java访问权限整理
类实例化成对象之后，可以通过对象加上"."操作符访问和操纵该对象的域和方法，但是这种访问是有限制的，通过public、protected、default（啥都不写）、private来控制。
先看一个实验的例子：（不注释表示可以访问，注释掉表示无法访问）
package packageA;
import packageB.SubB;
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);
SubA subA=new SubA();
System.out.println(" subA." + subA.publicStr);
System.out.println(" subA." + subA.protectedStr);
System.out.println(" subA." + subA.defaultStr);
// System.out.println(" subA." + subA.privateStr);
SubB subB=new SubB();
System.out.println(" subB." + subB.publicStr);
System.out.println(" subB." + subB.protectedStr);
// System.out.println(" subB." + subB.defaultStr);
// System.out.println(" subB." + subB.privateStr);
}
}
package packageA;
import packageB.SubB;
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);
SubA subA=new SubA();
System.out.println(" subA." + subA.publicStr);
System.out.println(" subA." + subA.protectedStr);
System.out.println(" subA." + subA.defaultStr);
// System.out.println(" subA." + subA.privateStr);
SubB subB=new SubB();
System.out.println(" subB." + subB.publicStr);
System.out.println(" subB." + subB.protectedStr);
// System.out.println(" subB." + subB.defaultStr);
// System.out.println(" subB." + subB.privateStr);
}
}
package packageA;
import packageB.SubB;
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);
SubA subA=new SubA();
System.out.println(" subA." + subA.publicStr);
System.out.println(" subA." + subA.protectedStr);
System.out.println(" subA." + subA.defaultStr);
// System.out.println(" subA." + subA.privateStr);
SubB subB=new SubB();
System.out.println(" subB." + subB.publicStr);
System.out.println(" subB." + subB.protectedStr);
// System.out.println(" subB." + subB.defaultStr);
// System.out.println(" subB." + subB.privateStr);
}
}
package packageB;
import packageA.Base;
import packageA.SubA;
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);
SubA subA=new SubA();
System.out.println(" subA." + subA.publicStr);
// System.out.println(" subA." + subA.protectedStr);
// System.out.println(" subA." + subA.defaultStr);
// System.out.println(" subA." + subA.privateStr);
SubB subB=new SubB();
System.out.println(" subB." + subB.publicStr);
System.out.println(" subB." + subB.protectedStr);
// System.out.println(" subB." + subB.defaultStr);
// System.out.println(" subB." + subB.privateStr);
}
}
package packageB;
import packageA.Base;
import packageA.SubA;
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);
SubA subA=new SubA();
System.out.println(" subA." + subA.publicStr);
// System.out.println(" subA." + subA.protectedStr);
// System.out.println(" subA." + subA.defaultStr);
// System.out.println(" subA." + subA.privateStr);
SubB subB=new SubB();
System.out.println(" subB." + subB.publicStr);
// System.out.println(" subB." + subB.protectedStr);
// System.out.println(" subB." + subB.defaultStr);
// System.out.println(" subB." + subB.privateStr);
}
}
总结：
1).不考虑继承的情况下：
一个类在 ”某处“实例化并访问域和方法，或者直接访问类域和类方法：
a). 在类定义出：四种类型都可访问。（见Base中的b对象）
b). 与类定义在同一包中：default、protected、public可访问。（见AnotherA中b对象）
c). 与类定义在不同包中：仅可访问public类型。（见AnotherB中b对象）
2).有继承关系
一个子类在 ”某处“实例化并访问父类继承的域和方法（子类新定义的域和方法参加上面不考虑继承的情况），或者直接访问父类类域和类方法：
a).定义在包内的子类（如上例的SubA）
A).在子类定义处、同包内：default、protected、public可访问。
B).在包外：只可访问public属性
b).定义在包外的子类（如上例的SubB）
A).在子类定义处、父类同包内：protected及public可访问。
B).在子类包内（非子类定义处）：只可访问public属性
