blocks|key|594781|text|私有接口方法是Java9的一部分，也是JEP-213的一部分。由于Java8中的接口可以具有default+methods，因此私有方法允许多个默认方法使用一个共享的私有方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|594782|entityMap|0|LINK|mutability|MUTABLE|url|http://openjdk.java.net/jeps/213|1|http://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html^0|J|7|0|1A|F|1|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@$A|O|B|P|1|Q]|$A|R|B|S|1|T]]|C|$]]|$1|D|3|-4|5|6|7|U|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]|L|$5|G|H|I|C|$J|M]]]]

Private interface methods are part of Java 9 as part of <a href="http://openjdk.java.net/jeps/213" rel="noreferrer">JEP-213</a>. Since interfaces in Java 8 can have <a href="http://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html" rel="noreferrer">default methods</a>, private methods allow for multiple default methods to use a shared private method.

blocks|key|594894|text|从Java+8开始，接口可以有默认方法，从Java+9开始，接口可以有私有方法，只能由同一接口中的默认方法访问。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|594895|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|$]]

As of Java 8, interfaces can have default methods, and as of Java 9, an interface is allowed to have a private methods which can only be accessed by default methods in the same interface.

blocks|key|378422|text|接口用于描述由实现该接口的任何类提供的API。由于定义中的接口没有状态，因此在其中声明字段成员没有任何用处。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|378423|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|$]]

An interface is used for describing an API which is provided by any class implementing the interface. Since an interface from its definition has no state there is no use of declaring field members in it.

blocks|key|378573|text|没有办法实现这样的接口。An+answer+to+a+question+I+posed强烈建议(如果不从根本上改变规则)实现具有私有方法的接口是不可能的-这留下了一个问题，为什么不允许保护和包私有方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|378574|class+OuterClass
{
+++++void+run+(+MyInterface+x+)
+++++{
+++++++++++x+.+publicMethod+(+)+;++//+why+not?
+++++++++++x+.+protectedMethod+(+)+;+//+why+not?
+++++++++++x+.+packagePrivateMethod+(+)+;+//+why+not?
+++++++++++x+.+privateMethod+(+)+;+//+why+not?
+++++}

+++++interface+MyInterface
+++++{
+++++++++++public+abstract+void+publicMethod+(+)+;+//+OK

+++++++++++protected+abstract+void+protectedMethod+(+)+;+//+why+not?

+++++++++++abstract+void+packagePrivateMethod+(+)+;+//+in+interface+default+is+public,+but+why+not+package+private

+++++++++++private+void+privateMethod+(+)+;+//+impossible+to+implement
+++++}

+++++class+MyImpl+implements+MyInterface
+++++{
+++++++++++public+void+publicMethod+(+)+{+}+//+ok

+++++++++++protected+void+protectedMethod+(+)+{+}+//+no+sweat

+++++++++++void+packagePrivateMethod+(+)+{+}+//+no+sweat

+++++++++++private+void+privateMethod+(+)+{+}+//+not+happening
+++++}
}|code-block|syntax|javascript|378575|下面的代码应该可以达到预期的效果。尽管所有方法都是公共的，但只有公共方法是有效的公共方法。被保护的方法得到了有效的保护。packagePrivateMethod实际上就是packagePrivate。privateMethod实际上是私有的。|378576|class+WorkAround
{
+++++void+run+(+MyPrivateInterface+x+)
+++++{
+++++++++++x+.+publicMethod+(+)+;++
+++++++++++x+.+protectedMethod+(+)+;+
+++++++++++x+.+packagePrivateMethod+(+)+;+
+++++++++++x+.+privateMethod+(+)+;+
+++++}

+++++public+interface+MyPublicInterface+{+void+publicMethod+(+)+;+}

+++++protected+interface+MyProtectedInterface+extends+MyPublicInterface+{+void+protectedMethod+(+)+;+}

+++++interface+MyPackagePrivateInterface+extends+MyProtectedInterface+{+void+packagePrivateMethod+(+)+;+}

