JAVA基础教程4.ppt

1、Reusing classes,华南理工大学 计算机科学与工程学院 韦佳 ,Composition syntax,Simply place object references inside new classes. /SprinklerSystem.java Reference initialization. /Bath.java At the point the objects are defined; In the constructor for that class; Right before you actually need to use the object. (lazy init

2、ialization) Using instance initialization.,Inheritance syntax,extends keyword. /Detergent.java “this new class is like that old class.” You automatically get all the fields and methods in the base class. super keyword. Refers to the “superclass” that the current class inherits.,Initializing the base

3、 class,When you create an object of the derived class, it contains within it a subobject of the base class. Java automatically inserts calls to the base-class constructor in the derived-class constructor. /Cartoon.java,Constructors with arguments,If there is no default base-class constructor, or if

4、you want to call a base-class constructor that has arguments, you must explicitly write a call to the base-class constructor using the super keyword and the appropriate argument list. /Chess.java,Delegation,Delegation is midway between inheritance and composition. You can place a member object in th

5、e class youre building. You can also expose all the methods from the member object in your new class. /SpaceShipControls.java; SpaceShipDelegation.java,Combining composition and inheritance,It is very common to use composition and inheritance together. /PlaceSetting.java Although the compiler forces

6、 you to initialize the base classes, it doesnt watch over you to make sure that you initialize the member objects, so you must remember to pay attention to that.,Guaranteeing proper cleanup,If you want something cleaned up for a class, you must explicitly write a special method to do it. /CADSystem.

7、java First perform all the cleanup work specific to your class, in the reverse order of creation. Then call the base-class cleanup method.,Name hiding,If a Java base class has a method name thats overloaded several times, redefining that method name in the derived class will not hide any of the base

8、-class version. /Hide.java Override,Choosing composition vs. inheritance,Composition is generally used when you want the functionality of an existing class inside your new class, but not its interface. /Car.java When you inherit, you take an existing class and make a special version of it. The is-a

9、relationship is expressed with inheritance, and the has-a relationship is expressed with composition.,protected,protected: available to anyone who inherits from this class or anyone else in the same package. /Orc.java Although its possible to create protected fields, the best approach is to leave th

10、e fields private; you should always preserve your right to change the underlying implementation.,Upcasting,/Wind.java Casting from a derived type to a base type moves up on the inheritance diagram, so its commonly referred to as upcasting. Upcasting is always safe.,The final keyword,final: “This can

11、not be changed” There are three places where final can be used: data, methods, and classes.,final data,A constant is useful for two reasons: It can be a compile-time constant that wont ever change. It can be a value initialized at run time that you dont want changed. In java, these sorts of constant

12、s must be primitives and are expressed with the final keyword. A value must be given at the time of definition of such a constant.,A field that is both static and final has only one piece of storage that cannot be changed. With an object reference, final makes the reference a constant. Once the refe

13、rence is initialized to an object, it can never be changed to point to another object. /FinalData.java,Blank finals,Java allows the creation of blank finals, which are fields that are declared as final but are not given an initialization value. The blank final must be initialized before it is used.

14、A blank final field inside a class can be different for each object. /BlankFinal.java,final arguments,Java allows you to make arguments final by declaring them as such in the argument list. This means that inside the method you cannot change what the argument reference points to. /FinalArguments.java,final methods,There are two reasons for final methods: Prevent any inheriting class from changing its meaning. Efficiency. Any private methods in a class are implicitly final. Confusion. /FinalOverridingIllusion.java,final classes,When you say that an entire