受保护/公共内部类

发布于 2024-07-14 05:41:14 字数 1054 浏览 14 评论 0原文

有人可以向我解释一下 protected / public Inner 类之间有什么区别吗？

我知道 public 内部类应尽可能避免（如文章）。

但据我所知，使用 protected 或 public 修饰符之间没有区别。

看一下这个例子：

public class Foo1 {
 public Foo1() { }

 protected class InnerFoo {
  public InnerFoo() {
   super();
  }
 }
}

...

public class Foo2 extends Foo1 {
 public Foo2() {
  Foo1.InnerFoo innerFoo = new Foo1.InnerFoo();
 }
}

...

public class Bar {
 public Bar() {
  Foo1 foo1 = new Foo1();
  Foo1.InnerFoo innerFoo1 = foo1.new InnerFoo();

  Foo2 foo2 = new Foo2();
  Foo2.InnerFoo innerFoo2 = foo2.new InnerFoo();
 }
}

无论我声明 InnerFoo protected 还是 public，所有这些都会编译并且有效。

我缺少什么？请指出使用 protected 或 public 之间存在差异的情况。

谢谢。

原文

Can someone please explain to me what is the difference between protected / public Inner classes?

I know that public inner classes are to avoid as much as possible (like explained in this article).

But from what I can tell, there is no difference between using protected or public modifiers.

Take a look at this example:

public class Foo1 {
 public Foo1() { }

 protected class InnerFoo {
  public InnerFoo() {
   super();
  }
 }
}

...

public class Foo2 extends Foo1 {
 public Foo2() {
  Foo1.InnerFoo innerFoo = new Foo1.InnerFoo();
 }
}

...

public class Bar {
 public Bar() {
  Foo1 foo1 = new Foo1();
  Foo1.InnerFoo innerFoo1 = foo1.new InnerFoo();

  Foo2 foo2 = new Foo2();
  Foo2.InnerFoo innerFoo2 = foo2.new InnerFoo();
 }
}

All of this compiles and is valid whether I declare InnerFoo protected or public.

What am I missing? Please, point me out a case where there's a difference in using protected or public.

Thanks.

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海风掠过北极光 2024-07-21 05:41:14

protected 访问修饰符将限制来自同一包及其子类以外的类的访问。

在所示示例中，public 和 protected 将具有相同的效果，因为它们位于同一包中。

有关访问修饰符的详细信息，请参阅控制对类成员的访问 Java 教程的页面可能会感兴趣。

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脸赞 2024-07-21 05:41:14

你可以认为受保护的内部类是受保护的成员，所以它只能访问类、包、子类，但不能访问世界。

另外，对于外部类，它只有两个访问修饰符。只是公开和打包。

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风尘浪孓 2024-07-21 05:41:14

java 中奇怪的事情：

纯 Java： 你不能从公共 getter 返回一个私有内部类。

在 JSP 中：您不能从公共 getter 返回非公共内部类。

您可以运行的Java演示：

public class ReturnInnerClass{
    public static void main(String []args){
        MyClass inst = new MyClass("[PROP_VAL]");
        System.out.println(

            inst.get().myProperty()

        );;    
    };;
};;

class MyClass{ 
    //:If JSP: MUST be public
    //:Pure Java: 
    //:     public,protected,no-access-modifier
    //:     Will all work.
    //:Private fails in both pure java & jsp.
    protected class Getters{
        public String 
        myProperty(){ return(my_property); }
    };;

    //:JSP EL can only access functions:
    private Getters _get;
    public  Getters  get(){ return _get; }

    private String 
    my_property;

    public MyClass(String my_property){
        super();
        this.my_property    = my_property;
        _get = new Getters();
    };;
};;

//:How to run this example:
//:  1: Put this code in file called: "ReturnInnerClass.java"
//:  2: Put ReturnInnerClass.java into it's own folder.
//:     ( Folder name does not matter.)
//:  3: Open the folder.
//:  4: Right-Click --> GitBashHere
//:  5: In command prompt within folder:
//:     5.1: javac ReturnInnerClass.java
//:     5.2: java  ReturnInnerClass
//:     ( javac: java compiler              )
//:     ( java : runs compiled java program )

//:  EXPECTED OUTPUT:
//:  [PROP_VAL]

对于JSP，仅将上面的类代码放入文件夹：com/myPackage/MyClass
并将“import com.myPackage.MyClass”作为源代码的第一行。然后使用以下源代码创建一个新的 .jsp 页面：

<%@ taglib uri   ="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@ page import="com.myPackage.MyClass" %>
<%
    MyClass inst = new MyClass("[PROP_VALUE]");
    pageContext.setAttribute("my_inst", inst ); 
%><html lang="en"><body>
    ${ my_inst.get().myProperty() }
</body></html>

使用的堆栈：
Java8 + Tomcat9

Weird thing in java:

Pure Java: You cannot return a private inner class from a public getter.

In JSP : You cannot return a non-public inner class from a public getter.

Java Demo You Can Run:

public class ReturnInnerClass{
    public static void main(String []args){
        MyClass inst = new MyClass("[PROP_VAL]");
        System.out.println(

            inst.get().myProperty()

        );;    
    };;
};;

class MyClass{ 
    //:If JSP: MUST be public
    //:Pure Java: 
    //:     public,protected,no-access-modifier
    //:     Will all work.
    //:Private fails in both pure java & jsp.
    protected class Getters{
        public String 
        myProperty(){ return(my_property); }
    };;

    //:JSP EL can only access functions:
    private Getters _get;
    public  Getters  get(){ return _get; }

    private String 
    my_property;

    public MyClass(String my_property){
        super();
        this.my_property    = my_property;
        _get = new Getters();
    };;
};;

//:How to run this example:
//:  1: Put this code in file called: "ReturnInnerClass.java"
//:  2: Put ReturnInnerClass.java into it's own folder.
//:     ( Folder name does not matter.)
//:  3: Open the folder.
//:  4: Right-Click --> GitBashHere
//:  5: In command prompt within folder:
//:     5.1: javac ReturnInnerClass.java
//:     5.2: java  ReturnInnerClass
//:     ( javac: java compiler              )
//:     ( java : runs compiled java program )

//:  EXPECTED OUTPUT:
//:  [PROP_VAL]

For JSP, put only the class code above into folder: com/myPackage/MyClass
and put "import com.myPackage.MyClass" as first line of source code. Then create a new .jsp page with this source code:

<%@ taglib uri   ="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@ page import="com.myPackage.MyClass" %>
<%
    MyClass inst = new MyClass("[PROP_VALUE]");
    pageContext.setAttribute("my_inst", inst ); 
%><html lang="en"><body>
    ${ my_inst.get().myProperty() }
</body></html>

Stack Used:
Java8 + Tomcat9

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