+++++private+interface+MyPrivateInterface+extends+MyPackagePrivateInterface+{+void+privateMethod+(+)+;+}
}|378577|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/6863600/348975^0|C|V|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@$A|V|B|W|1|X]]|C|$]]|$1|D|3|E|5|F|7|Y|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|Z|8|@]|9|@]|C|$]]|$1|K|3|L|5|F|7|10|8|@]|9|@]|C|$G|H]]|$1|M|3|-4|5|6|7|11|8|@]|9|@]|C|$]]]|N|$O|$5|P|Q|R|C|$S|T]]]]

There would be no way to implement such an interface. <a href="https://stackoverflow.com/a/6863600/348975">An answer to a question I posed</a> strongly suggests that it would be impossible (without radically changing the rules) to implement an interface with private methods - this leaves open the question of why protected and package private methods are not allowed.

<pre><code>class OuterClass
{
 void run ( MyInterface x )
 {
 x . publicMethod ( ) ; // why not?
 x . protectedMethod ( ) ; // why not?
 x . packagePrivateMethod ( ) ; // why not?
 x . privateMethod ( ) ; // why not?
 }

 interface MyInterface
 {
 public abstract void publicMethod ( ) ; // OK

 protected abstract void protectedMethod ( ) ; // why not?

 abstract void packagePrivateMethod ( ) ; // in interface default is public, but why not package private

 private void privateMethod ( ) ; // impossible to implement
 }

 class MyImpl implements MyInterface
 {
 public void publicMethod ( ) { } // ok

 protected void protectedMethod ( ) { } // no sweat

 void packagePrivateMethod ( ) { } // no sweat

 private void privateMethod ( ) { } // not happening
 }
}
</code></pre>

The below code should achieve the desired result. Even though all methods are public, only public method is effectively public. protected method is effectively protected. packagePrivateMethod is effectively packagePrivate. privateMethod is effectively private.

<pre><code>class WorkAround
{
 void run ( MyPrivateInterface x )
 {
 x . publicMethod ( ) ; 
 x . protectedMethod ( ) ; 
 x . packagePrivateMethod ( ) ; 
 x . privateMethod ( ) ; 
 }

 public interface MyPublicInterface { void publicMethod ( ) ; }

 protected interface MyProtectedInterface extends MyPublicInterface { void protectedMethod ( ) ; }

 interface MyPackagePrivateInterface extends MyProtectedInterface { void packagePrivateMethod ( ) ; }

 private interface MyPrivateInterface extends MyPackagePrivateInterface { void privateMethod ( ) ; }
}
</code></pre>

blocks|key|594676|text|根据Java编程语言，private+members的范围仅限于在其中声明它的class，并且只能由该class的方法访问。但是inteface没有方法体，因此在interface中声明私有成员是没有用的。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|594677|entityMap^0|2|4|B|F|13|5|1F|5|1S|8|29|9|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]|$9|S|A|T|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|U|8|@]|D|@]|E|$]]]|G|$]]

According to the <code>Java</code> programming language scope of the <code>private members</code> is limited to the <code>class</code> in which it is declared and can be accessed only by methods of that <code>class</code>. But <code>inteface</code> doesn't have a method body hence there is no use of declaring private members inside an <code>interface</code>.

blocks|key|378454|text|这是因为它们将是无用的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|378455|没有办法调用私有方法。|378456|私有成员是一个实现细节。接口是关于类可以承担的公共角色。|378457|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

It is because they would be useless.

There would be no way to call a private method.

Private members are an implementation detail. An interface is about the public role that a class can take on.

blocks|key|378711|text|私有字段并不是完全无用的，因为其他字段和内部类可以访问它们。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|378712|然而，即使在嵌套类中，私有方法也无法实现，这使得它们几乎毫无用处。您可以使用反射来读取它们，但这是一种边缘情况。|378713|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

private fields would not be completely useless as other fields and inner classes could access them.

