1.Java有三种访问权限修饰符，分别为public、protected、private，还有一种为缺省权限修饰符的情况，记为default。其中，可以由public和default来修饰类；这四种修饰符都可以修饰成员变量和成员方法。每一种修饰符对应不同的访问范围，下面以下图为例详细说明。

文章图片

图1

• private只可以覆盖圆1，即只有本类可以访问；
• default可以覆盖到圆3，即本类、同包子类、同包其他类都可以访问，简单说就是与本类同包的所有类都可以访问；
• protected可以覆盖到圆4，即本类、同包子类、同包其他类、其他包中的子类都可以访问，简单说就是与本类同包的类及其他包中的子类都可以访问；
• public可以覆盖到圆5，即本类、同包子类、同包其他类、其他包子类、其他包其他类都可以访问，简单说就是所有类都可以访问；

注：在与父类不同包的子类中，如果通过子类对象访问和调用父类中由protected修饰的变量和方法，确实可以；但如果通过父类的对象访问和调用的话，则不可以访问protected修饰的变量和方法，具体见下文的(6)和(7)。具体原因还未了解。

2.下面以简单的程序验证上述结论。
前提：

• 包human中定义了类Person，Student，DustMan；其中，Student是Person的子类，DustMan不是Person的子类。
• 包teacher中定义了类Teacher，GateMan；其中，Teacher是Person的子类，GateMan不是Person的子类。
• Person中定义了四个成员变量和四个成员方法，分别以public，protected，缺省，private修饰，详见下面代码：

String name;
public String education; private String hobby; protected String residence; public void testModifierPublic() { System.out.println("Public"); } protected void testModifierProtected() { System.out.println("Protected"); } void testModifierDefault() { System.out.println("Default"); } private void testModifierPrivate() { System.out.println("Private"); }

(1),在Person类中定义Person类对象pOwn，分别访问和调用这些成员变量和成员方法，详见下面的代码：

public static void main(String[] args) { Person pOwn = new Person("xi",20,"female","bachelor","recite","NJ"); System.out.println("public education: " + pOwn.education); System.out.println("protected residence: " + pOwn.residence); System.out.println("default name: " + pOwn.name); System.out.println("private hobby: "+ pOwn.hobby); pOwn.testModifierPublic(); pOwn.testModifierProtected(); pOwn.testModifierDefault(); pOwn.testModifierPrivate(); }

【Java学习笔记10---访问权限修饰符如何控制成员变量、成员方法及类的访问范围...】输出结果为：

public education: bachelor protected residence: NJ default name: xi private hobby: recite Public Protected Default Private

结果分析：Person类对象可以在本类中访问和调用由public、protected、default、private修饰的成员变量和成员方法。

(2).在Student类中分别定义Student类对象sSamePackChild和Person类对象pSamePackChild，并分别访问和调用这些成员变量和成员方法，详见下面的代码：

public static void main(String[] args) { Student sSamePackChild = new Student("xi",20,"female","bachelor","recite","NJ"); Person pSamePackChild = new Person("xi",20,"female","bachelor","recite","NJ"); System.out.println("public education: " + sSamePackChild.education); System.out.println("protected residence: " + sSamePackChild.residence); System.out.println("default name: " + sSamePackChild.name); System.out.println("private hobby: "+ sSamePackChild.hobby); sSamePackChild.testModifierPublic(); sSamePackChild.testModifierProtected(); sSamePackChild.testModifierDefault(); sSamePackChild.testModifierPrivate(); System.out.println("public education: " + pSamePackChild.education); System.out.println("protected residence: " + pSamePackChild.residence); System.out.println("default name: " + pSamePackChild.name); System.out.println("private hobby: "+ pSamePackChild.hobby); pSamePackChild.testModifierPublic(); pSamePackChild.testModifierProtected(); pSamePackChild.testModifierDefault(); pSamePackChild.testModifierPrivate(); }

输出结果为：

Exception in thread "main" java.lang.Error: Unresolved compilation problems: The field Person.hobby is not visible The method testModifierPrivate() from the type Person is not visible The field Person.hobby is not visible The method testModifierPrivate() from the type Person is not visible at human.Student.main(Student.java:108)

