Thinking in Java - 4th Edition
Disciplina:Programação Orientada A Objetos1.901 materiais • 39.309 seguidores
“object references.” And (he goes on) everything is actually pass by value. So you’re not passing by reference, you’re “passing an object reference by value.” One could argue for the precision of such convoluted explanations, but I think my approach simplifies the understanding of the concept without hurting anything (well, the language lawyers may claim that I’m lying to you, but I’ll say that I’m providing an appropriate abstraction). the room 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 television. That is, just because you have a reference doesn’t mean there’s 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 you’ve created only the reference, not an object. If you decided to send a message to s at this point, you’ll get an error 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 operator. 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, Java comes with a plethora of ready-made types in addition to String. What’s more important is that you can create your own types. In fact, creating new types is the fundamental activity in Java programming, and it’s what you’ll be learning about in the rest of this book. Where storage lives 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 five different places to store data: 1. 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 as they are needed. You don’t have direct control, nor do you see any evidence in your programs that registers even exist (C & C++, on the other hand, allow you to suggest register allocation to the compiler). 2. 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 moved up to release that memory. This is an extremely fast and efficient way to allocate storage, second only to registers. The Java system must know, while it is creating the program, the exact lifetime of all the items that are stored on the stack. 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. 42 Thinking in Java Bruce Eckel Everything Is an Object 43 3. The heap. This is a general-purpose 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 compiler doesn’t need to know 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 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 may take more time to allocate and clean up heap storage than stack storage (if you even could create objects on the stack in Java, as you can in C++). 4. 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.2 5. Non-RAM storage. If data lives completely outside a program, it can exist while the program is not running, outside the control of the program. The two primary examples of this are streamed objects, in which objects are turned into streams of bytes, generally to be sent to another machine, and persistent objects, in which the objects are placed on disk so they will hold their state even when the program is terminated. The trick with these types of storage is turning the objects into something that can exist on the other medium, and yet can be resurrected into a regular RAM- based object when necessary. Java provides support for lightweight persistence, and mechanisms such as JDBC and Hibernate provide more sophisticated support for storing and retrieving object information in databases. Special case: primitive types One group of types, which you’ll use quite often in your programming, gets special treatment. You can think of these as “primitive” types. The reason for the special treatment is that to create an object with new—especially a small, simple variable—isn’t very efficient, because new places objects on the heap. For these types Java falls back on the approach taken by C and C++. That is, instead of creating the variable by using new, an “automatic” variable is created that is not a reference. The variable holds the value directly, and it’s placed on the stack, so it’s much more efficient. Java determines the size of each primitive type. These sizes don’t change from one machine architecture to another as they do in most languages. This size invariance is one reason Java programs are more portable than programs in most other languages. Primitive type Size Minimum Maximum Wrapper type boolean — — — Boolean char 16 bits Unicode 0 Unicode 216- 1 Character byte 8 bits -128 +127 Byte short 16 bits -215 +215-1 Short int 32 bits -231 +231-1 Integer long 64 bits -263 +263-1 Long float 32 bits IEEE754 IEEE754 Float double 64 bits IEEE754 IEEE754 Double void — — — Void 2 An example of this is the string pool. All literal strings and string-valued constant expressions are interned automatically and put into special static storage. All numeric types are signed, so don’t look for unsigned types. The size of the boolean type is not explicitly specified; it is only defined to be able to take the literal values true or false. The “wrapper” classes for the primitive data types allow you to make a non-primitive object on the heap to represent that primitive type. For example: char c = ‘x’; Character ch = new Character(c); Or you could also use: Character ch = new Character(‘x’); Java SE5 autoboxing will automatically convert from a primitive to a wrapper type: Character ch = ‘x’; and back: char c = ch; The reasons for wrapping primitives will be shown in a later chapter. High-precision numbers Java includes two classes for performing high-precision arithmetic: BigInteger and BigDecimal. Although these approximately fit into the same category as the “wrapper” classes, neither one has a primitive analogue. Both classes have methods that provide analogues for the operations that you perform on primitive types. That is, you can do anything with a BigInteger or BigDecimal that you can with an int or float, it’s just that you must use method calls instead of operators. Also, since there’s more involved, the operations will be slower. You’re exchanging speed for accuracy. BigInteger supports arbitrary-precision integers.