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.

这里有一些不明显的内存保留问题需要考虑。由于非静态内部类维护对其“外部”类的隐式引用，如果内部类的一个实例被强引用，那么外部实例也会被强引用。当外部类没有被垃圾收集时，这可能会导致一些令人费解的问题，即使看起来没有任何东西引用它。

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;
    }
}

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:)

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。

内部类的优势——

一次性使用 支持和改进封装 readibility 私有字段访问

没有外部类的存在，内部类就不存在。

class car{
    class wheel{

    }
}

内部类有四种类型。

正常的内部类 局部内部类 匿名内部类 静态内部类

点——

from static inner class ,we can only access static member of outer class. Inside inner class we cananot declare static member . inorder to invoke normal inner class in static area of outer class. Outer 0=new Outer(); Outer.Inner i= O.new Inner(); inorder to invoke normal inner class in instance area of outer class. Inner i=new Inner(); inorder to invoke normal inner class in outside of outer class. Outer 0=new Outer(); Outer.Inner i= O.new Inner(); inside Inner class This pointer to inner class. this.member-current inner class outerclassname.this--outer class for inner class applicable modifier is -- public,default, final,abstract,strictfp,+private,protected,static outer$inner is the name of inner class name. inner class inside instance method then we can acess static and instance field of outer class.

10.内部类在静态方法中，那么我们只能访问的静态字段

外部类。

class outer{

    int x=10;
    static int y-20;

    public void m1() {
        int i=30;
        final j=40;

        class inner{

            public void m2() {
                // have accees x,y and j
            }
        }
    }
}