計(jì)算機(jī)科學(xué)與技術(shù)外文資料翻譯-其他專業(yè)(編輯修改稿)

　

【文章內(nèi)容簡(jiǎn)介】 object. If you decided to send a message to s at this point, you?ll get an error (at run time) because s isn?t actually attached to anything (there?s 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. Normally, 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 example, 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 es with a plethora of readymade types. What?s more important is that you can create your own types. In fact, that?s the fundamental activity in Java programming, and it?s what you?ll be learning about in the rest of this book. It?s useful to visualize some aspects of how things are laid out while the program is running—in particular how memory is arranged. There are six different places to store data: 1. Registers. This is the fastest storage because it exists in a place different from tha t of other storage: inside the processor. However, the number of registers is severely limited, so registers are allocated by the piler according to its needs. You don?t have direct control, nor do you see any evidence in your programs that registers even exist. 2. The stack. This lives in the general randomaccess 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 moved up to release that memory. This is an extremely fast and efficient way to allocate storage, second only to registers. The Java piler 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 move the stack pointer up and down. This constraint places limits on the flexibility of your programs, so while some Java storage exists on the stack—in particular, object references—Java objects themselves are not placed on the stack. 3. The heap. This is a generalpurpose pool of memory (also in the RAM area) where all Java objects live. The nice thing about the heap is that, unlike the stack, the piler doesn?t need to know how much storage it needs to allocate from the heap or how long that storage must stay on the heap. Thus, there?s a great deal of flexibility in using storage on the heap. Whenever 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 there?s a price you pay for this flexibility. It takes more time to allocate heap storage than it does to allocate stack storage (if you even could create objects on the stack in Java, as you can in C++). 4. Static storage. “Static” is used here in the sense of “in a fixed location” (although it?s 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 specify that a particular element of an object is static, but Java objects themselves are never placed in static storage. 5. Constant storage. Constant values are often placed direc