blocks|key|118458|text|使用枚举执行单例的一种简单而有效的方法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|118459|public+enum+Singleton+{
++++++++INSTANCE;
++++++++public+void+execute+(String+arg)+{
++++++++++++//...+perform+operation+here+...
++++++++}
}|code-block|syntax|javascript|118460|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

An easy and efficient way to do Singleton with an Enum:

<pre><code>public enum Singleton {
 INSTANCE;
 public void execute (String arg) {
 //... perform operation here ...
 }
}
</code></pre>

blocks|key|1762846|text|只要将类本身重构为单例即可。私有构造函数等。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1762847|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Just refactor class itself as singleton. Private constructor and etc.

blocks|key|1762887|text|Java没有像C%2B%2B那样的“朋友”概念|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1762888|您提到了嵌套类(真正的内部类将不起作用，因为它们需要外部类)和包。|1762889|除了创建实例的方法之外，我还不知道保护其他类的其他方法。|1762890|但一般来说，没有理由通过帮助器类来构建单例。|1762891|您可以使用枚举或静态最终变量构建单例|offset|length|style|BOLD|1762892|What+is+the+best+approach+for+using+an+Enum+as+a+singleton+in+Java?|1762893|public+enum+Elvis+implements+HasAge+{
++++INSTANCE;
++++private+int+age;

++++@Override
++++public+int+getAge()+{
++++++++return+age;
++++}
}|code-block|syntax|javascript|1762894|1762895|class+X+{
+++++public+static+final+X+instance+=+new+X+();

+++++private+X+()+{
+++++}
+++++...
}++|1762896|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/427902/what-is-the-best-approach-for-using-an-enum-as-a-singleton-in-java^0|0|0|0|0|5|2|8|6|0|0|1V|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|15|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|16|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|17|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|18|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|19|8|@$J|1A|K|1B|L|M]|$J|1C|K|1D|L|M]]|9|@]|A|$]]|$1|N|3|O|5|6|7|1E|8|@]|9|@$J|1F|K|1G|1|1H]]|A|$]]|$1|P|3|Q|5|R|7|1I|8|@]|9|@]|A|$S|T]]|$1|U|3|-4|5|6|7|1J|8|@]|9|@]|A|$]]|$1|V|3|W|5|R|7|1K|8|@]|9|@]|A|$S|T]]|$1|X|3|-4|5|6|7|1L|8|@]|9|@]|A|$]]]|Y|$Z|$5|10|11|12|A|$13|14]]]]

Java has no "friend" concept like C++

You mentioned nested classes (real inner classes will not work because they need the outer) and packages.

Other approaches to protected other classes but one from creating an instance are not known to me.

But in general there is no reason to build a singleton by an helper class.

You could build singleton using enums or static final vars

<a href="https://stackoverflow.com/questions/427902/what-is-the-best-approach-for-using-an-enum-as-a-singleton-in-java">What is the best approach for using an Enum as a singleton in Java?</a>

<pre><code>public enum Elvis implements HasAge {
 INSTANCE;
 private int age;

 @Override
 public int getAge() {
 return age;
 }
}
</code></pre>

<hr>

<pre><code>class X {
 public static final X instance = new X ();

 private X () {
 }
 ...
} 
</code></pre>

blocks|key|118442|text|在示例场景中，使用您的API，，我是否需要声明？：|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|118443|ToBeInvisibleClass+instance+=+TheSingleton.getThatInvisibleInstnace();|code-block|syntax|javascript|118444|如果答案是肯定的，那么你的问题的答案是否定的，因为我需要声明一个变量，为此我需要类型是可见的。如果答案是否定的，那么使用内部/嵌套类似乎是一种正确的方法，或者使类本身成为单例。|118445|entityMap^0|F|8|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|N|$]]

In a sample scenario, using your API, do I need to declare?:

<pre><code>ToBeInvisibleClass instance = TheSingleton.getThatInvisibleInstnace();
</code></pre>

If the answer is Yes, then the answer to your question is No since I need to declare a variable and for that I need the type to visible. If the answer is No, then using inner/nested class seems to be a proper approach or making the class itself the singleton.

blocks|key|2129296|text|要确保仅通过类方法进行实例化，可以执行以下操作：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2129297|2129298|将默认构造函数设为私有|unordered-list-item|2129299|将单例实例保存在私有方法中|2129300|使用公共静态方法将实例提供给客户端：|2129301|2129302|In+this+site有一个很好的例子：|offset|length|2129303|public+class+MySingleton+{+
++++private+static+MySingleton+_instance+=+new+MySingleton();+

+++++private+MySingleton()+{+
++++++++//+construct+object+.+.+.+
++++}+

++++public+static+MySingleton+getInstance()+{+
++++++++return+_instance;+
++++}+|code-block|syntax|javascript|2129304|entityMap|0|LINK|mutability|MUTABLE|url|http://java.sun.com/developer/technicalArticles/Programming/singletons/^0|0|0|0|0|0|0|0|C|0|0|0^^$0|@$1|2|3|4|5|6|7|11|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|12|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|13|8|@]|9|@]|A|$]]|$1|F|3|G|5|E|7|14|8|@]|9|@]|A|$]]|$1|H|3|I|5|E|7|15|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|16|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|17|8|@]|9|@$M|18|N|19|1|1A]]|A|$]]|$1|O|3|P|5|Q|7|1B|8|@]|9|@]|A|$R|S]]|$1|T|3|-4|5|6|7|1C|8|@]|9|@]|A|$]]]|U|$V|$5|W|X|Y|A|$Z|10]]]]

To assure that instantiation only occurs through your class method, you can do the following:

<ul>
<li>Make the default constructor private</li>
<li>Save your singleton instance in a private method</li>
<li>Use a public static method to provide the instance to the clients:</li>
</ul>

<a href="http://java.sun.com/developer/technicalArticles/Programming/singletons/" rel="nofollow">In this site</a> there's a nice example:

<pre><code>public class MySingleton { 
 private static MySingleton _instance = new MySingleton(); 

 private MySingleton() { 
 // construct object . . . 
 } 

 public static MySingleton getInstance() { 
 return _instance; 
 } 
</code></pre>

Basically I have a class which an instance of is created via a Singleton class. The class should never been instantiated via any other means than the singleton class. My question is can the class be effectively 'not seen' by other classes, apart from Singleton.

I know inner classes and different pacakages etc would help, but I'm curious to see if anyone has a nice solution to this.

Thanks for replies

Hiding the visibility of a class to other classes

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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基本上我有一个类，它的一个实例是通过Singleton类创建的。除了单例类之外，不应该通过任何其他方式实例化该类。我的问题是，除了Singleton之外，其他类是否可以有效地“看不到”这个类。我知道内部类和不同的包等会有帮助，但我很好奇是否有人有一个很好的解决方案。感谢您的回复

问隐藏类对其他类的可见性
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