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IO technology

For any programming language, the input-output (Input/Output) system is a very core function. The operation of the program needs data, and the acquisition of data often requires communication with external systems, which may be files, databases, other programs, networks, IO devices and so on.

The external system is more complex and changeable, so it is necessary for us to abstract and shield external differences by some means, so as to achieve more convenient programming.

Input means that a program can obtain data from an external system (the core meaning is "read" and read external data). Common applications:

1) read the contents of the file on the hard disk to the program. For example, the player opens a video file and word opens a doc file.

2) read the content from a location on the network to the program. For example: after entering the URL in the browser, open the web page content corresponding to the URL; download the file of a URL on the network.

3) read the data of the database system to the program.

4) read some hardware system data to the program. For example: on-board computer reads radar scan information to program; temperature control system

Output means that a program outputs data to an external system so that it can operate the external system (the core meaning is "write", writing out the data to the external system). Common applications are:

5) write the data to the hard disk. For example, after we finish editing an word document, we write the contents to the hard drive and save them.

6) write data to the database system. For example, when we register a website member, the background program actually writes a record to the database.

7) write the data to some hardware systems. For example: × × system navigation program outputs the new path to the flight control subsystem, and the flight control subsystem modifies the flight path according to the data.

The java.io package provides us with the relevant API and implements the input and output operations to all external systems, which is the technology we will learn in this chapter.

Basic Concepts and introduction to IO

Data source

The data source data source, which provides the original medium of the data.

Common: databases, files, other programs, memory, network connections, IO devices.

The data source is divided into source device and target device.

Source device: provides data for the program, usually corresponding to the input stream.

Target device: the destination of program data, usually corresponding to the output stream.

The concept of flow

A data source is like a water tank, a stream is like a water flow in a pipe, and the program is our final user. Flow is an abstract and dynamic concept, which is a series of continuous and dynamic data sets.

Figure 1 relationship between stream and source data source and destination data source

The first simple IO stream program and in-depth

When the program needs to read the data from the data source, it will open a stream to the data source through the IO stream object, and the data in the stream can be read sequentially through this IO stream object-related method.

[example 1] read data from a file through a stream object (for testing only, irregular writing)

Import java.io.*

Public class Demo01 {

Public static void main (String [] args) {

Try {

FileInputStream fis = new FileInputStream ("d:/a.txt"); / / the file content is: abc

Int S1 = fis.read (); / / print the ascii code value for the input character a 97

Int S2 = fis.read (); / / print the ascii code value 98 corresponding to the input character a

Int S3 = fis.read (); / / print the ascii code value 99 for the input character a

Int S4 = fis.read (); / / since the contents of the file have been read, return-1

System.out.println (S1)

System.out.println (S2)

System.out.println (S3)

System.out.println (S4)

Fis.close ()

/ / after using the stream object, it must be closed! Otherwise, the total occupation of system resources will eventually cause the system to crash!

} catch (Exception e) {

E.printStackTrace ()

}

}

}

[example 2] read data from a file through a stream object (classic code, be sure to master it)

Import java.io.*

Public class Demo02 {

Public static void main (String [] args) {

FileInputStream fis = null

Try {

Fis = new FileInputStream ("d:/a.txt"); / / content is: abc

StringBuilder sb = new StringBuilder ()

Int temp = 0

While ((temp=fis.read ())! =-1) {

Sb.append ((char) temp)

}

System.out.println (sb)

} catch (Exception e) {

E.printStackTrace ()

} finally {

Try {

If (fissure null) {

Fis.close ()

/ / Stream object, must be closed after use! This way of writing ensures that the stream object will be closed even if an exception is encountered.

}

} catch (IOException e) {

E.printStackTrace ()

}

}

}

}

The old bird suggested:

As the above code is a very typical IO stream code, the use of other stream objects is basically the same pattern!

Let's stop here in this section, and we'll continue to talk in the next section.

"full Stack Java Notes" is a series of Java engineer notes that can help you grow from zero to one. The author, known as Mr. G, has 10 years of experience in Java research and development. He has been engaged in software design and development in a research and development center of China Digital and Aerospace Academy, and has gradually become an engineer, senior engineer and architect. Proficient in Java platform software development, proficient in JAVAEE, familiar with various popular development frameworks.

The notes include six parts from shallow to deep:

Getting started with A-Java

B-database from entry to proficiency

C-hand blade moving front end and Web front end

D-J2EE from knowledge to actual combat

E-Java high-level framework refinement

F-Linux and Hadoop

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