blocks|key|207838|text|如果您有一个需要提供Callable<T>对象的对象(如ExecutorService)，您可以向它传递一个Callable<Void>，以指示您的Callable不返回任何内容。Callable<T>必须在某些类型上参数化，因此提供Void来指示缺少类型。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|207839|entityMap^0|A|B|S|F|1I|E|2I|B|39|4|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|G|$]]

If you have an object (like an <code>ExecutorService</code>) that requires you to provide a <code>Callable&lt;T&gt;</code> object, you can pass it a <code>Callable&lt;Void&gt;</code> to indicate that your Callable doesn't return anything. <code>Callable&lt;T&gt;</code> has to be parameterized on some type, so <code>Void</code> is provided to indicate a lack of type.

blocks|key|208396|text|-与其他包装器不同，Void类本身并不存储void类型的值，因此它是，而不是真正的essence包装器。|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|CODE|entityRanges|data|208397|-根据javadoc的Void类之所以存在，是因为有时我们可能需要将关键字表示为对象。|208398|-，但同时我们不能使用新的创建空类的实例，因为operator.This中的构造函数已经被声明为私有。此外，Void类是最后一个类，这意味着我们无法继承这个类。|208399|-所以Void类存在的唯一目的是反射，在这里我们可以获得方法的返回类型为void。|208400|entityMap^0|0|1|F|4|Y|E|A|4|L|4|0|0|1|3|7|Y|9|B|4|0|0|28|0|0|15|3|4|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|D]|$9|X|A|Y|B|D]]|E|@]|F|$]]|$1|G|3|H|5|6|7|Z|8|@$9|10|A|11|B|C]|$9|12|A|13|B|C]|$9|14|A|15|B|C]|$9|16|A|17|B|D]]|E|@]|F|$]]|$1|I|3|J|5|6|7|18|8|@$9|19|A|1A|B|C]]|E|@]|F|$]]|$1|K|3|L|5|6|7|1B|8|@$9|1C|A|1D|B|C]|$9|1E|A|1F|B|D]]|E|@]|F|$]]|$1|M|3|-4|5|6|7|1G|8|@]|E|@]|F|$]]]|N|$]]

- Unlike the other wrappers <code>Void</code> class doesn't store a value of type <code>void</code> in itself and hence is not a wrapper in true essence.

- The <code>Void</code> class according to javadoc exists because of the fact that some time we may need to represent the void keyword as an object. 

- But at the same point we cannot create an instance of the Void class using the new operator.This is because the constructor in Void has been declared as private.
Moreover the Void class is a final class which means that there is no way we can inherit this class.

- So the only purpose that remains for the existence of the <code>Void</code> class is reflection, where we can get the return type of a method as void.

blocks|key|208349|text|在Java反射中使用Void类。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|208350|请参阅Method#getReturnType|208351|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/6/docs/api/java/lang/reflect/Method.html#getReturnType%2528%2529^0|A|4|0|3|K|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@$9|Q|A|R|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|S|8|@]|D|@$9|T|A|U|1|V]]|E|$]]|$1|H|3|-4|5|6|7|W|8|@]|D|@]|E|$]]]|I|$J|$5|K|L|M|E|$N|O]]]]

<code>Void</code> class is used in Java-reflection. 
see <a href="http://docs.oracle.com/javase/6/docs/api/java/lang/reflect/Method.html#getReturnType%28%29">Method#getReturnType</a>

blocks|key|209998|text|正如Luke+Woodward指出的那样，java反射API使用Void.TYPE来表示返回空的方法的返回类型。(我之前认为返回Void.class会导致方法返回声明java.lang.Void返回类型不明确)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|209999|+void+test(){...};

