我查看了LinkedList的Java代码，注意到它使用了一个静态嵌套类Entry。
public class LinkedList<E> ... {
...
private static class Entry<E> { ... }
}
为什么要使用静态嵌套类，而不是普通的内部类?
我能想到的唯一原因是，Entry不能访问实例变量，所以从OOP的角度来看，它有更好的封装。
但我想可能还有其他原因，可能是表现。可能是什么?
请注意。我希望我的术语是正确的，我将它称为静态内部类，但我认为这是错误的:http://java.sun.com/docs/books/tutorial/java/javaOO/nested.html
当前回答
在构建器模式中使用静态内部类。静态内部类可以实例化只有私有构造函数的外部类。你不能对内部类做同样的事情，因为你需要在访问内部类之前创建外部类的对象。
class OuterClass {
private OuterClass(int x) {
System.out.println("x: " + x);
}
static class InnerClass {
public static void test() {
OuterClass outer = new OuterClass(1);
}
}
}
public class Test {
public static void main(String[] args) {
OuterClass.InnerClass.test();
// OuterClass outer = new OuterClass(1); // It is not possible to create outer instance from outside.
}
}
这将输出x: 1
其他回答
静态嵌套类就像任何其他外部类一样，因为它不能访问外部类成员。
为了方便打包，我们可以将静态嵌套类合并到一个外部类中以提高可读性。除此之外，静态嵌套类没有其他用例。
这种用法的例子，你可以在Android R.java(资源)文件中找到。 android的Res文件夹包含布局(包含屏幕设计)，可绘制文件夹(包含用于项目的图像)，值文件夹(包含字符串常量)等。
因为所有的文件夹都是Res文件夹的一部分，android工具生成一个R.java(资源)文件，其中内部包含许多静态嵌套类的每个内部文件夹。
下面是在android中生成的R.java文件的外观和感觉: 这里他们只是为了包装方便而使用。
/* AUTO-GENERATED FILE. DO NOT MODIFY.
*
* This class was automatically generated by the
* aapt tool from the resource data it found. It
* should not be modified by hand.
*/
package com.techpalle.b17_testthird;
public final class R {
public static final class drawable {
public static final int ic_launcher=0x7f020000;
}
public static final class layout {
public static final int activity_main=0x7f030000;
}
public static final class menu {
public static final int main=0x7f070000;
}
public static final class string {
public static final int action_settings=0x7f050001;
public static final int app_name=0x7f050000;
public static final int hello_world=0x7f050002;
}
}
JVM knows no nested classes. Nesting is just syntactic sugar. Below images shows Java file: Below images show class files representation of the java file : Notice that 2 class files are generated, one for parent and another for nested class. Non-static nested class' objects have access to the enclosing scope. That access to the enclosing scope is maintained by holding an implicit reference of the enclosing scope object in the nested object Nested class is a way to represent the intent that the nested class type represents a component of the parent class. public class Message { private MessageType messageType; // component of parent class public enum MessageType { SENT, RECEIVE; } } class Otherclass { public boolean isSent(Message message) { if (message.getMessageType() == MessageType.SENT) { // accessible at other places as well return true; } return false; } } private static nested class represents Point#3 & the fact the nested type can only be the subcomponent to the parent class. It can't be used separately. public class Message { private Content content; // Component of message class private static class Content { // can only be a component of message class private String body; private int sentBy; public String getBody() { return body; } public int getSentBy() { return sentBy; } } } class Message2 { private Message.Content content; // Not possible } More details here.
在我看来，当你看到一个内部类时，问题应该是反过来的——它真的需要是一个内部类吗?它有额外的复杂性和隐式(而不是显式和更清晰的)引用包含类的实例?
请注意，作为一个c#爱好者，我有偏见——c#没有等价的内部类，尽管它有嵌套类型。我还不能说我错过了内部类:)
从http://docs.oracle.com/javase/tutorial/java/javaOO/whentouse.html:
如果需要访问，请使用非静态嵌套类(或内部类) 到封闭实例的非公共字段和方法。使用静电 嵌套类，如果您不需要此访问。
你链接到的Sun页面在这两者之间有一些关键的区别:
A nested class is a member of its enclosing class. Non-static nested classes (inner classes) have access to other members of the enclosing class, even if they are declared private. Static nested classes do not have access to other members of the enclosing class. ... Note: A static nested class interacts with the instance members of its outer class (and other classes) just like any other top-level class. In effect, a static nested class is behaviorally a top-level class that has been nested in another top-level class for packaging convenience.
不需要LinkedList。Entry是顶级类，因为它只被LinkedList使用(还有一些其他接口也有名为Entry的静态嵌套类，例如Map。入口-相同的概念)。因为它不需要访问LinkedList的成员，所以它是静态的是有意义的——这是一种更简洁的方法。
正如Jon Skeet所指出的，我认为如果你正在使用一个嵌套类，最好从它是静态的开始，然后根据你的使用情况决定它是否真的需要是非静态的。
