Java Lecture 1

Chapter 1 Java Language Overview

Through this chapter, learn the development, characteristics of Java language, and the differences between Java and C , master how the general structure of Java applications and how the Applet applet is generated. Through skill training, master the basic use of Java operating environments.

1.1 Java language profile

1.1.1 Development of Java Language

The predecessor of Java language is known as Oak language. It is a software project called "Green", developed by SUN Microsystem in 1991. The original intention of the project is to develop a platform-independent of the consumer electronics. Software technology. Since then, Oak language has been considered to be tools for developing consumer electronics and interactive TV controllers. In 1994, the two developers of Sun created the first version of Hotjava on the basis of Oak. At that time, it was called WebRunner, ie the graphical browser used on the web. It is called Java after a period of time. In May 1995, Sun has officially released Java language, and then immediately got a strong support, and joined the Applet applet on the browser (using a small application written in Java language) and through the Internet around the world. Communicate.

So far, there are 4 main versions of the Java language: Java 1.0 (1995), Java 1.1 (1997), Java 2 (1998) with SDK1.2 and Java 2 with SDK1.3 ( 2000).

In fact, Java language is a new type of network language that is developing synchronously with the Internet, which is one of the most meaningful progress in computer software environments in the past 20 years. The Java language is as important as the HypertExt Markup Language in the network. The Java language is a powerful network programming language that maximizes network resources, and the Applet applet can run across platforms, cross-operative systems, cross-network operations. Due to small applet code, it is easy to download and easy to download and Send and have new features that can increase the web page without modifying applications. Java also provides many public system interfaces for programming technicians. As Internet is widely popular in the world, as well as penetration in various fields, Java languages ​​have been accepted by people from all walks of life.

1.1.2 Characteristics of Java Language

The Java language is widely recognized because it has many advanced technical features:

1. Good transplantability

We know that there are tens of thousands of computer systems on the Internet online, and the computer is soft and hardware. To make the application can run normally in any computer system on the network, it must ensure that the portability of the application software is better. That is, the application software itself is not limited by the computer hardware and the operating system (ie, software code can run normally in different computer environments. The Java language is a platform-independent programming language, and the transplantable programming language is guaranteed to have the basic data type and the platform. The binary code generated by the Java source program is a collection of instructions that are independent of the specific instructions, through the Java Virtual Machine, JVM, can run on a different platform.

2. Pure object-oriented technology

Java is a programming language for completely faced objects (ORIENTED Obiect Programming, OOP). The Java language code consists in form of a class. Java deletes the non-objective characteristics of C , making the application development very easy. The characteristics of Java object-oriented technology include encapsulation, inheritance, and polymorphism, which will be detailed in the future. 3. distributed

The Java language provides a library that supports HTTP and FTP and other TCP / IP protocols, through these class libraries, Java applications can open and access objects on the network via URL. Therefore, it is convenient to use the Java language to easily write applications suitable for Internet and distributed environments.

4. safety

Distributed computing environment requires software with high stability and security. The Java language has a powerful security structure and security strategy. Java has high quality security because of the following measures:

(1) Cancel the pointer operation and not allowed to operate directly.

(2) Realize memory management automation, the memory layout is determined by JVM.

(3) Provide a bytecode inspection to ensure that the program code accepts a layer of security check during compiling and running, which prevents illegal programs or viruses.

(4) Provide a file access control mechanism to strictly control access to program code.

(5) Provides a user interface for a variety of network software protocols, users can use multiple encryption techniques in network transmission to ensure the security and integrity of network transmission.

5. Combination of compilation and interpretation

Java is a combination of compilation and interpretation. A Java language source program must be run, and must be compiled by a Java compiler (bytecode), ie the file extension is .class. This word code file is not the final executor, cannot run on a specific platform, and must be translated into a machine language on the byte interpreter on the running system, reaching the purpose of performing the program while the translation. Since this interpretation process is performed on the compiled bytecode, the running speed is still quite fast.

1.1.3 Comparison of Java and C

The C language is adapted to the object-oriented new trend of the software engineering, but C does not really meet object-oriented programs, which retains many non-domain features. The Java language realizes the characteristics of pure object-oriented, making it easier than C languages ​​and is more acceptable.