+Method+handleForTest+=+...;
+assert(handleForTest.getReturnType()+==+Void.TYPE);|code-block|syntax|javascript|210000|可以在泛型类中使用Post+java1.5void来指示它们不返回值-实现仍然必须返回null，但它可以被忽略，因为这是Void类型的唯一有效值。|210001|interface<T>+Example{
++++++//Implementations+may+do+something+and+return+a+result
++++++T+doSomeThing();
}

class+ExampleVoid+implements+Example<Void>{
++++++//does+something+without+returning+a+result
++++++Void+doSomething(){return+null;}
}|210002|此外，java允许在重写方法时提供更具体的返回类型，这与上面的泛型示例基本相同，只是为了说明如果原始返回类型是Object，它不需要泛型就可以工作。(它仅限于Object，因为Void遵循与其他所有java类型相同的规则，遗憾的是，没有办法为此使用无名的null-type+)|210003|interface+Example{
++++++//Implementations+may+do+something+and+return+a+result
++++++Object+doSomeThing();
}

class+ExampleVoid+implements+Example{
++++++//does+something+without+returning+a+result
++++++Void+doSomething(){return+null;}
}|210004|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

The java reflection API uses Void.TYPE to represent the return type of a method returning void as Luke Woodward points out. (Returning Void.class as I previously thought would cause an ambiguity with methods returning declaring a java.lang.Void return type). 

<pre><code> void test(){...};

 Method handleForTest = ...;
 assert(handleForTest.getReturnType() == Void.TYPE);
</code></pre>

Post java 1.5 Void can be used in generic classes to indicate that they do not return a value - the Implementation still has to return null, but it can be ignored since that is the only valid value for the type Void.

<pre><code>interface&lt;T&gt; Example{
 //Implementations may do something and return a result
 T doSomeThing();
}

class ExampleVoid implements Example&lt;Void&gt;{
 //does something without returning a result
 Void doSomething(){return null;}
}
</code></pre>

Also java allows to give a more specific return type when overwriting a method, this is basically the same as the generic example above, just to show that it works without generics if the original return type is Object. (it is limited to Object since Void follows the same rules as every other java type, sadly there is no way to use the nameless null-type for this)

<pre><code>interface Example{
 //Implementations may do something and return a result
 Object doSomeThing();
}

class ExampleVoid implements Example{
 //does something without returning a result
 Void doSomething(){return null;}
}
</code></pre>

