8.5

Java language

Program : a description of the implementation process of the complete thing.

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello World");
    }
}

public common 
} mathematics, the set {behalf. {} Where the code can be written.

inlet public static void main (String [] args) program 
main door 
command　　System.out.println ( "Hello World");　　the output statement
 System System print out the println

javac  followed afterwards by file name java file, for example HelloWorld.java. This command is used to compile the source files to java byte code class files, such as:  the javac HelloWorld.java .

After running javac command, if successful compilation no errors, file a HelloWorld.class will appear.

java  followed afterwards by the class name java file, for example HelloWorld is the name of the class, such as: java HelloWorld.

Note : java command behind do not add .class.

Java is divided into three systems:

• JavaSE (J2SE) (Java2 Platform Standard Edition, java Platform, Standard Edition)
• JavaEE (J2EE) (Java 2 Platform, Enterprise Edition, java platform enterprise edition)
• JavaME (J2ME) (Java 2 Platform Micro Edition, java Platform, Micro Edition).

version version

• Object : An object is an instance of a class, have state and behavior. For example, a dog is an object that states are: color, name, breed; behavior: wagging its tail, called, eat and so on.
• Class : The class is a template that describes the behavior and state of a class object.
• Methods : The method is behavior, a class can have a number of ways. Logical operation, and all the data modification operation is done in the process.
• Examples of variables : each object has a unique instance variables, the state of the object is determined by the instance variables.

The basic syntax

When writing a Java program, you should note the following:

• Case sensitive : Java is case-sensitive, which means that the identifier Hello and hello are different.
• Class Name : For all classes, the class name first letter should be capitalized. If the class name composed of several words, the first letter of each word should be capitalized, such as  MyFirstJavaClass .
• Method name : All method name should begin with a lowercase letter. If the method name contains several words, each word is capitalized later.
• Source File name : source file name must be the same as the class name. When saving files, you should use the class name as the file name to save (Remember that Java is case-sensitive), the file name extension is  .java . (If the file name and class name are not the same will result in a compilation error).
• Method main entrance : all the Java programs  public static void main (String [] args)  method to begin.

Java identifier

All components of the Java name is required. Class names, method names and variable names are called tags.

About Java identifiers, the following points should be noted:

• All identifiers should begin with a letter (AZ or az), the dollar sign ($) or an underscore (_)
• After the first character may be a letter (AZ or az), any combination of characters dollar sign ($), underscore (_) or digital
• Keywords can not be used as an identifier　　keyword is given a special programming language word meaning
• Identifiers are case sensitive
• Legal identifier, for example: age, $ salary, _value, __ 1_value
• Illegal identifier example: 123abc, -salary

Java modifiers

Like other languages, Java classes can be modified in the methods and properties using the modifier. There are two types of modifiers:

• Access control modifiers: default, public, protected, private
• Non-access control modifiers: final, abstract, static, synchronized

In later chapters we will discuss in depth Java modifiers.

Java keywords

Java keywords are listed below. These reserved words can not be used to name constants, variables, and any identifiers.

category	Keyword	Explanation
Access control	private	private
	protected	under protection
	public	public
Class, method and variable modifiers	abstract	Abstract statement
	class	class
	extends	Expansion, succession
	final	The final value can not be changed
	implements	Implement (Interface)
	interface	interface
	native	本地，原生方法（非 Java 实现）
	new	新,创建
	static	静态
	strictfp	严格,精准
	synchronized	线程,同步
	transient	短暂
	volatile	易失
程序控制语句	break	跳出循环
	case	定义一个值以供 switch 选择
	continue	继续
	default	默认
	do	运行
	else	否则
	for	循环
	if	如果
	instanceof	实例
	return	返回
	switch	根据值选择执行
	while	循环
错误处理	assert	断言表达式是否为真
	catch	捕捉异常
	finally	有没有异常都执行
	throw	抛出一个异常对象
	throws	声明一个异常可能被抛出
	try	捕获异常
包相关	import	引入
	package	包
基本类型	boolean	布尔型
	byte	字节型
	char	字符型
	double	双精度浮点
	float	单精度浮点
	int	整型
	long	长整型
	short	短整型
变量引用	super	父类,超类
	this	本类
	void	无返回值
保留关键字	goto	是关键字，但不能使用
	const	是关键字，但不能使用
	null	空