1、Object landscapes and lifetimes Technically, OOP is just about abstract data type, inheritance and polymorphism, but other issues can be at least as important. The remainder of this section will cover these issues. One of the most important factors is the way objects are created and destroyed. Where
2、 is the data for an object and how is the lifetime of the object controlled? There are different philosophies at work here. C+ takes the approach that control of efficiency is the most important issue, so it gives the programmer a choice. For maximum run-time speed, the storage and lifetime can be d
3、etermined when the program is being written, by placing the objects on the stack (these are sometimes called automatic or scoped variables) or in the static storage area. This places a priority on the speed of storage allocation and release, and control of these can be very valuable in some situatio
4、ns. However, you sacrifice flexibility because you must know the exact quantity, lifetime, and type of objects while youre writing the program. If you are trying to solve a more general problem such as computer-aided design, warehouse management, or air-traffic control, this is too restrictive. The
5、second approach is to create objects dynamically in a pool of memory called the heap. In this approach, you dont know how many objects you need, what their lifetime is, or what their exact type is until run-time. Those are determined at the spur of the moment while the program is running. If you nee
6、d a new object, you simply make it on the heap at the point that you need it. Because the storage is managed dynamically, at run-time, the amount of time required to allocate storage on the heap is significantly longer than the time to create storage on the stack. (Creating storage on the stack is o
7、ften a single assembly instruction to move the stack pointer down, and another to move it back up.) The dynamic approach makes the generally logical assumption that objects tend to be complicated, so the extra overhead of finding storage and releasing that storage will not have an important impact o
8、n the creation of an object. In addition, the greater flexibility is essential to solve the general programming problem. C+ uses the second approach, exclusively. Every time you want to create an object, you use the new keyword to build a dynamic instance of that object. Theres another issue, howeve
9、r, and thats the lifetime of an object. With languages that allow objects to be created on the stack, the compiler determines how long the object lasts and can automatically destroy it. However, if you create it on the heap the compiler has no knowledge of its lifetime. In a language like C+, you mu
10、st determine programmatically when to destroy the object, which can lead to memory leaks if you dont do it correctly (and this is a common problem in C+ programs). Java provides a feature called a garbage collector that automatically discovers when an object is no longer in use and destroys it. A ga
11、rbage collector is much more convenient because it reduces the number of issues that you must track and the code you must write. The rest of this section looks at additional factors concerning object landscapes and lifetimes.1 Collections and iteratorsIf you dont know how many objects youre going to
12、 need to solve a particular problem, or how long they will last, you also dont know how to store those objects. How can you know how much space to create for those objects? You cant, since that information isnt known until run-time. The solution to most problems in object-oriented design seems flipp
13、ant: you create another type of object. The new type of object that solves this particular problem holds references to other objects. Of course, you can do the same thing with an array, which is available in most languages. But theres more. This new object, generally called a container (also called
14、a collection, but the Java library uses that term in a different sense so this book will use “container”), will expand itself whenever necessary to accommodate everything you place inside it. So you dont need to know how many objects youre going to hold in a container. Just create a container object
15、 and let it take care of the details. Fortunately, a good OOP language comes with a set of containers as part of the package. In C+, its part of the Standard C+ Library and is sometimes called the Standard Template Library (STL). Object Pascal has containers in its Visual Component Library (VCL). Sm
16、alltalk has a very complete set of containers. Java also has containers in its standard library. In some libraries, a generic container is considered good enough for all needs, and in others (Java, for example) the library has different types of containers for different needs: a vector (called an Ar
17、rayList in Java) for consistent access to all elements, and a linked list for consistent insertion at all elements, for example, so you can choose the particular type that fits your needs. Container libraries may also include sets, queues, hash tables, trees, stacks, etc. All containers have some wa