blocks|key|2845823|text|JLS#14.8.+Expression+Statements|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2845824|与C和C%2B%2B不同，|2845825|编程语言只允许将某些形式的表达式用作表达式语句。注意，Java编程语言不允许“强制转换为空”--空不是类型--所以编写表达式语句的传统C技巧如下：|blockquote|BOLD|2845826|2845827|2845828|(void)...+;++//+incorrect!|code-block|syntax|javascript|2845829|2845830|不工作。另一方面，Java编程语言允许表达式语句中所有最有用的表达式类型，并且它不需要用作表达式语句的方法调用来调用void方法，因此几乎不需要这样的技巧。如果需要技巧，可以使用赋值语句(§15.26)或局部变量声明语句(§14.4)。|2845831|2845832|2845833|JLS#15.8.2.+Class+Literals|2845834|2845835|+void.class+(§8.4.5)的类型为Class<Void>。|2845836|2845837|2845838|JLS#8.4.5.+Method+Return+Type|2845839|2845840|方法声明的结果或者声明方法返回的值的类型(返回类型)，或者使用关键字来指示方法不返回值。|2845841|2845842|2845843|Java+doc+Void|2845844|2845845|类是一个不可实例化的占位符类，用来保存对表示关键字void的类对象的引用。|2845846|2845847|2845848|当您想要检查方法返回类型时，它用于反射目的|2845849|if+(getClass().getMethod("someMethod").getReturnType()+==+Void.TYPE)|2845850|这同样适用于泛型方法。|2845851|+final+Callable<Void>+callable+=+new+Callable<Void>()+{
++++++++public+Void+call()+{
+++++++++++++return+null;
++++++++}
++++};|2845852|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/specs/jls/se7/html/jls-14.html#jls-14.8|1|http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.8.2|2|http://docs.oracle.com/javase/specs/jls/se7/html/jls-8.html#jls-8.4.5|3|http://docs.oracle.com/javase/7/docs/api/java/lang/Void.html^0|0|V|0|V|0|0|0|1C|3|0|0|0|0|0|0|0|0|0|Q|0|Q|1|0|0|O|B|0|0|0|0|T|0|T|2|0|0|0|0|0|0|D|0|D|3|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|24|8|@$9|25|A|26|B|C]]|D|@$9|27|A|28|1|29]]|E|$]]|$1|F|3|G|5|6|7|2A|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|2B|8|@$9|2C|A|2D|B|K]]|D|@]|E|$]]|$1|L|3|-4|5|6|7|2E|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|2F|8|@]|D|@]|E|$]]|$1|N|3|O|5|P|7|2G|8|@]|D|@]|E|$Q|R]]|$1|S|3|-4|5|6|7|2H|8|@]|D|@]|E|$]]|$1|T|3|U|5|J|7|2I|8|@]|D|@]|E|$]]|$1|V|3|-4|5|6|7|2J|8|@]|D|@]|E|$]]|$1|W|3|-4|5|6|7|2K|8|@]|D|@]|E|$]]|$1|X|3|Y|5|6|7|2L|8|@$9|2M|A|2N|B|C]]|D|@$9|2O|A|2P|1|2Q]]|E|$]]|$1|Z|3|-4|5|6|7|2R|8|@]|D|@]|E|$]]|$1|10|3|11|5|J|7|2S|8|@$9|2T|A|2U|B|C]]|D|@]|E|$]]|$1|12|3|-4|5|6|7|2V|8|@]|D|@]|E|$]]|$1|13|3|-4|5|6|7|2W|8|@]|D|@]|E|$]]|$1|14|3|15|5|6|7|2X|8|@$9|2Y|A|2Z|B|C]]|D|@$9|30|A|31|1|32]]|E|$]]|$1|16|3|-4|5|6|7|33|8|@]|D|@]|E|$]]|$1|17|3|18|5|J|7|34|8|@]|D|@]|E|$]]|$1|19|3|-4|5|6|7|35|8|@]|D|@]|E|$]]|$1|1A|3|-4|5|6|7|36|8|@]|D|@]|E|$]]|$1|1B|3|1C|5|6|7|37|8|@$9|38|A|39|B|C]]|D|@$9|3A|A|3B|1|3C]]|E|$]]|$1|1D|3|-4|5|6|7|3D|8|@]|D|@]|E|$]]|$1|1E|3|1F|5|J|7|3E|8|@]|D|@]|E|$]]|$1|1G|3|-4|5|6|7|3F|8|@]|D|@]|E|$]]|$1|1H|3|-4|5|6|7|3G|8|@]|D|@]|E|$]]|$1|1I|3|1J|5|6|7|3H|8|@]|D|@]|E|$]]|$1|1K|3|1L|5|P|7|3I|8|@]|D|@]|E|$Q|R]]|$1|1M|3|1N|5|6|7|3J|8|@]|D|@]|E|$]]|$1|1O|3|1P|5|P|7|3K|8|@]|D|@]|E|$Q|R]]|$1|1Q|3|-4|5|6|7|3L|8|@]|D|@]|E|$]]]|1R|$1S|$5|1T|1U|1V|E|$1W|1X]]|1Y|$5|1T|1U|1V|E|$1W|1Z]]|20|$5|1T|1U|1V|E|$1W|21]]|22|$5|1T|1U|1V|E|$1W|23]]]]

<a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-14.html#jls-14.8" rel="nofollow"><code>JLS#14.8. Expression Statements</code></a>

<blockquote>
 Unlike C and C++, the Java programming language allows only certain forms of expressions to be used as expression statements. Note that the Java programming language does not allow a "cast to void" - void is not a type - so the traditional C trick of writing an expression statement such as:
</blockquote>

<pre><code>(void)... ; // incorrect!
</code></pre>

<blockquote>
 does not work. On the other hand, the Java programming language allows all the most useful kinds of expressions in expressions statements, and it does not require a method invocation used as an expression statement to invoke a void method, so such a trick is almost never needed. If a trick is needed, either an assignment statement (§15.26) or a local variable declaration statement (§14.4) can be used instead. 
</blockquote>

<a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.8.2" rel="nofollow"><code>JLS#15.8.2. Class Literals</code></a>

