1、 外文资料 Object landscapes and lifetimes Technically, OOP is just about abstract data typing, 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 destroy
2、ed. Where 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
3、 can be determined while 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 som
4、e situations. 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 restric
5、tive. The second approach is to create objects dynamically in a pool of memory called the heap. In this approach, you dont know until run-time how many objects you need, what their lifetime is, or what their exact type is. Those are determined at the spur of the moment while the program is running.
6、If you need 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
7、stack is often 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 importan
8、t impact on the creation of an object. In addition, the greater flexibility is essential to solve the general programming problem. Java 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 i
9、ssue, however, 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 lik
10、e C+, you must 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 destr
11、oys it. A garbage collector is much more convenient because it reduces the number of issues that you must track and the code you must write. More important, the garbage collector provides a much higher level of insurance against the insidious problem of memory leaks (which has brought many a C+ proj
12、ect to its knees). The rest of this section looks at additional factors concerning object lifetimes and landscapes. 1 Collections and iterators If you dont know how many objects youre going to need to solve a particular problem, or how long they will last, you also dont know how to store those objec
13、ts. 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 flippant: you create another type of object. The new type of object that solves this particular problem holds ref
14、erences 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 a collection, but the Java library uses that term in a different sense so this book will use “container”), w
15、ill expand itself whenever necessary to accommodate everything you place inside it. So you dont need to know how manyobjects youre going to hold in a container. Just create a container object and let it take care of the details. Fortunately, a good OOP language comes with a set of containers as part
16、 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). Smalltalk has a very complete set of containers. Java also has containers in its standard library. In some libr
17、aries, 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 ArrayList in Java) for consistent access to all elements, and a linked list for consistent insertion at all ele
18、ments, 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 way to put things in and get things out； there are usually functions to add elements to a container, and others
19、 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 elements in the container instead of just one? The solution is an iterator, which is an object whose job is to sele
20、ct 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 the container from the code thats accessing that container. The container, via the iterator, is abstracted to
21、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 else. This gives you the flexibility to easily change the underlying data structure without disturbing the cod
22、e 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 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