18、y to put things in and get things out; there are usually functions to add elements to a container, and others to fetch those elements back out. But fetching elements can be more problematic, because a single-selection function is restrictive. What if you want to manipulate or compare a set of elemen
19、ts in the container instead of just one? The solution is an iterator, which is an object whose job is to select the elements within a container and present them to the user of the iterator. As a class, it also provides a level of abstraction. This abstraction can be used to separate the details of t
20、he container from the code thats accessing that container. The container, via the iterator, is abstracted to be simply a sequence. The iterator allows you to traverse that sequence without worrying about the underlying structurethat is, whether its an ArrayList, a LinkedList, a Stack, or something e
21、lse. This gives you the flexibility to easily change the underlying data structure without disturbing the code in your program. Java began (in version 1.0 and 1.1) with a standard iterator, called Enumeration, for all of its container classes. Java 2 has added a much more complete container library
22、that contains an iterator called Iterator that does more than the older Enumeration. From a design standpoint, all you really want is a sequence that can be manipulated to solve your problem. If a single type of sequence satisfied all of your needs, thered be no reason to have different kinds. There
23、 are two reasons that you need a choice of containers. First, containers provide different types of interfaces and external behavior. A stack has a different interface and behavior than that of a queue, which is different from that of a set or a list. One of these might provide a more flexible solut
24、ion to your problem than the other. Second, different containers have different efficiencies for certain operations. The best example is an ArrayList and a LinkedList. Both are simple sequences that can have identical interfaces and external behaviors. But certain operations can have radically diffe
25、rent costs. Randomly accessing elements in an ArrayList is a constant-time operation; it takes the same amount of time regardless of the element you select. However, in a LinkedList it is expensive to move through the list to randomly select an element, and it takes longer to find an element that is
26、 further down the list. On the other hand, if you want to insert an element in the middle of a sequence, its much cheaper in a LinkedList than in an ArrayList. These and other operations have different efficiencies depending on the underlying structure of the sequence. In the design phase, you might
27、 start with a LinkedList and, when tuning for performance, change to an ArrayList. Because of the abstraction via iterators, you can change from one to the other with minimal impact on your code. In the end, remember that a container is only a storage cabinet to put objects in. If that cabinet solve
28、s all of your needs, it doesnt really matter how it is implemented (a basic concept with most types of objects). If youre working in a programming environment that has built-in overhead due to other factors, then the cost difference between an ArrayList and a LinkedList might not matter. You might n
29、eed only one type of sequence. You can even imagine the “perfect” container abstraction, which can automatically change its underlying implementation according to the way it is used. 2 The singly rooted hierarchyOne of the issues in OOP that has become especially prominent since the introduction of
30、C+ is whether all classes should ultimately be inherited from a single base class. In Java (as with virtually all other OOP languages) the answer is “yes” and the name of this ultimate base class is simply Object. It turns out that the benefits of the singly rooted hierarchy are many. All objects in
31、 a singly rooted hierarchy have an interface in common, so they are all ultimately the same type. The alternative (provided by C+) is that you dont know that everything is the same fundamental type. From a backward-compatibility standpoint this fits the model of C better and can be thought of as les
32、s restrictive, but when you want to do full-on object-oriented programming you must then build your own hierarchy to provide the same convenience thats built into other OOP languages. And in any new class library you acquire, some other incompatible interface will be used. It requires effort (and po
33、ssibly multiple inheritance) to work the new interface into your design. Is the extra “flexibility” of C+ worth it? If you need itif you have a large investment in Cits quite valuable. If youre starting from scratch, other alternatives such as Java can often be more productive. All objects in a sing
