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();
Core Java Volume I — 3.1. A Simple Java Program的更多相关文章
- Core Java Volume I — 1.2. The Java "White Paper" Buzzwords
1.2. The Java "White Paper" BuzzwordsThe authors of Java have written an influential White ...
- Core Java Volume I — 3.8. Control Flow
3.8. Control FlowJava, like any programming language, supports both conditional statements and loops ...
- Core Java Volume I — 5.1. Classes, Superclasses, and Subclasses
5.1. Classes, Superclasses, and SubclassesLet's return to the Employee class that we discussed in th ...
- Core Java Volume I — 4.7. Packages
4.7. PackagesJava allows you to group classes in a collection called a package. Packages are conveni ...
- Core Java Volume I — 4.6. Object Construction
4.6. Object ConstructionYou have seen how to write simple constructors that define the initial state ...
- Core Java Volume I — 4.1. Introduction to Object-Oriented Programming
4.1. Introduction to Object-Oriented ProgrammingObject-oriented programming, or OOP for short, is th ...
- ASP.NET CORE(C#)与Spring Boot MVC(JAVA)
干货分享:ASP.NET CORE(C#)与Spring Boot MVC(JAVA)异曲同工的编程方式总结 目录 C# VS JAVA 基础语法类比篇: 一.匿名类 二.类型初始化 三.委托(方 ...
- Java (JVM) Memory Model – Memory Management in Java
原文地址:http://www.journaldev.com/2856/java-jvm-memory-model-memory-management-in-java Understanding JV ...
- 0031 Java学习笔记-梁勇著《Java语言程序设计-基础篇 第十版》英语单词
第01章 计算机.程序和Java概述 CPU(Central Processing Unit) * 中央处理器 Control Unit * 控制单元 arithmetic/logic unit /ə ...
随机推荐
- [转]z-order引出的问题
在窗口与窗口之间毫无重叠的情况下,根本不需要关心z-order.然而,当窗口之间出现重叠时,系统就需要通过一个标准来确定窗口的显示顺序.这个标准就是z-order.存在多个因素影响一个窗口的z-ord ...
- spring中配置jdbc数据源
1.加入jdbc驱动器包,mysql-connector-java.jar 2.加入commons-dbcp.jar配置数据源 3.在classpath下新建文件jdbc.properties,配置j ...
- 表单_post提交方式和get的区别,元素集
提交方式及表单域的name属性 使用form表单一种是post提交方式,一种是get提交方式,它们以method属性来定义,如果没有指定method属性,默认get方式提交. 表单域必须配合name属 ...
- mysqldump使用语法
复制代码 代码如下: mysqldump -u user -p db tab1 tab2 > db.sql 恢复 复制代码 代码如下: mysql -u user -p db < db ...
- How to Run a .Jar Java File
.jar files are used for archiving, archive unpacking. One of the essential features of jar file is l ...
- Oracle合并函数内容
--MINUS去差集,取第一个集合有的而第二集合没有的,并以第一个字段排序select t.bumenbm from T_HQ_BM t minus select b.bumenbm from t_h ...
- dancing link 学习资源导航+心得
dancing link简直是求解数独的神器,NOIP2009最后一题靶形数独,DFS 各种改变搜索顺序 都没法过,最后还是用了卡时过得.用dancing link写,秒杀所有数据,总时间才400ms ...
- NOIP 2001解题报告
第一题: 有形如:ax3+bx2+cx+d=0 这样的一个一元三次方程.给出该方程中各项的系数(a,b,c,d 均为实数),并约定该方程存在三个不同实根(根的范围在-100至100之间),且根与 ...
- net IL的一些探索
查看IL有2个工具比较好用,一个是大名鼎鼎的Reflector,但已经转向收费,另一个是开源的ILSpy,强大好用,对linq和lambda表达示的支持很好.相关的书籍也很多,比如这本Microsof ...
- Apache Jmeter(2)
上一节中,我们了解了jmeter的一此主要元件,那么这些元件如何使用到性能测试中呢.这一节创建一个简单的测试计划来使用这些元件.该计划对应的测试需求. 1)测试目标网站是fnng.cnblogs.co ...