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Java All-in-One Desk Reference For Dummies phần 5

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Tại bề mặt, một lớp bên trong chỉ đơn giản là một lớp chứa bên trong một lớp khác. Tuy nhiên, có nhiều đến nó hơn. Là một số điểm chính về các lớp bên trong: ✦ Một lớp bên trong sẽ tự động truy cập vào tất cả các lĩnh vực và phương pháp của lớp bên ngoài, thậm chí cả các lĩnh vực tư nhân và phương pháp. | 330 Declaring Inner Classes Understanding inner classes At the surface an inner class is simply a class that s contained inside another class. However there s more to it than that. Here are some key points about inner classes An inner class automatically has access to all the fields and methods of the outer class even private fields and methods. Thus an inner class has more access to its outer class than a subclass has to its superclass. A subclass can access public and protected members of its superclass but not private members. An inner class carries with it a reference to the current instance of the outer class that enables it to access instance data for the outer class. Because of the outer class instance reference you can t create or refer to an inner class from a static method of the outer class. You can however create a static inner class as I describe in the section Using Static Inner Classes later in this chapter. One of the main reasons for creating an inner class is to create a class that s only of interest to the outer class. As a result you usually declare inner classes to be private so other classes can t access them. Occasionally code in an inner class needs to refer to the instance of its outer class. To do that you list the name of the outer class followed by the dot operator and this. For example if the outer class is named MyOuterClass you would use MyOuterClass.this to refer to the instance of the outer class. An example Book III Chapter 5 introduces an application that uses the Timer class in the Swing package javax.swing.Timer that displays the lines Tick. and Tock. on the console at a one second interval. It uses a class named Ticker that implements the ActionListener interface to handle the Timer object s clock events. In this chapter you see a total of three different versions of this application. You may want to quickly review Book III Chapter 5 if you re unclear on how this application uses the Timer class to display the Tick. and Tock. .