However private methods could not be implemented, even in nested classes, making them almost useless. You could read them using reflection, but that is rather an edge case.

blocks|key|378483|text|私有成员在接口中没有意义。接口是一种访问具有已定义方法的类的方法，您不需要查看该类的内部函数。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|378484|私人成员不同意这一点。|378485|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

Private members don't make sense in interface. Interface is a way to access a class with defined methods where you don't need to see the inners of that class. 

Private members disagree to that.

blocks|key|594849|text|是啊，不能这么做。对于那些评论为什么它不应该的人：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|594850|假设我有一个类A，它利用接口I。类B，扩展了类A，因此也继承了A中的所有接口方法。|594851|现在，假设我想要在A类中有一个私有方法，但也想要为其他类(可能是C类，它不一定扩展B类或A类)定义它。|594852|也许是一个“初始化”方法，我希望所有使用i接口的类都使用这个方法。但很明显，我不希望初始化方法是公开的...因为它应该只使用一次，或者类认为有必要使用它，而不仅仅是因为你想随意使用它。|594853|唯一的解决方案是一种变通办法，或者只是在没有接口的情况下将init方法强制到类本身中。|594854|我不太理解其中的原因，但仍然，它有时会派上用场。显然，Oracle对此表示同意，因为他们在JDK+9中允许私有接口方法。|594855|无论如何，我所做的是放置一个简单的布尔变量，这样接口方法(应该是私有的)在设置一次之后就可以被标记为true+(initialized+=+true)。然后，当再次被调用时，该方法什么也不做。通过这种方式，接口方法可以实现为public，但由于(我的类)的构造函数首先调用该方法，这会将变量设置为true，因此不能再次调用它。|594856|否则，如果您只想让类的内部工作使用它，则必须尝试不同的解决方法。也许一个方法本身在使用它时会设置一个开关标志。当标志为false时，该方法什么也不做(这是当有人从类外部调用它时)。然而，当类自己的方法调用它时，它们会迅速将标志设置为true，然后调用该方法，然后将标志设置为false?？|594857|最后是一种哑巴。也许现在更好的做法是简单地将私有类放在类本身中，并完全去掉接口。|594858|entityMap^0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|T|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|U|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|V|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|W|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|X|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|Y|8|@]|9|@]|A|$]]|$1|L|3|M|5|6|7|Z|8|@]|9|@]|A|$]]|$1|N|3|O|5|6|7|10|8|@]|9|@]|A|$]]|$1|P|3|Q|5|6|7|11|8|@]|9|@]|A|$]]|$1|R|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|S|$]]

Yep, can't do that. For all those commenting on why it shouldn't:

Imagine I have Class A, which utilizes interface I. Class B, extends Class A, therefore also inheriting all interface methods in A.

Now, imagine I want a private method in Class A, but want it contractually defined for other classes as well (Maybe a class C, which doesn't necessarily extend Class B or A).

Perhaps for an "initialization" method, that I want for all classes using an I interface. But obviously I don't want an initialization method to be public.... since it should only be used once, or as the class deems necessary, not just because you want to use it all willy-nilly.

The only solution is a workaround, or by simply forcing the init method into the classes themselves without an interface.

I understand the reason not too, for sure, but still, it can come in handy sometimes. Clearly Oracle agrees as they're allowing private interface methods in JDK 9.

What I did, for mine anyway, was place a simple boolean variable, that way the interface method (which should be private) can be flagged as true (initialized = true) after being set once. Then when called again the method simply does nothing. This way the interface method can be implemented as public, but since the constructor (of my class) calls the method first, this sets the variable to true, and so it can't be called again.

Otherwise you'd have to try a different workaround if you only want the inner workings of the class to use it.... perhaps a method itself sets a flag on and off as it uses it. When the flag is false, the method does nothing (this would be when someone calls it from outside the class). However, when the classes own methods call it, they quickly set the flag to true, then call the method, then set the flag to false??

In the end kind of mute. Probably just better for now to simply place the private class into the class itself and cut-out the interface altogether.

Why doesn't Java allow private members in interface? Is there any particular reason?

Why doesn't Java allow private members in interface?

Java

为什么Java不允许在接口中使用私有成员？有什么特别的原因吗？

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