>NullPointerException</code>.</p> <p>The easy "fix" is to cast the <code>null</code> explicitly to <code>Object</code> as follows:</p> <pre><code>System.out.println(String.valueOf((Object) null)); // prints "null" </code></pre> <h3>Related questions</h3> <ul> <li><a href="https://stackoverflow.com/questions/2659496/how-does-polymorph-ambiguity-distinction-work/">How does polymorph ambiguity distinction work?</a></li> <li><a href="https://stackoverflow.com/questions/1545501/which-overload-will-get-selected-for-null-in-java">Which overload will get selected for null in Java?</a></li> </ul> <hr> <h3>Moral of the story</h3> <p>There are several important ones:</p> <ul> <li><em>Effective Java 2nd Edition, Item 41: Use overloading judiciously</em> <ul> <li>Just because you can overload, doesn't mean you should every time</li> <li>They can cause confusion (especially if the methods do wildly different things)</li> </ul></li> <li>Using good IDE, you can check which overload is selected at compile time <ul> <li>With Eclipse, you can mouse-hover on the above expression and see that <em>indeed</em>, the <code>valueOf(char[])</code> overload is selected!</li> </ul></li> <li>Sometimes you want to explicitly cast <code>null</code> (examples to follow)</li> </ul> <h3>See also</h3> <ul> <li><a href="https://stackoverflow.com/questions/154577/polymorphism-vs-overriding-vs-overloading">Polymorphism vs Overriding vs Overloading</a></li> <li><a href="https://stackoverflow.com/questions/248222/method-overloading-can-you-overuse-it">Method Overloading. Can you overuse it?</a></li> </ul> <hr> <h3>On casting <code>null</code></h3> <p>There are at least two situations where explicitly casting <code>null</code> to a specific reference type is necessary:</p> <ul> <li>To select overloading (as given in above example)</li> <li>To give <code>null</code> as a single argument to a vararg parameter</li> </ul> <p>A simple example of the latter is the following:</p> <pre><code>static void vararg(Object... os) { System.out.println(os.length); } </code></pre> <p>Then, we can have the following:</p> <pre><code>vararg(null, null, null); // prints "3" vararg(null, null); // prints "2" vararg(null); // throws NullPointerException! vararg((Object) null); // prints "1" </code></pre> <h3>See also</h3> <ul> <li><a href="http://java.sun.com/j2se/1.5.0/docs/guide/language/varargs.html" rel="noreferrer">Java Language Guide/varargs</a> - to understand how it's implemented</li> </ul> <h3>Related questions</h3> <ul> <li><a href="https://stackoverflow.com/questions/315846/why-null-cast">Why null cast?</a></li> <li><a href="https://stackoverflow.com/questions/2888305/difference-between-double-and-double-in-formal-parameter-type-declaration/">Difference between double… and double[] in formal parameter type declaration</a></li> </ul>
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    1. POWhy does String.valueOf(null) throw a NullPointerException?
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    1. COAnother suggestion: When you overload a method and one argument could be called on two overloads (such as `null` in this case), make sure that both overloads act the same with regard to that value!
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      1. USJoachim Sauer
    2. CO@Joachim: I just read the item, and was pleasantly surprised to find that these two methods were explicitly discussed! Bloch went a step further and claim that since `String.valueOf(Object)` and `valueOf(char[])` do completely different things anyway (irrespective of `null` or not) that perhaps they shouldn't have been overloads in the first place.
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