java 中 private public protected default 访问权限的区别

看到网上有很多关于java访问权限的文章，发现要么是错的，要么就是讲的不清不楚，我特地去国外论坛看了下，现将清晰且正确的说法展示给大家，希望各位友人少走弯路。

不同元素的访问权限

class \ have access to	private elements	default elements (no modifier)	protected elements	public elements
own class (Base)	yes	yes	yes	yes
subclass - same package (SubA)	no	yes	yes	yes
class - same package (AnotherA)	no	yes	yes	yes
subclass - another package (SubB)	no	no	yes/no*	yes
class - another package (AnotherB)	no	no	no	yes

*与基类不在同一个包中的子类，只能访问自身从基类继承而来的受保护成员，而不能访问基类实例本身的受保护成员。

举个例子说明:

package package1;

public Class Base{

           protected int i=5;}

package package2;

public Class SubB extends Base{

           public static void main(String arg[]){

                       SubB subb = new SubB();

                       subb.i=6;

                       System.out.println(sub.i);// 正确     与基类不在同一个包中的子类，可以访问自身从基类继承而来的受保护成员

                       Base base=new Base();

                       base.i=6;

                       System.out.println(base.i);//错误     与基类不在同一个包中的子类，不能访问基类实例本身的受保护成员

}

}

其实default（friendly）与protected的区别就在于，与基类不在同一个包中的子类可以继承基类的protected元素，却无法继承default元素。看到很多人说

	public	protected	default	private
同类	T	T	T	T
同包	T	T	T
子类(不同包)	T	T
不同包中无继承关系的类	T

其实这种说法是错误的，读过thinging in java的人都知道，里面有一句，“若新建一个包，并从另一个包中的某个类
中继承，这唯一能够访问的成员就是原来那个包中的public成员”，可见不同包中的子类是不能访问基类的protected成员的，只能继承。这是最容易混淆的地方。

其他如private和public我想大家应该清楚，我就不复述了。

参考资料（英文）：http://bmanolov.free.fr/javaprotection.php#subb

Code Example

All lines with not accessible fields are commented.

package packageA;

public class Base {
public String publicStr = "publicString";
protected String protectedStr = "protectedString";
    String defaultStr = "defaultString";
private String privateStr = "privateString";

public void print() {
        System.out.println("packageA.Base has access to");
        System.out.println(" " + publicStr);
        System.out.println(" " + protectedStr);
        System.out.println(" " + defaultStr);
        System.out.println(" " + privateStr);

        Base b = new Base(); // -- other Base instance
        System.out.println("    b." + b.publicStr);
        System.out.println("    b." + b.protectedStr);
        System.out.println("    b." + b.defaultStr);
        System.out.println("    b." + b.privateStr);
    }
}

--------------------------------------------------------------------------------

package packageA;

public class SubA extends Base {
public void print() {
        System.out.println("packageA.SubA has access to");
        System.out.println("    " + publicStr + " (inherited from Base)");
        System.out.println("    " + protectedStr + " (inherited from Base)");
        System.out.println("    " + defaultStr + " (inherited from Base)");
// -- not accessible - private elements are even not inherited
        // System.out.println(privateStr);

        Base b = new Base(); // -- other Base instance
        System.out.println("    b." + b.publicStr);
        System.out.println("    b." + b.protectedStr);
        System.out.println("    b." + b.defaultStr);
// -- not accessible
        // System.out.println(b.privateStr);
    }
}

--------------------------------------------------------------------------------

package packageA;

public class AnotherA {
public void print() {
        System.out.println("packageA.AnotherA has access to");
        Base b = new Base();
        System.out.println("    b." + b.publicStr);
        System.out.println("    b." + b.protectedStr);
        System.out.println("    b." + b.defaultStr);
// System.out.println(b.privateStr);
    }
}

--------------------------------------------------------------------------------

package packageB;
import packageA.Base;

public class SubB extends Base {
public void print() {
        System.out.println("packageB.SubB has access to");
        System.out.println("    " + publicStr + " (inherited from Base)");
// -- protectedStr is inherited element -> accessible
        System.out.println("    " + protectedStr + " (inherited from Base)");
// -- not accessible
        // System.out.println(defaultStr);
        // System.out.println(privateStr);

        Base b = new Base(); // -- other Base instance
        System.out.println("    b." + b.publicStr);
// -- protected element, which belongs to other object -> not accessible
        // System.out.println(b.protectedStr);

        // -- not accessible
        // System.out.println(b.defaultStr);
        // System.out.println(b.privateStr);
    }
}

--------------------------------------------------------------------------------

package packageB;
import packageA.Base;

public class AnotherB {
public void print() {
        System.out.println("packageB.AnotherB has access to");
        Base b = new Base();
        System.out.println("    b." + b.publicStr);
// -- not accessible
        // System.out.println(b.protectedStr);
        // System.out.println(b.defaultStr);
        // System.out.println(b.privateStr);
    }
}

--------------------------------------------------------------------------------

import packageA.*;
import packageB.*;

// -- testing class
public class TestProtection {
public static void main(String[] args) {
// -- all classes are public, so class TestProtection
        // -- has access to all of them
new Base().print();
new SubA().print();
new AnotherA().print();
new SubB().print();
new AnotherB().print();
    }
}