Variables变量

As you learned in the previous lesson, an object stores its state in fields.如前一课所述，对象将其状态存储在字段中。

int cadence = 0;
int speed = 0;
int gear = 1;

The What Is an Object? discussion introduced you to fields, but you probably have still a few questions, such as: What are the rules and conventions for naming a field?什么是对象？的讨论向您介绍了字段，但您可能还有一些问题，例如：命名字段的规则和约定是什么？Besides int, what other data types are there? Do fields have to be initialized when they are declared?除了int，还有哪些其他数据类型？声明字段时是否必须初始化字段？Are fields assigned a default value if they are not explicitly initialized?如果字段未显式初始化，是否为其指定了默认值？We'll explore the answers to such questions in this lesson, but before we do, there are a few technical distinctions you must first become aware of.在本课中，我们将探讨这些问题的答案，但在此之前，您必须首先了解一些技术区别。In the Java programming language, the terms "field" and "variable" are both used; this is a common source of confusion among new developers, since both often seem to refer to the same thing.在Java编程语言中，术语“字段”和“变量”都被使用；这在新开发人员中是一个常见的混淆源，因为两者似乎经常提到同一件事。

The Java programming language defines the following kinds of variables:Java编程语言定义了以下类型的变量：

• Instance Variables (Non-Static Fields)实例变量（非静态字段） Technically speaking, objects store their individual states in "non-static fields", that is, fields declared without the static keyword.从技术上讲，对象将其各自的状态存储在“非静态字段”中，即在声明字段时不使用static关键字。Non-static fields are also known as instance variables because their values are unique to each instance of a class (to each object, in other words); the currentSpeed of one bicycle is independent from the currentSpeed of another.非静态字段也称为实例变量，因为它们的值对于类的每个实例（换句话说，对于每个对象）都是唯一的；一辆自行车的当前速度与另一辆自行车的当前速度无关。
• Class Variables (Static Fields)类变量（静态字段） A class variable is any field declared with the static modifier; this tells the compiler that there is exactly one copy of this variable in existence, regardless of how many times the class has been instantiated.类变量是使用static修饰符声明的任何字段；这会告诉编译器，不管类被实例化了多少次，该变量只有一个副本存在。A field defining the number of gears for a particular kind of bicycle could be marked as static since conceptually the same number of gears will apply to all instances.定义特定类型自行车档位数量的字段可以标记为static，因为概念上相同的档位数量将应用于所有实例。The code static int numGears = 6; would create such a static field.代码static int numGears=6;将创建这样一个静态字段。Additionally, the keyword final could be added to indicate that the number of gears will never change.此外，还可以添加关键字final，表示档位的数量永远不会改变。
• Local Variables局部变量 Similar to how an object stores its state in fields, a method will often store its temporary state in local variables.与对象在字段中存储其状态的方式类似，方法通常将其临时状态存储在局部变量中。The syntax for declaring a local variable is similar to declaring a field (for example, int count = 0;).声明局部变量的语法类似于声明字段（例如，int count = 0;）。There is no special keyword designating a variable as local; that determination comes entirely from the location in which the variable is declared — which is between the opening and closing braces of a method.没有将变量指定为局部变量的特殊关键字；该确定完全来自变量声明的位置—它位于方法的左大括号和右大括号之间。As such, local variables are only visible to the methods in which they are declared; they are not accessible from the rest of the class.因此，局部变量只对声明它们的方法可见；他们不能从班上的其他人那里得到。
• Parameters参数 You've already seen examples of parameters, both in the Bicycle class and in the main method of the "Hello World!" application.您已经在Bicycle类和“Hello World！”的main方法中看到了参数示例应用。Recall that the signature for the main method is public static void main(String[] args).回想一下，main方法的签名是public static void main(String[] args)。Here, the args variable is the parameter to this method.这里，args变量是此方法的参数。The important thing to remember is that parameters are always classified as "variables" not "fields".要记住的重要一点是，参数总是分类为“变量”而不是“字段”。This applies to other parameter-accepting constructs as well (such as constructors and exception handlers) that you'll learn about later in the tutorial.这也适用于其他接受参数的构造（如构造函数和异常处理程序），您将在本教程稍后部分了解这些构造。