34、ly rooted hierarchy (such as Java provides) can be guaranteed to have certain functionality. You know you can perform certain basic operations on every object in your system. A singly rooted hierarchy, along with creating all objects on the heap, greatly simplifies argument passing (one of the more
35、complex topics in C+). A singly rooted hierarchy makes it much easier to implement a garbage collector (which is conveniently built into Java). The necessary support can be installed in the base class, and the garbage collector can thus send the appropriate messages to every object in the system. Wi
36、thout a singly rooted hierarchy and a system to manipulate an object via a reference, it is difficult to implement a garbage collector. Since run-time type information is guaranteed to be in all objects, youll never end up with an object whose type you cannot determine. This is especially important
37、with system level operations, such as exception handling, and to allow greater flexibility in programming. 3 Collection libraries and support for easy collection useBecause a container is a tool that youll use frequently, it makes sense to have a library of containers that are built in a reusable fa
38、shion, so you can take one off the shelf Because a container is a tool that youll use frequently, it makes sense to have a library of containers that are built in a reusable fashion, so you can take one off the shelf and plug it into your program. Java provides such a library, which should satisfy m
39、ost needs. Downcasting vs. templates/genericsTo make these containers reusable, they hold the one universal type in Java that was previously mentioned: Object. The singly rooted hierarchy means that everything is an Object, so a container that holds Objects can hold anything. This makes containers e
40、asy to reuse. To use such a container, you simply add object references to it, and later ask for them back. But, since the container holds only Objects, when you add your object reference into the container it is upcast to Object, thus losing its identity. When you fetch it back, you get an Object r
41、eference, and not a reference to the type that you put in. So how do you turn it back into something that has the useful interface of the object that you put into the container? Here, the cast is used again, but this time youre not casting up the inheritance hierarchy to a more general type, you cas
42、t down the hierarchy to a more specific type. This manner of casting is called downcasting. With upcasting, you know, for example, that a Circle is a type of Shape so its safe to upcast, but you dont know that an Object is necessarily a Circle or a Shape so its hardly safe to downcast unless you kno
43、w thats what youre dealing with. Its not completely dangerous, however, because if you downcast to the wrong thing youll get a run-time error called an exception, which will be described shortly. When you fetch object references from a container, though, you must have some way to remember exactly wh
44、at they are so you can perform a proper downcast. Downcasting and the run-time checks require extra time for the running program, and extra effort from the programmer. Wouldnt it make sense to somehow create the container so that it knows the types that it holds, eliminating the need for the downcas
45、t and a possible mistake? The solution is parameterized types, which are classes that the compiler can automatically customize to work with particular types. For example, with a parameterized container, the compiler could customize that container so that it would accept only Shapes and fetch only Sh
46、apes. Parameterized types are an important part of C+, partly because C+ has no singly rooted hierarchy. In C+, the keyword that implements parameterized types is “template.” Java currently has no parameterized types since it is possible for it to get byhowever awkwardlyusing the singly rooted hiera
47、rchy. However, a current proposal for parameterized types using a syntax that is strikingly similar to C+ templates. 对象的创建和生命周期从技术角度说，OOP（面向对象程序设计）只是涉及抽象的数据类型、继承以及多形性，但另一些问题也可能显得非常重要。本节将就这些问题进行探讨。最重要的问题之一是对象的创建及破坏方式。对象需要的数据位于哪儿，如何控制对象的“存在时间”呢？针对这个问题，解决的方案是各异其趣的。C+认为程序的执行效率是最重要的一个问题，所以它允许程序员作出选择。为获得最
48、快的运行速度，存储以及存在时间可在编写程序时决定，只需将对象放置在堆栈（有时也叫作自动或定域变量）或者静态存储区域即可。这样便为存储空间的分配和释放提供了一个优先级。某些情况下，这种优先级的控制是非常有价值的。然而，我们同时也牺牲了灵活性，因为在编写程序时，必须知道对象的准确的数量、存在时间、以及类型。如果要解决的是一个较常规的问题，如计算机辅助设计、仓储管理或者空中交通控制，这一方法就显得太局限了。第二个方法是在一个内存池中动态创建对象，该内存池亦叫“堆”或者“内存堆”。若采用这种方式，除非进入运行期，否则根本不知道到底需要多少个对象，也不知道它们的存在时间有多长，以及准确的类型是什么。这些
49、参数都在程序正式运行时才决定的。若需一个新对象，只需在需要它的时候在内存堆里简单地创建它即可。由于存储空间的管理是运行期间动态进行的，所以在内存堆里分配存储空间的时间比在堆栈里创建的时间长得多（在堆栈里创建存储空间一般只需要一个简单的指令，将堆栈指针向下或向下移动即可）。由于动态创建方法使对象本来就倾向于复杂，所以查找存储空间以及释放它所需的额外开销不会为对象的创建造成明显的影响。除此以外，更大的灵活性对于常规编程问题的解决是至关重要的。C+运用第二种方法。每次当你要创建一个对象时，你就用new关键字来为对象创建一个动态实体。但还要考虑另外一个问题，亦即对象的“存在时间”或者“生存时间”（Lifetime）。若在堆栈或者静态存储空间里创建一个对象，编译器会判断对象的持续时间有多长，到时会自动“破坏”或者“清除”它。然而，如果你把它创建在堆里，编译器将不会知道它的存在时间。在C+语言中，你必须用程序化的方式决定何时破坏对象，如果不正确处理这也可能会导致出现存碎片（这是C+程序中常出现的问题）。 Java确实提供
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