<blockquote>
 The type of void.class (§8.4.5) is <code>Class&lt;Void&gt;</code>. 
</blockquote>

<a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-8.html#jls-8.4.5" rel="nofollow"><code>JLS#8.4.5. Method Return Type</code></a>

<blockquote>
 The result of a method declaration either declares the type of value that the method returns (the return type), or uses the keyword void to indicate that the method does not return a value. 
</blockquote>

<a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Void.html" rel="nofollow"><code>Java doc Void</code></a>

<blockquote>
 The Void class is an uninstantiable placeholder class to hold a reference to the Class object representing the Java keyword void.
</blockquote>

It is used for reflection purpose when you are want to check return type of a method

<pre><code>if (getClass().getMethod("someMethod").getReturnType() == Void.TYPE)
</code></pre>

Same is applicable with Generic methods.

<pre><code> final Callable&lt;Void&gt; callable = new Callable&lt;Void&gt;() {
 public Void call() {
 return null;
 }
 };
</code></pre>

blocks|key|3058764|text|原因|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3058765|引入Void类型是为了允许回调和与外部系统的接口(JNI、来自applet的JavaScript调用，等等)，并且从Java1.1时代起就已经存在，用于接口类型不为其他接口所知或强制执行的情况(因此不能使用void+keyword)。Java被设计成一种“一切都是对象”的语言，需要一个虚拟的包装器/占位符来指示预期类型的缺失。|offset|length|style|CODE|3058766|定义|3058767|由JLS+(§8.4.5)的method+return+type部分定义的方法需要在返回类型中正式声明：|3058768|返回方法声明的结果要么声明该方法返回的值的类型(+|3058769|类型)，要么使用关键字void来指示该方法不返回值。|blockquote|3058770|3058771|3058772|正如在Class+Literals+(§15.8.2)的JLS部分中提到的：|3058773|3058774|+void.class+(§8.4.5)的类型为Class<Void>|3058775|3058776|3058777|通用用法|3058778|随着Java+5中引入泛型，它的用法变得更加突出，因为它允许这些接口的定义使用泛型，并允许对这些接口和回调的使用进行类型检查。以前，有时实现者需要知道没有传递的对象或返回的对象，例如，在使用AccessController的特权执行块的情况下，通常在小程序中用来定义安全代码路径：|3058779|++somemethod()+{
+++++//+...normal+code+here...
+++++AccessController.doPrivileged(new+PrivilegedAction<Void>()+{
+++++++++public+Void+run()+{
+++++++++++++//+privileged+code+goes+here,+for+example:
+++++++++++++System.loadLibrary("awt");
+++++++++++++return+null;+//+nothing+to+return
+++++++++}
+++++});
+++++//+...normal+code+here...
+}|code-block|syntax|javascript|3058780|注意与Java+1.4+version+of+this+same+code的不同之处，调用者不清楚(而且是类型安全的)不会返回任何内容：|3058781|+++somemethod()+{
++++++++//+...normal+code+here...
++++++++AccessController.doPrivileged(new+PrivilegedAction()+{
++++++++++++public+Object+run()+{
++++++++++++++++//+privileged+code+goes+here,+for+example:
++++++++++++++++System.loadLibrary("awt");
++++++++++++++++return+null;+//+nothing+to+return
++++++++++++}
++++++++});
+++++++//+...normal+code+here...