结果分析：出现编译错误，提示private修饰的hobby和testModifierPrivate()不可见。

(3).根据(2)提示的错误，注释掉相关的行，再次执行，详见代码：

public static void main(String[] args) { Student sSamePackChild = new Student("xi",20,"female","bachelor","recite","NJ"); Person pSamePackChild = new Person("xi",20,"female","bachelor","recite","NJ"); System.out.println("public education: " + sSamePackChild.education); System.out.println("protected residence: " + sSamePackChild.residence); System.out.println("default name: " + sSamePackChild.name); //System.out.println("private hobby: "+ sSamePackChild.hobby); sSamePackChild.testModifierPublic(); sSamePackChild.testModifierProtected(); sSamePackChild.testModifierDefault(); //sSamePackChild.testModifierPrivate(); System.out.println("public education: " + pSamePackChild.education); System.out.println("protected residence: " + pSamePackChild.residence); System.out.println("default name: " + pSamePackChild.name); //System.out.println("private hobby: "+ pSamePackChild.hobby); pSamePackChild.testModifierPublic(); pSamePackChild.testModifierProtected(); pSamePackChild.testModifierDefault(); //pSamePackChild.testModifierPrivate(); }

输出结果为：

public education: bachelor protected residence: NJ default name: xi Public Protected Default public education: bachelor protected residence: NJ default name: xi Public Protected Default

结果分析：
注释掉private修饰的行后，成功执行；
Person类对象可以在与父类同包的子类Student中访问和调用由public、protected、default修饰的成员变量和成员方法，不能访问由private修饰的变量和方法；
在子类中定义的Student类对象也拥有同样的访问权限。

(4).在DustMan类中定义Person类对象pSamePackNonChild，分别访问和调用这些成员变量和成员方法，详见下面的代码：

package human; public class DustMan { public static void main(String[] args) { Person pSamePackNonChild = new Person("xi",20,"female","bachelor","recite","NJ"); System.out.println("public education: " + pSamePackNonChild.education); System.out.println("protected residence: " + pSamePackNonChild.residence); System.out.println("default name: " + pSamePackNonChild.name); System.out.println("private hobby: "+ pSamePackNonChild.hobby); pSamePackNonChild.testModifierPublic(); pSamePackNonChild.testModifierProtected(); pSamePackNonChild.testModifierDefault(); pSamePackNonChild.testModifierPrivate(); } }

输出结果为：

Exception in thread "main" java.lang.Error: Unresolved compilation problems: The field Person.hobby is not visible The method testModifierPrivate() from the type Person is not visible at human.DustMan.main(DustMan.java:19)

结果分析：出现编译错误，提示private修饰的hobby和testModifierPrivate()不可见。

(5).根据(4)提示的错误，注释掉相关的行，再次执行，详见代码：

package human; public class DustMan { public static void main(String[] args) { Person pSamePackNonChild = new Person("xi",20,"female","bachelor","recite","NJ"); System.out.println("public education: " + pSamePackNonChild.education); System.out.println("protected residence: " + pSamePackNonChild.residence); System.out.println("default name: " + pSamePackNonChild.name); //System.out.println("private hobby: "+ pSamePackNonChild.hobby); pSamePackNonChild.testModifierPublic(); pSamePackNonChild.testModifierProtected(); pSamePackNonChild.testModifierDefault(); //pSamePackNonChild.testModifierPrivate(); } }

输出结果为：

public education: bachelor protected residence: NJ default name: xi Public Protected Default

结果分析：
注释掉private修饰的行后，成功执行；
Person类对象可以在与Person同包的非子类中访问和调用由public、protected、default修饰的成员变量和成员方法，不能访问由private修饰的变量和方法。

(6).在Teacher类中定义Teacher类对象tDiffPackChild和Person类对象pDiffPackChild，并分别访问和调用这些成员变量和成员方法，详见下面的代码：

package teacher; import human.Person; public class Teacher extends human.Person { String duty; public Teacher() { } public Teacher(String d) { super(); this.duty = d; } public Teacher(String n, int a, String g, String e, String h, String r) { super(n,a,g,e,h,r); } public static void main(String[] args) { Teacher tDiffPackChild = new Teacher("xi",20,"female","bachelor","recite","NJ"); Person pDiffPackChild = new Person("xi",20,"female","bachelor","recite","NJ"); System.out.println("public education: " + tDiffPackChild.education); System.out.println("protected residence: " + tDiffPackChild.residence); System.out.println("default name: " + tDiffPackChild.name); System.out.println("private hobby: "+ tDiffPackChild.hobby); tDiffPackChild.testModifierPublic(); tDiffPackChild.testModifierProtected(); tDiffPackChild.testModifierDefault(); tDiffPackChild.testModifierPrivate(); System.out.println("public education: " + pDiffPackChild.education); System.out.println("protected residence: " + pDiffPackChild.residence); System.out.println("default name: " + pDiffPackChild.name); System.out.println("private hobby: "+ pDiffPackChild.hobby); pDiffPackChild.testModifierPublic(); pDiffPackChild.testModifierProtected(); pDiffPackChild.testModifierDefault(); pDiffPackChild.testModifierPrivate(); } }

