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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
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