++}|3058782|进一步阅读和类似的问题|3058783|3058784|Can+void+be+Considered+a+Primitive|unordered-list-item|3058785|Java+Generics+and+Void+Types|3058786|Uses+for+the+Java+Void+Reference+Type|3058787|3058788|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/7/docs/api/java/lang/Void.html|1|https://stackoverflow.com/questions/4681568/in-java-can-void-be-considered-a-primitive-type|2|http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-8.4.5|3|4|http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.8.2|5|6|7|http://docs.oracle.com/javase/7/docs/api/java/security/AccessController.html|8|http://docs.oracle.com/javase/1.4.2/docs/api/java/security/AccessController.html|9|10|https://stackoverflow.com/questions/5568409/java-generics-void-void-types|11|https://stackoverflow.com/questions/643906/uses-for-the-java-void-reference-type^0|0|2|4|2V|4|2|4|0|2V|C|1|0|0|6|6|2|E|I|3|0|0|B|4|0|0|0|3|E|4|J|7|5|0|0|1|A|O|B|D|6|6|0|0|0|0|2N|G|2N|G|7|0|0|8|T|8|0|0|0|0|4|4|0|Y|9|0|0|S|A|0|0|11|B|0|0^^$0|@$1|2|3|4|5|6|7|2D|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|2E|8|@$D|2F|E|2G|F|G]|$D|2H|E|2I|F|G]]|9|@$D|2J|E|2K|1|2L]|$D|2M|E|2N|1|2O]]|A|$]]|$1|H|3|I|5|6|7|2P|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|2Q|8|@]|9|@$D|2R|E|2S|1|2T]|$D|2U|E|2V|1|2W]]|A|$]]|$1|L|3|M|5|6|7|2X|8|@]|9|@]|A|$]]|$1|N|3|O|5|P|7|2Y|8|@$D|2Z|E|30|F|G]]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|31|8|@]|9|@]|A|$]]|$1|R|3|-4|5|6|7|32|8|@]|9|@]|A|$]]|$1|S|3|T|5|6|7|33|8|@]|9|@$D|34|E|35|1|36]|$D|37|E|38|1|39]]|A|$]]|$1|U|3|-4|5|6|7|3A|8|@]|9|@]|A|$]]|$1|V|3|W|5|P|7|3B|8|@$D|3C|E|3D|F|G]|$D|3E|E|3F|F|G]]|9|@$D|3G|E|3H|1|3I]]|A|$]]|$1|X|3|-4|5|6|7|3J|8|@]|9|@]|A|$]]|$1|Y|3|-4|5|6|7|3K|8|@]|9|@]|A|$]]|$1|Z|3|10|5|6|7|3L|8|@]|9|@]|A|$]]|$1|11|3|12|5|6|7|3M|8|@$D|3N|E|3O|F|G]]|9|@$D|3P|E|3Q|1|3R]]|A|$]]|$1|13|3|14|5|15|7|3S|8|@]|9|@]|A|$16|17]]|$1|18|3|19|5|6|7|3T|8|@]|9|@$D|3U|E|3V|1|3W]]|A|$]]|$1|1A|3|1B|5|15|7|3X|8|@]|9|@]|A|$16|17]]|$1|1C|3|1D|5|6|7|3Y|8|@]|9|@]|A|$]]|$1|1E|3|-4|5|6|7|3Z|8|@]|9|@]|A|$]]|$1|1F|3|1G|5|1H|7|40|8|@$D|41|E|42|F|G]]|9|@$D|43|E|44|1|45]]|A|$]]|$1|1I|3|1J|5|1H|7|46|8|@]|9|@$D|47|E|48|1|49]]|A|$]]|$1|1K|3|1L|5|1H|7|4A|8|@]|9|@$D|4B|E|4C|1|4D]]|A|$]]|$1|1M|3|-4|5|6|7|4E|8|@]|9|@]|A|$]]|$1|1N|3|-4|5|6|7|4F|8|@]|9|@]|A|$]]]|1O|$1P|$5|1Q|1R|1S|A|$1T|1U]]|1V|$5|1Q|1R|1S|A|$1T|1W]]|1X|$5|1Q|1R|1S|A|$1T|1Y]]|1Z|$5|1Q|1R|1S|A|$1T|1Y]]|20|$5|1Q|1R|1S|A|$1T|21]]|22|$5|1Q|1R|1S|A|$1T|21]]|23|$5|1Q|1R|1S|A|$1T|1Y]]|24|$5|1Q|1R|1S|A|$1T|25]]|26|$5|1Q|1R|1S|A|$1T|27]]|28|$5|1Q|1R|1S|A|$1T|1W]]|29|$5|1Q|1R|1S|A|$1T|2A]]|2B|$5|1Q|1R|1S|A|$1T|2C]]]]

<h1>Reasons</h1>

The <a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Void.html" rel="nofollow noreferrer"><code>Void</code></a> type was introduced to allow for call-backs and interfaces with external systems (JNI, JavaScript calls from applets, etc...), and has been around since the days of Java 1.1 for such cases where the interface type isn't known or enforced by the other interface (and thus where you can't use the <a href="https://stackoverflow.com/questions/4681568/in-java-can-void-be-considered-a-primitive-type"><code>void</code> keyword</a>). Java having been designed - except for primitives and null references - as an "everything is an object" kind of language, a dummy wrapper / placeholder was needed to indicate the absence of expected types.

