【正文】 } void nothing() { return。 The type of the return value must be patible with the type of x. This act of calling a method is monly referred to as sending a message to an object. In the preceding example, the message is f( ) and the object is a. Objectoriented programming is often summarized as simply “sending messages to objects.” The argument list The method argument list specifies what information you pass into the method. As you might guess, this information—like everything else in Java—takes the form of objects. So, what you must specify in the argument list are the types of the objects to pass in and the name to use for each one. As in any situation in Java where you seem to be handing objects around, you are actually passing The type of the reference must be correct, however. If the argument is supposed to be a String, you must pass in a String or the piler will give an error. Consider a method that takes a String as its argument. Here is the definition, which must be placed within a class definition for it to be piled: int storage(String s) { return () * 2。 The DataOnly class cannot do much of anything except hold data, because it has no methods. To understand how those work, you must first understand arguments and return values, which will be described shortly. Default values for primitive members When a primitive data type is a member of a class, it is guaranteed to get a default value if you do not initialize it: The default values are only what Java guarantees when the variable is used as a member of a class. This ensures that member variables of primitive types will always be initialized (something C++ doesn?t do), reducing a source of bugs. However, this initial value may not be correct or even legal for the program you are writing. It?s best to always explicitly initialize your variables. This guarantee doesn?t apply to local variables—those that are not fields of a class. Thus, if within a method definition you have: int x。 You can assign values to the fields, but you must first know how to refer to a member of an object. This is acplished by stating the name of the object reference, followed by a period (dot), followed by the name of the member inside the object: object Reference. Member or example: data. i = 47。 ordinary fields are not shared among objects. Here is an example of a class with some fields: class DataOnly {int i。 {int x = 96。 you can use these for accurate moary calculations, for example. Consult the JDK documentation for details about the constructors and methods you can call for these two classes. Arrays in Java Virtually all programming languages support some kind of arrays. Using arrays in C and C++ is perilous because those arrays are only blocks of memory. If a program accesses the array outside of its memory block or uses the memory before initialization (mon programming errors), there will be unpredictable results. One of the primary goals of Java is safety, so many of the problems that plague programmers in C and C++ are not repeated in Java. A Java array is guaranteed to be initialized and cannot Thinking in Java be accessed outside of its range. The range checking es at the price of having a small amount of memory overhead on each array as well as verifying the index at run time, but the assumption is that the safety and increased productivity are worth the expense (and Java can sometimes optimize these operations). When you create an array of objects, you are really creating an array of references, and each of those references is automatically initialized to a special value with its own keyword: null. When Java sees null, it recognizes that the reference in question isn?t pointing to an object. You must assign an object to each reference before you use it, and if you try to use a reference that?s still null, the problem will be reported at run time. Thus, typical array errors are prevented in Java. You can also create an array of primitives. Again, the piler guarantees initialization because it zeroes the memory for that will be covered in detail in later chapters. You never need todestroy an object In most programming languages, the concept of the lifetime of a variable occupies a significant portion of the programming effort. How long does the variable last? If you are supposed to destroy it, when should you? Confusion over variable lifetimes can lead to a lot of bugs, and this section shows how Java greatly simplifies the issue by doing all the cleanup work for you. Scoping Most procedural languages have the concept of scope. This determines both the visibility and lifetime of the names defined within that scope. In C, C++, and Java, scope is determined by the placement of curly braces {}. So for example: { int x = 12。 Or you could also use: Character ch = new Character(?x?)。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 es with a plethora of readymade 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 amp。至于本章，讀者只需要理解消息發(fā)送就足夠了。因此，沒有必要到方法結(jié)束時才離開，可在任何地