From the origin and development of Java language, on the one hand, it is developed by C , and its language style is very similar to C ; on the other hand, Java is simpler than C , it deletes C difficult to understand, easy to cause security hazards Content. The difference between Java and C is mainly manifested in the following aspects:

1. pointer

The most effective characteristics of the pointer and memory address are also the most hazardous features in C . Incorrect pointer operations will cause many errors. The pointer is not allowed in Java.

2. Memory allocation

Memory allocation in C is the same danger, and its memory allocation is implemented through the malloc () and Free () library methods and new operators in New and Delete, and programmers need to release space. There is no Malloc and Free functions in the Java language. Since each complex data structure is an object, they allocate space through the New operator, once the object is no longer accessed, the occupied memory space will be automatically recovered, and there is no need to intervene.

3. Global variable

The C global variable is not well packaged as the status signal of the program. Java language is only a class, it is impossible to create a global variable that does not belong to any class. 4. type of data

Different C compilers are assigned to the data type according to the actual configuration of the different machines to different word lengths. The basic data type given by the Java language determines a reasonable word length and remains unchanged. This strict and hardware-independent data type of the Java language interpreter is difficult to optimize and implement the code. The data type of Java language is Very fragile, but this is the only way to ensure cross-platform implementation.

5. Goto

Before C introduces exception handling, GOTO is often used to jump out of the loop in an exception handling. The Java language does not have a goto statement. It strictly defined the exception handling mechanism to make GOTO no longer exist, canceling this random jump statement facilitates optimizing the code and maintaining the stability and security of the system.

6. head File

C has the header file referenced by the include, the Java language has no header file, but the Java language has a package introduced by IMPORT.

7. Multiple inheritance

C has multiple inheritance characteristics, Java language does not directly support multiple inheritance.

1.1.4 Several basic concepts for object-oriented programming

Object-oriented programming technology is one of the technologies commonly used in today's software development. The Java language is a new type of pure object-oriented programming language, which uses a software object called "class", which can be reused and scalable, which can make these classes consisting of variables and methods. As a template, adding other functions on these templates to create the new classes you need yourself, no need to override a lot of code, so the development of applications is easy. Several main concepts that are often involved in object-oriented programming are as follows:

1. Object

Object (Object) can be any object in real life, such as a book, people or moving plants, computers, etc. are objects in real life, some invisible affairs, rules are also objective. Any object includes two basic aspects, one is an internal configuration of an object (e.g., various components that make up aircraft), and the other is the behavior of the object, that is, the operation of the object constitutes the operation or exchange of external information (for example , Launch, takeoff, etc.). Corresponding to the two fundamentals of the object, the objects in the OOP also have two basic components: data and methods. Data represents the composition of the object, the method represents the behavior of the object.

2. class

Class (Class) is a set of objects with the same basic ingredients. For example, university consists of many specific schools, university is a class, and the specific Peking University is an object of university class.

Category is the most basic, most important data type. As a data type, the corresponding type of data must be defined in the program, and the data defined by the class is referred to as an instance or object of this class. There are two basic components: data and methods, they are packaged in the body, separated from the outside world. Data is a member of the class, which can be a basic type of data (eg, int, char, etc.) or an example of a class. The method is also a member of the class, which is used to handle the data of the class, the method of the class is similar to the functions in other languages, and has the name, parameters, method and return value of the method.

3. Encapsulate

Encapsulation is to encapsulate the basic components (data and methods) of classes (or objects) within the type (or object), so that it is separated from the outside.

Generally, the mechanism with hidden complexity in the class. Each method or variable in the class can be defined as public or private. The public part of the class can let the outside user know or must know that the public instance variable and method are a class and an external interface, the application uses the functionality of this interface; and define the private method and instance data cannot be classified Access to the outside of other code. When performing OOP programming, first need to define classes, and point out the name, data member, and method of the class, determine the permissions of the class member to be accessed, etc .; Secondly, use the class to create the corresponding instance (or object); finally, in the program run Information is exchanged with objects as needed by the system.

4. inherit

Inherit refers to establishing a new class based on existing classes. The new class automatically has all methods and instance variables of the parent class, and then add the method or instance variable required to add new tasks as needed.

