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[英]Return a class, not an instance, of a concrete class that extends an abstract base class?
[英]Accessing Protected Variable Of An Instance Of A Class That Extends An Abstract Base Class?
我有一个名为MyAction的抽象类,其中包含一个受保护的枚举变量。 该类的定义如下:
package mypackage;
public abstract class MyAction {
public enum ActionId {
ACTION1, ACTION2;
}
protected ActionId actionId;
// constructor
public MyAction(ActionId actionId) {
this.actionId = actionId;
}
public ActionId getActionId() {
return actionId;
}
...
...
}
我创建了一个特定的动作MyAction1,该动作扩展了MyAction:
package mypackage;
public class MyAction1 extends MyAction {
public MyAction1() {
super(ActionId.ACTION1);
}
...
...
}
我有一个单例实用程序类(在同一包中),该类创建MyAction1的实例并将其存储在HashMap中:
package mypackage;
public class MyActionFactory {
private static MyActionFactory theInstance;
private HashMap<ActionId, MyAction> actions;
private MyActionFactory() {
actions = new HashMap<ActionId, MyAction>();
MyAction1 myAction1 = new MyAction1();
actions.put(myAction1.actionId, myAction1); // able to access protected variable actionId
}
public static VsActionFactory getInstance() {
if (theInstance == null)
theInstance = new VsActionFactory();
return theInstance;
}
...
...
}
请注意,在action.put( myAction1.actionId ,myAction1)方法中,我可以访问受保护的成员actionId 。
为什么我能MyAction1实例的访问受保护的成员actionId(包含在基类MyAction)? 我认为受保护的成员只能访问子类。
与MyActionFactory与其他软件包位于同一软件包中有什么关系吗?
受保护的访问修饰符允许在同一个包中访问+在其他包中的子类。您可以将其记住为默认访问加继承。
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