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
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。
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();
}
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。