A class that is not born from other classes is called a base class; a recent father (its nearest superior class) is called the parent class of this class; the class of derived class is called the subclass of such class. In the Java language, a parent class can have multiple subclasses, but a subclass can only inherit from a parent class, that is, a subclass cannot have more than one parent class, which means that the Java language does not support multiple inheritance. . In order to solve multiple inheritance issues, the concept of interfaces introduced in Java language and overcomes multiple inheritance complexity.

Reasonable use inheritance can reduce much repetitive labor. If a special feature is implemented in a class, these features can be reused in its derived class without re-programming. You can use the Java language built-in class to create a class, or you can establish a class for the class you created yourself.

Inheritance and packaging have good cooperative. If a given class encapsulates some properties, then any subclass will inherit these properties.

5. Polymorphism

Polymophism means that there are multiple names in a class, but the type and number of parameters are different. When these methods are called, the parameter type and quantity of the method determine which method is called, which not only simplifies the implementation and call of the method, but also for memory.

The overlay (or rewrite) of the method means that the subclass can redefine certain methods of the parent class in the process of inheritance, and implement themselves.

In the Java language, polymorphisms are implemented by Overloading and overridding. Method Overloading refers to a plurality of methods have the same name, but the parameters of each method are different. When the method is called, the system automatically selects the corresponding method according to the actual parameters.

1.1.5 Use of Java language

Java language is increasingly affected and recognized by developers because of the characteristics and complete object-oriented characteristics, and it has also determined the special purpose of the Java language. The area of ​​Java language applications include a lot, but its side weight is different from other languages. It is hoped that the reader has mastered the knowledge related to the main application below through this book.

1. Java language can easily increase the function of the Internet / Intranet page

The performance form of the Applet applet execution result is based on the web page based on the web page, which can be easily embedded in the HTML code and is easily called and implemented. At present, people are very familiar with the web page, you can use some popular tools to make a web page. If you use the Java Applet applet to expand your web page, you can enrich the content of the webpage.

2. Java language can easily implement parallel work in multiple tasks

Many languages ​​have functions that implement multitasking parallel work, but the Java language is based on its network applications, plus multi-threaded programming technology provided by Java languages, making it easy to use Java language-based multitasking multitasking program. For example, complete the operation task for monitoring multiple tasks while simultaneously on the web page. 3. Java language can easily access network resources

Using Java language will easily develop procedures for network resources. For example, to access the information of a web page, including the domain name, IP address, information termination date, information modification date, length of information; to get the content of a file on a network server; design network client and server application Programs (such as postage, dialogue, etc.), etc., Java language presents a good network language.

4. Java language can easily implement web-based database access

The Java language provides technology for accessing the database. Using JDBC technology in Java language, users can easily develop web-based database access programs to expand the functionality of network applications.

5. Java language can easily implement distributed applications

The Java language provides many technologies to facilitate distributed applications. Using techniques such as JSP, RMI, CORBA, and EJB in Java languages, users can easily implement distributed applications.

1.2 Java program introduction

The Java source program consists of class definitions, so it can only be programmed in the source program. A Java source program can contain a class definition or multiple class definitions. The Java source program is saved with .java as the suffix name. When the Java source code is compiled, each class is saved in the output file that is .class as the suffix name. Before you start learning Java language programming, you should learn about the basic composition of the Java source, and how to write, compile, and run the Java program to create a general impression.

Generally, Java programs are divided into Java applications and two Java-based Applet applets.

1.2.1 A simple Java application

Write an application with Java language, and then get the result, you need to pass three processes, namely, write source programs, compile and run.

1. Write the source program

A simple Java application is shown in Example 1.1.

[Example 1.1] A simple Java application.

// File name: HelloChina.java

Public Class Hellochina, PUBLIC CLASS HelloCHINA

{

Public static void main (string [] args)

{

// Output a string

System.out.println ("Hello, China!");

}

}

Through this program, we can see the Java app is still relatively simple, the structure is not complicated. The reason why it is called an application, its flag is a main method, and there is only a Java program containing the main method to run with a Java command. The programming principle of Java language must be followed when writing a Java program. The basic format of a simple Java application has the following points:

(1) The Java program is an unformatted plain text file that can be written in any text editor (eg, Word, Word board) to write Java programs.

