Java basic syntax

A Java program can be thought of as a collection of objects, and these objects work together by calling each other's methods. The following briefly introduces the concepts of classes, objects, methods and instance variables.

• ##Object: An object is an instance of a class and has state and behavior. For example, a dog is an object. Its status includes: color, name, and breed; its behaviors include: wagging its tail, barking, eating, etc.

• Class: A class is a template that describes the behavior and status of a type of object.

• Method: Method is behavior, and a class can have many methods. Logical operations, data modification, and all actions are completed in methods.

• Instance variables: Each object has unique instance variables, and the state of the object is determined by the values ​​of these instance variables.

The first Java program

Hello World .

public class HelloWorld {
    /* 第一个Java程序
     * 它将打印字符串 Hello World
     */
    public static void main(String []args) {
        System.out.println("Hello World"); // 打印 Hello World
    }
}

• ##Open Notepad and add the above code;

• Save the file name as: HelloWorld.java;

• Open the cmd command window , enter the location of the target file, assuming it is C:\

• Type javac HelloWorld.java in the command line window and press the enter key to compile the code. If there are no errors in the code, the cmd command prompt will go to the next line. (Assuming the environment variables are all set).

• Type java HelloWorld and press Enter to run the program

You will be in The window sees Hello World.

C : > javac HelloWorld.java
C : > java HelloWorld 
Hello World

When writing Java programs, you should pay attention to the following points:

• ##Case Sensitive: Java is case sensitive, which means that the identifiers Hello and hello are different.

• Class name: For all classes, the first letter of the class name should be capitalized. If the class name consists of several words, the first letter of each word should be capitalized, for example, MyFirstJavaClass .

• Method name: All method names should start with a lowercase letter. If the method name contains several words, the first letter of each subsequent word is capitalized.

• Source file name: The source file name must be the same as the class name. When saving the file, you should use the class name as the filename (remember Java is case-sensitive) and the filename suffix .java. (If the file name and class name are different, a compilation error will occur).

• Main method entrance: All Java programs consist of public static void main(String []args)method Begin execution.

All components of Java need names.

Class names, variable names, and method names are all called identifiers.

## Regarding Java identifiers, there are the following points to note:

All identifiers should start with letters (A-Z or a-z), dollar signs ($), or underscores (_)Start
The first character can be followed by any combination of characters
key Words cannot be used as identifiers
Identifiers are case-sensitive
Legal Examples of identifiers: age, $salary, _value, __1_value

• ## Examples of illegal identifiers: 123abc, -salary

Like other languages, Java can use modifiers to modify methods and properties in a class. There are two main types of modifiers:

(scope)

• 访问控制修饰符 : default, public , protected, private

• 非访问控制修饰符 : final, abstract, strictfp 

Java变量

Java中主要有如下几种类型的变量

• 局部变量

• 类变量（静态变量）

• 成员变量（非静态变量）

Java数组

数组是储存在堆上的对象，可以保存多个同类型变量。在后面的章节中，我们将会学到如何声明、构造以及初始化一个数组。

Java枚举

Java 5.0引入了枚举，枚举限制变量只能是预先设定好的值。使用枚举可以减少代码中的bug。

例如，我们为果汁店设计一个程序，它将限制果汁为小杯、中杯、大杯。这就意味着它不允许顾客点除了这三种尺寸外的果汁。

注意：枚举可以单独声明或者声明在类里面。方法、变量、构造函数也可以在枚举中定义。

class FreshJuice {
   enum FreshJuiceSize{ SMALL, MEDIUM , LARGE }
   FreshJuiceSize size;
}
 
public class FreshJuiceTest {
   public static void main(String []args){
      FreshJuice juice = new FreshJuice();
      juice.size = FreshJuice. FreshJuiceSize.MEDIUM  ;
   }
}

##Java keyword

Java reserved words are listed below. These reserved words cannot be used in the names of constants, variables, and any identifiers.

Keyword	Description
abstract	Abstract method, abstract class modifier
assert	Assert whether the condition is met
boolean	Boolean data type
break	Break out of the loop or label code segment
byte	8-bit signed data type
case	A condition of the switch statement
catch	Combined with try to capture exception information
char	16-bit Unicode character data type
class	Definition class
const	Unused
continue	Do not execute the remainder of the loop
default	Default branch in switch statement
do	Loop statement, the loop body will be executed at least once
double	64-bit double precision floating point number
else	The branch executed when the condition is not true
enum	Enumeration type
extends	Indicates that one class is a subclass of another class
final	Indicates that a value cannot be changed after initialization Indicates that the method cannot be overridden, or a class cannot have subclasses
finally	Designed to complete the executed code, mainly for the robustness and stability of the program Integrity, code is executed regardless of whether an exception occurs.
float	32-bit single precision floating point number
for	for loop statement
goto	Not used
if	Conditional statement
implements	Indicates that a class implements the interface
import	Import class
instanceof	Test whether an object is an instance of a class
int	32-bit integer
interface	Interface, an abstract type with only definitions of methods and constants
long	64-bit integer
native	Indicates that the method is implemented in non-java code
new	Allocate a new class instance
package	A series of related classes form a package
private	indicates that private fields, methods, etc. can only be accessed from within the class
protected	indicates that the field can only be accessed from within the class Access via class or its subclass Subclasses or other classes in the same package
public	represents shared properties or methods
return	Method return value
short	16-digit number
static	means in Class level definition, shared by all instances
strictfp	Floating point comparison uses strict rules
super	Indicates the base class
switch	Select statement
synchronized	Indicates only the same time A block of code that can be accessed by a thread
this	represents a call to the current instance Or call another constructor
throw	throws exception
throws	define method possible Thrown exception
transient	Modify fields not to be serialized
try	Indicates the code If the block needs to handle exceptions or cooperate with finally to indicate whether an exception is thrown, the code in finally will be executed
void	The marked method does not return any value
volatile	Marked fields may be accessed by multiple threads at the same time without synchronization
while	while loop

#Java Notes

## Similar to C/C++ , Java also supports single-line and multi-line comments. The characters

in comments will be ignored by the Java compiler

.

##Blank lines or lines with comments will be ignored by the Java compiler.

#Inheritance

#In Java, a class Can be derived from other classes. If you are creating a class and there is already a class that has the properties or methods you need, then you can inherit the newly created class from that class.

Using inherited methods, you can reuse methods and properties of existing classes , without having to rewrite these codes. The inherited class is called a super class, and the derived class is called a subclass.

In Java, an interface can be understood as a protocol for communication between objects. Interfaces play a very important role in inheritance.

The interface only defines the methods to be used by the derived class, but the specific implementation of the method completely depends on the derived class.

#Java source program and compilation The difference in operation

## is shown in the figure below:

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