输出结果为：

Exception in thread "main" java.lang.Error: Unresolved compilation problems: The field Person.name is not visible The field Person.hobby is not visible The method testModifierDefault() from the type Person is not visible The method testModifierPrivate() from the type Person is not visible The field Person.residence is not visible The field Person.name is not visible The field Person.hobby is not visible The method testModifierProtected() from the type Person is not visible The method testModifierDefault() from the type Person is not visible The method testModifierPrivate() from the type Person is not visible at teacher.Teacher.main(Teacher.java:39)

结果分析：
出现编译错误，对于定义的Teacher类对象tDiffPackChild而言，对其的变量访问和方法调用提示，default修饰的name和testModifierDefault()、private修饰的hobby和testModifierPrivate()不可见；
对于定义的Person类对象pDiffPackChild而言，对其的变量访问和方法调用提示，protected修饰的residence和testModifierProtected()、default修饰的name和testModifierDefault()、private修饰的hobby和testModifierPrivate()不可见。

(7).根据(6)提示的错误，注释掉相关的行，再次执行，详见代码：

package teacher; import human.Person; public class Teacher extends human.Person { String duty; public Teacher() { } public Teacher(String d) { super(); this.duty = d; } public Teacher(String n, int a, String g, String e, String h, String r) { super(n,a,g,e,h,r); } public static void main(String[] args) { Teacher tDiffPackChild = new Teacher("xi",20,"female","bachelor","recite","NJ"); Person pDiffPackChild = new Person("xi",20,"female","bachelor","recite","NJ"); System.out.println("public education: " + tDiffPackChild.education); System.out.println("protected residence: " + tDiffPackChild.residence); //System.out.println("default name: " + tDiffPackChild.name); //System.out.println("private hobby: "+ tDiffPackChild.hobby); tDiffPackChild.testModifierPublic(); tDiffPackChild.testModifierProtected(); //tDiffPackChild.testModifierDefault(); //tDiffPackChild.testModifierPrivate(); System.out.println("public education: " + pDiffPackChild.education); //System.out.println("protected residence: " + pDiffPackChild.residence); //System.out.println("default name: " + pDiffPackChild.name); //System.out.println("private hobby: "+ pDiffPackChild.hobby); pDiffPackChild.testModifierPublic(); //pDiffPackChild.testModifierProtected(); //pDiffPackChild.testModifierDefault(); //pDiffPackChild.testModifierPrivate(); } }

输出结果为：

public education: bachelor protected residence: NJ Public Protected public education: bachelor Public

结果分析：
注释掉相关的行后，成功执行；
如果在与Person不同包的子类Teacher中定义了Teacher类对象，则通过该对象可以访问和调用Person中public和protected修饰的成员变量和成员方法，default、private修饰的成员变量和成员方法都不可以访问或调用；
如果定义了Person类对象，则通过该对象只可以访问和调用Person中public修饰的成员变量和成员方法，protected、default、private修饰的成员变量和成员方法都不可以访问或调用。
疑问：不明白为什么会有这样的差异。

(8).在GateMan类中定义定义Person类对象pDiffPackNonChild，分别访问和调用这些成员变量和成员方法，详见下面的代码：

package teacher; import human.Person; public final class GateMan { int gateNumber; public GateMan() { } public GateMan(int g) { this.gateNumber = g; } public static void main(String[] args) { Person pDiffPackNonChild = new Person("xi",20,"female","bachelor","recite","NJ"); System.out.println("public education: " + pDiffPackNonChild.education); System.out.println("protected residence: " + pDiffPackNonChild.residence); System.out.println("default name: " + pDiffPackNonChild.name); System.out.println("private hobby: "+ pDiffPackNonChild.hobby); pDiffPackNonChild.testModifierPublic(); pDiffPackNonChild.testModifierProtected(); pDiffPackNonChild.testModifierDefault(); pDiffPackNonChild.testModifierPrivate(); } }