(2) The file name must be consistent with the name of the name, and the case is consistent. For example, class name Hellochina, then the file name at the time of the market should be Hellochina.java.

(3) First, use the keyword class to define a new class, and the class name is followed. Here PUBLIC indicates a public class, which can be omitted, and Hellochina is the class name defined in this example.

(4) A Java program can have multiple classes, each class can have multiple methods, but only one public class is available. (5) For an application, there must be a main () method, and can only have a main () method. This method marks the starting point when performing an application. The keyword public indicates that all classes can call this method. The keyword Static indicates that the method is a static method. The keyword Void represents the main () method without return value.

(6) There are multiple statements in any method. In this main () method of this example, only one statement is:

System.out.println ("Hello, China!");

This statement is used to output a "Hello, China!" String on the screen, and the functionality of System.out.Println () is the same as the Printf () function in the C language.

(7) Each statement in the Java program is to end with a semicolon ";", including the type descriptions, which appears in the later program.

(8) In order to increase the readability of the program, some annotation lines can be added, for example, using the "//" row.

See Chapter 2 for Note Definitions for Java Language.

This example is just a simplest Java program, only one class, only one method is included in the class, and does not include data. If the data is included in the class, there is also a type of description and other statements in the structure.

2. Compile

When the Java program is written, you must translate the .java source program into a two-input bytecode file that is independent of the processor complicated by the Java compiler. In the Java language environment, perform the following command to compile the HelloChina.java program, which will automatically get the HelloChina.class bytecode file after compiling.

Javac HelloChina.java

3. run

The bytecode file with .class is not an executable program. It must be translated by the bytecode detector and Java interpreter to run, through a special operating environment, can form .exe's executable . In the Java language environment, execute the following command to run the HelloChina.class program:

Java HelloChina

If everything is normal, the results will be shown in Figure 1.1.

Figure 1.1 HelloChina program operation results

1.2.2 Write the application as an Applet applet

The Applet applet based on Java language can be embedded in the HTML language and explained the execution of the program with a WWW browser (such as Internet Explorer, Netscape Communicator, etc.) with a Java interpreter. It is not possible to use the Java interpreter. carried out. The reason why the applet is called a small program is because its code is small and easy to download through Internet / Intranet.

Generally, a feature is implemented with an Applet applet. The last result is required to write Applet applets, compile the applet, write an HTML file, and perform HTML code programs. There are two ways to execute the HTML program, one is to perform an HTML program by running the browser; the other is the appletViewer command provided by the Java package executes an HTML program.

1. Write Applet applet

[Example 1.2] Applet small program example

// File name: HelloWorld.java

IMPORT JAVA.Applet.Applet;

Import java.awt.graphics;

Public Class HelloWorld Extends Applet

{

Public void init ()

{

Resize (400, 300);

}

Public void Paint (Graphics G)

{

g.drawstring ("Hello, the World!", 60, 40);

}

}

From Example 1.2, we see that the basic structure based on Java-based Applets is similar to Java applications, and different places are as follows:

(1) At the beginning of the small program, two system classes must be introduced into two system classes: java.applet.applet and java.awt.grapgics, the methods in these two classes are required in the program. When introducing the class provided by the system, you should use keyword import, which is the same as the #include function in the C language.

(2) Keywords EXTENDS indicates that the HelloWorld class is inherited by the Applet class.

(3) The init () method and the Paint (Graphics G) method are the public methods defined in the Applet class and the Graphics class, respectively, respectively, for the initialization and display output of the applet.

(4) All defined Applet techniques must be derived from the Applet class and cannot have the main () method.

For details on applet applet, please see Chapter 5.

2. Compile Applet applet

After the Applet applet HelloWorld.java is written, you need to compile it, automatically generate the bytecode file helloworld.class, which is the same as the Java application.

3. Write HTML code

The bytecode HelloWorld.class for the Applet applet must be embedded in the HTML code to complete the functionality of the applet. So, you must also write an HTML code file for HelloWorld.class, and introduce the bytecode program. Below is an HTML code file, the file name is HelloWorld.html. About HTML language, not the main content of this book, interested readers, please refer to relevant books.

[Example 1.3] Embed an HTML program example of the HelloWorld.class code, the file name is HelloWorld.html.