1、英文原文 Everything is an Object Although it is based on C+, Java is more of a “pure” object-oriented language. Both C+ and Java are hybrid languages, but in Java the designers felt that the hybridization was not as important as it was in C+. A hybrid language allows multiple programming styles； the rea
2、son C+ is hybrid is to support backward compatibility with the C language. Because C+ is a superset of the C language, it includes many of that languages undesirable features, which can make some aspects of C+ overly complicated. The Java language assumes that you want to do only object-oriented pro
3、gramming. This means that before you can begin you must shift your mindset into an object-oriented world (unless its already there). The benefit of this initial effort is the ability to program in a language that is simpler to learn and to use than many other OOP languages. Alown well see the basic
4、components of a Java program and well learn that everything in Java is an object, even a Java program. You manipulate objects with references Each programming language has its own means of manipulating data. Sometimes the programmer must be constantly aware of what type of manipulation is going on.
5、Are you manipulating the object directly, or are you dealing with some kind of indirect representation (a pointer in C or C+) that must be treated with a special syntax? All this is simplified in Java. You treat everything as an object, using a single consistent syntax. Although you treat everything
6、 as an object, the identifier you manipulate is actually a “reference” to an object.10 You might imagine this scene as a television (the object) with your remote control (the reference). As long as youre holding this reference, you have a connection to the television, but when someone says “change t
7、he channel” or “lower the volume,” what youre manipulating is the reference, which in turn modifies the object. If you want to move around the room 1 and still control the television, you take the remote/reference with you, not the television. Also, the remote control can stand on its own, with no t
8、elevision. That is, just because you have a reference doesnt mean theres necessarily an object connected to it. So if you want to hold a word or sentence, you create a String reference: String s； But here youve created only the reference, not an object. If you decided to send a message to s at this
9、point, youll get an error (at run time) because s isnt actually attached to anything (theres no television). A safer practice, then, is always to initialize a reference when you create it: String s = asdf； However, this uses a special Java feature: strings can be initialized with quoted text. Normal
10、ly, you must use a more general type of initialization for objects. You must create all the objects When you create a reference, you want to connect it with a new object. You do so, in general, with the new keyword. The keyword new says, “Make me a new one of these objects.” So in the preceding exam
11、ple, you can say: String s = new String(asdf)； Not only does this mean “Make me a new String,” but it also gives information about how to make the String by supplying an initial character string. Of course, String is not the only type that exists. Java comes with a plethora of ready-made types. What
12、s more important is that you can create your own types. In fact, thats the fundamental activity in Java programming, and its what youll be learning about in the rest of this book. Its useful to visualize some aspects of how things are laid out while the program is runningin particular how memory is
13、arranged. There are six different places to store data: Registers. This is the fastest storage because it exists in a place different from that of other storage: inside the processor. However, the number of registers is severely limited, so registers are allocated by the compiler according to its ne
14、eds. You 2 dont have direct control, nor do you see any evidence in your programs that registers even exist. The stack. This lives in the general random-access memory (RAM) area, but has direct support from the processor via its stack pointer. The stack pointer is moved down to create new memory and
15、 moved up to release that memory. This is an extremely fast and efficient way to allocate storage, second only to registers. The Java compiler must know, while it is creating the program, the exact size and lifetime of all the data that is stored on the stack, because it must generate the code to mo
16、ve the stack pointer up and down. This constraint places limits on the flexibility of your programs, so while some Java storage exists on the stackin particular, object referencesJava objects themselves are not placed on the stack. The heap. This is a general-purpose pool of memory (also in the RAM
17、area) where all Java objects live. The nice thing about the heap is that, unlike the stack, the compiler doesnt need to know how much storage it needs to allocate from the heap or how long that storage must stay on the heap. Thus, theres a great deal of flexibility in using storage on the heap. When
18、ever you need to create an object, you simply write the code to create it by using new, and the storage is allocated on the heap when that code is executed. Of course theres a price you pay for this flexibility. It takes more time to allocate heap storage than it does to allocate stack storage (if y
19、ou even could create objects on the stack in Java, as you can in C+). Static storage. “Static” is used here in the sense of “in a fixed location” (although its also in RAM). Static storage contains data that is available for the entire time a program is running. You can use the static keyword to spe
20、cify that a particular element of an object is static, but Java objects themselves are never placed in static storage. Constant storage. Constant values are often placed directly in the program code, which is safe since they can never change. Sometimes constants are cordoned off by themselves so that they can be optionally placed in read-only memory (ROM), in embedded systems. Non-RAM storage. If data lives completely outside a program, it can exist while
