a class called HelloWorld and an instance of that class, also called HelloWorld. This
instance is created on demand, the first time it is used.
细心(astute，机敏的，聪明的)的读者可能会注意到，main 方法并没有声明为 static。
这是因为 Scala 中不存在静态成员（无论方法还是属性，methods or fields）这一概念，
Scala 使用前述的单例对象（singleton objects）中的成员来代替静态成员。
The astute reader might have noticed that the
main method is not declared as static
here. This is because static members (methods or fields) do not exist in Scala. Rather
than defining static members, the Scala programmer declares these members in
singleton objects.
2.1 编译该示例 － Compiling the example
要编译上面写的例子，要 scalac 命令，这就是 Scala 的编译器。scalac 的工作流程和
多数编译器类似：从命令行上接收待编译的源文件名（source file）以及编译参数，生
成一个或者多个目标文件（object files，或者叫对象文件）。Scala 生成的目标文件是
标准的 java class 文件。
To compile the example, we use
scalac, the Scala compiler. scalac works like most
compilers: it takes a source file as argument, maybe some options, and produces one
or several object files. The object files it produces are standard Java class files.
假如我们将 HelloWorld 示例程序存放到 HelloWorld.scala 文件中，则可以用以下指
令进行编译（大于号’>’表示 shell/命令行 的提示符，不需要人工键入）：
If we save the above program in a file called
HelloWorld.scala, we can compile it by
issuing the following command (the greater-than sign ‘
>’ represents the shell prompt
and should not be typed):
> scalac HelloWorld.scala
该指令执行后，会在当前目录下生成几个 class 文件，其中一个是 HelloWorld.class，
该文件中包含一个可以直接被
scala 指令执行的类(class)，具体操作参见后续章节。
This will generate a few class files in the current directory. One of them will be called
HelloWorld.class, and contains a class which can be directly executed using the
scala command, as the following section shows.
2.2 运行该示例 － Running the example
代码编译通过以后，可以使用 scala 指令运行程序，scala 指令和 java 指令的用法非
常相似，甚至它们接受的命令行参数都是一样的。前面编译好的例子，可以用如下指令
运行，并输出预期的问候语：
Once compiled, a Scala program can be run using the
scala command. Its usage is
very similar to the
java command used to run Java programs, and accepts the same
options. The above example can be executed using the following command, which
produces the expected output:
> scala -classpath . HelloWorld