<h1>Definition</h1>

A method, as defined by the <a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-8.4.5" rel="nofollow noreferrer">method return type</a> section of the JLS (<a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-8.4.5" rel="nofollow noreferrer">§8.4.5</a>), needs has to formally declare at return type:

<blockquote>
 The result of a method declaration either declares the type of value
 that the method returns (the return type), or uses the keyword <code>void</code> to
 indicate that the method does not return a value.
</blockquote>

As mentioned in the JLS section for <a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.8.2" rel="nofollow noreferrer">Class Literals</a> (<a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.8.2" rel="nofollow noreferrer">§15.8.2</a>):

<blockquote>
 The type of <code>void.class</code> (<a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-8.4.5" rel="nofollow noreferrer">§8.4.5</a>) is <code>Class&lt;Void&gt;</code>
</blockquote>

<h1>General Use</h1>

Its usage became more prominent with the introduction of generics in Java 5, as it allowed the definition of these interfaces to use generics and would allow to type-check the use of these interfaces and callbacks. Before, it would sometimes have been up to the implementer to know there was no passed object or no return object, for instance in the case of a priviledged execution block using the <a href="http://docs.oracle.com/javase/7/docs/api/java/security/AccessController.html" rel="nofollow noreferrer"><code>AccessController</code></a>, very often used in Applet to define secure code paths:

<pre><code> somemethod() {
 // ...normal code here...
 AccessController.doPrivileged(new PrivilegedAction&lt;Void&gt;() {
 public Void run() {
 // privileged code goes here, for example:
 System.loadLibrary("awt");
 return null; // nothing to return
 }
 });
 // ...normal code here...
 }
</code></pre>

Note the difference with the Java <a href="http://docs.oracle.com/javase/1.4.2/docs/api/java/security/AccessController.html" rel="nofollow noreferrer">1.4 version of this same code</a>, where it wasn't clear (and type-safe) to the caller that nothing is returned:

<pre><code> somemethod() {
 // ...normal code here...
 AccessController.doPrivileged(new PrivilegedAction() {
 public Object run() {
 // privileged code goes here, for example:
 System.loadLibrary("awt");
 return null; // nothing to return
 }
 });
 // ...normal code here...
 }
</code></pre>

<h1>Further Reading and Similar Questions</h1>

<ul>
<li><a href="https://stackoverflow.com/questions/4681568/in-java-can-void-be-considered-a-primitive-type">Can <code>void</code> be Considered a Primitive</a></li>
<li><a href="https://stackoverflow.com/questions/5568409/java-generics-void-void-types">Java Generics and Void Types</a></li>
<li><a href="https://stackoverflow.com/questions/643906/uses-for-the-java-void-reference-type">Uses for the Java Void Reference Type</a></li>
</ul>

<blockquote>
 Possible Duplicate: 
 <a href="https://stackoverflow.com/questions/643906/uses-for-the-java-void-reference-type">Uses for the Java Void Reference Type?</a> 
</blockquote>



What is the real usage of <code>Void</code> class in real world problems? In which scenario can we use this class?

Why does the Void wrapper class exists in the JDK?

 可能重复：Void类在现实问题中的实际用法是什么？在什么情况下我们可以使用这个类？

问为什么在JDK中存在Void包装器类？
EN

回答 6

Stack Overflow用户

Stack Overflow用户

Stack Overflow用户

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问为什么在JDK中存在Void包装器类？EN

回答 6

Stack Overflow用户

Stack Overflow用户

Stack Overflow用户

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问为什么在JDK中存在Void包装器类？
EN