Core Java Volume I — 3.1. A Simple Java Program

Let’s look more closely at one of the simplest Java programs you can have—one that simply prints a message to console:

public class FirstSample
{
    public static void main(String[] args)
    {
        System.out.println("We will not use 'Hello, World!'");
    }
}

It is worth spending all the time you need to become comfortable with the framework of this sample; the pieces will recur in all applications. First and foremost, Java is case sensitive（大小写敏感）. If you made any mistakes in capitalization (such as typing Main instead of main), the program will not run.
Now let's look at this source code line by line. The keyword public is called an access modifier（访问修饰符）; these modifiers control the level of access other parts of a program have to this code. We have more to say about access modifiers in Chapter 5. The keyword class reminds you that everything in a Java program lives inside a class. Although we will spend a lot more time on classes in the next chapter, for now think of a class as a container for the program logic that defines the behavior of an application. As mentioned in Chapter 1, classes are the building blocks with which all Java applications and applets are built. Everything in a Java program must be inside a class.
Following the keyword class is the name of the class. The rules for class names in Java are quite generous. Names must begin with a letter, and after that, they can have any combination of letters and digits（类名需要以字母开头，不可以有下划线？）. The length is essentially unlimited（不限制长度）. You cannot use a Java reserved word (such as public or class) for a class name（不能使用Java保留字作为类名）. (See the Appendix for a list of reserved words.)
The standard naming convention (which we follow in the name FirstSample) is that class names are nouns that start with an uppercase letter. If a name consists of multiple words, use an initial uppercase letter in each of the words. (This use of uppercase letters in the middle of a word is sometimes called "camel case" or, self-referentially, "CamelCase".)（标准命名习惯，大写字母开头，驼峰命名法）
You need to make the file name for the source code the same as the name of the public class, with the extension .java appended（源码文件名需要和public class的名称一样）. Thus, you must store this code in a file called FirstSample.java. (Again, case is important—don’t use firstsample.java.)
If you have named the file correctly and not made any typos in the source code, then when you compile this source code, you end up with a file containing the bytecodes for this class. The Java compiler automatically names the bytecode file FirstSample.class and stores it in the same directory as the source file. Finally, launch the program by issuing the following command:

java FirstSample

(Remember to leave off the .class extension.) When the program executes, it simply displays the string

We will not use 'Hello, World!' on the console.

When you use

java ClassName

to run a compiled program, the Java virtual machine always starts execution with the code in the main method in the class you indicate（Java虚拟机从main方法开始执行）. (The term “method” is Java-speak for a function.) Thus, you must have a main method in the source file for your class for your code to execute. You can, of course, add your own methods to a class and call them from the main method. (We cover writing your own methods in the next chapter.)

Notice the braces { } in the source code. In Java, as in C/C++, braces delineate the parts (usually called blocks) in your program. In Java, the code for any method must be started by an opening brace { and ended by a closing brace }.
Brace styles have inspired an inordinate amount of useless controversy. We follow a style that lines up matching braces. As whitespace is irrelevant to the Java compiler, you can use whatever brace style you like. We will have more to say about the use of braces when we talk about the various kinds of loops.
For now, don’t worry about the keywords static void—just think of them as part of what you need to get a Java program to compile. By the end of Chapter 4, you will understand this incantation completely. The point to remember for now is that every Java application must have a main method that is declared in the following way:

public class ClassName
{
    public static void main(String[] args)
    {
        program statements
    }
}

Next, turn your attention to this fragment:

{
    System.out.println("We will not use 'Hello, World!'");
}

Braces mark the beginning and end of the body of the method. This method has only one statement in it. As with most programming languages, you can think of Java statements as sentences of the language. In Java, every statement must end with a semicolon（所有语句以分号结束）. In particular, carriage returns do not mark the end of a statement, so statements can span multiple lines if need be.
The body of the main method contains a statement that outputs a single line of text to the console.
Here, we are using the System.out object and calling its println method. Notice the periods used to invoke a method. Java uses the general syntax object.method(parameters) as its equivalent of a function call.
In this case, we are calling the println method and passing it a string parameter. The method displays the string parameter on the console. It then terminates the output line, so that each call to println displays its output on a new line. Notice that Java, like C/C++, uses double quotes to delimit strings（双引号-字符串）. (You can find more information about strings later in this chapter.)
Methods in Java, like functions in any programming language, can use zero, one, or more parameters (some programmers call them arguments). Even if a method takes no parameters, you must still use empty parentheses. For example, a variant of the println method with no parameters just prints a blank line. You invoke it with the call System.out.println();