Having said that, the remainder of this tutorial uses the following general guidelines when discussing fields and variables.话虽如此，本教程的其余部分在讨论字段和变量时使用以下一般准则。If we are talking about "fields in general" (excluding local variables and parameters), we may simply say "fields".如果我们谈论的是“一般字段”（不包括局部变量和参数），我们可以简单地说“字段”。If the discussion applies to "all of the above", we may simply say "variables".如果讨论适用于“上述所有”，我们可以简单地说“变量”。If the context calls for a distinction, we will use specific terms (static field, local variables, etc.) as appropriate.如果上下文需要区分，我们将酌情使用特定术语（静态字段、局部变量等）。You may also occasionally see the term "member" used as well.您偶尔也会看到使用“成员”一词。A type's fields, methods, and nested types are collectively called its members.类型的字段、方法和嵌套类型统称为其成员。

Naming命名

Every programming language has its own set of rules and conventions for the kinds of names that you're allowed to use, and the Java programming language is no different.对于允许使用的名称类型，每种编程语言都有自己的规则和约定，Java编程语言也不例外。The rules and conventions for naming your variables can be summarized as follows:命名变量的规则和约定可总结如下：

• Variable names are case-sensitive.变量名区分大小写。A variable's name can be any legal identifier — an unlimited-length sequence of Unicode letters and digits, beginning with a letter, the dollar sign "$", or the underscore character "_".变量名称可以是任何合法标识符—；Unicode字母和数字的无限长序列，以字母、美元符号“$”或下划线字符“_”开头。The convention, however, is to always begin your variable names with a letter, not "$" or "_".但是，惯例是始终以字母开头变量名，而不是“$”或“_”。Additionally, the dollar sign character, by convention, is never used at all.此外，根据惯例，美元符号字符从未使用过。You may find some situations where auto-generated names will contain the dollar sign, but your variable names should always avoid using it.您可能会发现某些情况下，自动生成的名称将包含美元符号，但您的变量名称应始终避免使用美元符号。A similar convention exists for the underscore character; while it's technically legal to begin your variable's name with "_", this practice is discouraged.下划线字符也有类似的约定；虽然从技术上讲，变量名称以“_”开头是合法的，但不鼓励这种做法。White space is not permitted.不允许使用空白。
• Subsequent characters may be letters, digits, dollar signs, or underscore characters.后续字符可以是字母、数字、美元符号或下划线字符。Conventions (and common sense) apply to this rule as well. When choosing a name for your variables, use full words instead of cryptic abbreviations.惯例（和常识）也适用于这条规则。为变量选择名称时，请使用完整的单词，而不是晦涩的缩写。Doing so will make your code easier to read and understand.这样做将使您的代码更易于阅读和理解。In many cases it will also make your code self-documenting; fields named cadence, speed, and gear, for example, are much more intuitive than abbreviated versions, such as s, c, and g.在许多情况下，它也会使您的代码自我记录；例如，名为cadence、speed和gear的字段比缩写版本（如s、c和g）直观得多。Also keep in mind that the name you choose must not be a keyword or reserved word.还要记住，您选择的名称不能是关键字或保留字。
• If the name you choose consists of only one word, spell that word in all lowercase letters.如果您选择的名称仅由一个单词组成，请使用所有小写字母拼写该单词。If it consists of more than one word, capitalize the first letter of each subsequent word.如果它由一个以上的单词组成，则将其后每个单词的第一个字母大写。The names gearRatio and currentGear are prime examples of this convention.名称gearRatio和currentGear是此约定的主要示例。If your variable stores a constant value, such as static final int NUM_GEARS = 6, the convention changes slightly, capitalizing every letter and separating subsequent words with the underscore character.如果您的变量存储常量值，例如static final int NUM_GEARS=6，则约定会略有更改，将每个字母大写，并用下划线字符分隔后续单词。By convention, the underscore character is never used elsewhere.按照惯例，下划线字符从不在其他地方使用。