输出结果为：

Exception in thread "main" java.lang.Error: Unresolved compilation problems: The field Person.residence is not visible The field Person.name is not visible The field Person.hobby is not visible The method testModifierProtected() from the type Person is not visible The method testModifierDefault() from the type Person is not visible The method testModifierPrivate() from the type Person is not visible at teacher.GateMan.main(GateMan.java:29)

结果分析：出现编译错误，提示protected修饰的residence和testModifierProtected()、default修饰的name和testModifierDefault()、private修饰的hobby和testModifierPrivate()不可见。

(9).根据(8)提示的错误，注释掉相关的行，再次执行，详见代码：

package teacher; import human.Person; public final class GateMan { int gateNumber; public GateMan() { } public GateMan(int g) { this.gateNumber = g; } public static void main(String[] args) { Person pDiffPackNonChild = new Person("xi",20,"female","bachelor","recite","NJ"); System.out.println("public education: " + pDiffPackNonChild.education); //System.out.println("protected residence: " + pDiffPackNonChild.residence); //System.out.println("default name: " + pDiffPackNonChild.name); //System.out.println("private hobby: "+ pDiffPackNonChild.hobby); pDiffPackNonChild.testModifierPublic(); //pDiffPackNonChild.testModifierProtected(); //pDiffPackNonChild.testModifierDefault(); //pDiffPackNonChild.testModifierPrivate(); } }

输出结果为：

public education: bachelor Public

结果分析：
注释掉protected、default、private修饰的行后，成功执行；
Person类对象可以在与Person不同包的非子类中访问和调用由public修饰的成员变量和成员方法，不能访问由protected、default、private修饰的变量和方法。

(10).把Person类定义为缺省访问权限修饰的类，即class Person{,,,,}。在teacher包里的Teacher类和GateMan类都出现编译错误，如下所示：

Exception in thread "main" java.lang.IllegalAccessError: class teacher.Teacher cannot access its superclass human.Person at java.lang.ClassLoader.defineClass1(Native Method) at java.lang.ClassLoader.defineClass(ClassLoader.java:763) at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142) at java.net.URLClassLoader.defineClass(URLClassLoader.java:467) at java.net.URLClassLoader.access$100(URLClassLoader.java:73) at java.net.URLClassLoader$1.run(URLClassLoader.java:368) at java.net.URLClassLoader$1.run(URLClassLoader.java:362) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:361) at java.lang.ClassLoader.loadClass(ClassLoader.java:424) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:335) at java.lang.ClassLoader.loadClass(ClassLoader.java:357) at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:495)

结果分析：说明缺省访问权限修饰符的类只能由本包中的类访问，其他包中的类都不可以访问。
转载于:https://www.cnblogs.com/chanchan/p/7752373.html

推荐阅读

• 

  雷尼替丁|重磅大品种，暂停销售使用
• 

  mysql限制binlog mysql限制sql执行时间
• 

  请问怀孕6周了去拍婚纱照要注意什么？化一整天的新娘妆对宝宝有影响吗？
• 

  梦到三个小女孩的意思 梦到三个小女孩什么意思
• 

  手机号拉黑名单怎么拉回来
• 

  香橙app怎么发布话题 具体方法介绍
• 

  荣耀崛起破解版无限抽，开会员抢TX游戏激活几率大吗
• 

  疤痕美容一般多少钱 青岛整容疤痕多少钱
• 

  吃草莓的好处
• 

  空腹吃柚子好不好
• 

  今日合肥土鸡苗价格今日价
• 

  趣故事怎么填写邀请码？邀请码大全汇总
• 

  遇到松下空调不制冷自动关机怎么办？3个处理方法帮助你
• 

  大家看过的哪部战争类型纪录片印象最深刻？
• 

  反加偏旁组词 反加偏旁组词有哪些
• 

  户口冻结是什么意思
• 

  工程管理系统软件，工程项目管理软件选什么好
• 

  洗衣机不转是什么原因造成的,来涨点知识
• 

  梅州客家人来源以哪里
• 

  从哪些方面选择儿童玩具好 从哪些方面选择儿童玩具好呢

• JAVA（抽象类与接口的区别&重载与重写&内存泄漏）
• EffectiveObjective-C2.0|EffectiveObjective-C2.0 笔记 - 第二部分
• 由浅入深理解AOP
• 继续努力，自主学习家庭Day135(20181015）
• python学习之|python学习之 实现QQ自动发送消息
• Android中的AES加密-下
• 事件代理
• 一起来学习C语言的字符串转换函数
• Java|Java OpenCV图像处理之SIFT角点检测详解
• 定制一套英文学习方案