What is Java Input/Output?

Java Input/Output (I/O) is used to process input and generate output in the form of files. Java uses the concept of streams, which allows for fast I/O operations.
With the java.io package, all input and output operations can be easily performed.

Handling Files in Java Using Input/Output

Streams

Streams can be defined as a sequence of data composed of bytes. It's called a stream because it is like a stream of water that continues to flow. There are two kinds of Streams:

Input Stream: Used to read data from a source. This could be a file, array, peripheral device, or socket.
Output Stream: Used to write data to a destination. This could be a file, array, peripheral device, or socket.
The flow for an input stream is illustrated below:

Byte to Stream

Java byte stream are used to perform input and output of 8-bit bytes. Following is an example which makes use of these two classes to copy an input file into an output file −

import java.io.FileInputStream; import java.io.FileNotFoundException; import java.io.FileOutputStream; import java.io.IOException;  public class LearnStream {     public static void main(String[] args) throws IOException {         var directory = "D://sample/stream/";        var fileInput = new FileInputStream(directory+"input.txt");        var fileOutput = new FileOutputStream(directory+"output.txt");         try{            int i;            while((i= fileInput.read())!=-1){                fileOutput.write(i);            }        } catch (IOException e) {            throw new RuntimeException(e);        }      }  }  

Now let's have a file input.txt with the following content

Dimas Priyandi Software Developer Java Angular  Spring Boot 

Running program and we get a file with name output.txt with the following content

Dimas Priyandi Software Developer Java Angular  Spring Boot 

Example

To better understand file input streams and file output streams, let's create a new sample where we have an input file named count.txt.

The contents of count.txt are as follows:

100 90 80 70 60 50 40 30 20 10 0 

When the File Input Stream reads the numerical data from the file count.txt, we will store it in an array and then perform a summation operation to calculate the total sum of the data. Please follow the program code below:

import java.io.*;  public class LeanCount {     public static void main(String[] args) throws FileNotFoundException {         var directory = "D://sample/stream/";         var fileInput = new FileInputStream(directory+"count.txt");         var fileOutput = new FileOutputStream(directory+"sum.txt");         Integer sum = 0;         try{             var reader = new BufferedReader(new InputStreamReader(fileInput));             var outputWriter = new BufferedWriter(new OutputStreamWriter(fileOutput));             String line;             while((line=reader.readLine()) !=null){                 sum+=Integer.parseInt(line);             }             reader.close();             outputWriter.write(sum.toString());             outputWriter.close();           } catch (IOException e) {             throw new RuntimeException(e);         }      }      }    

Output:

• we can see output of sum.txt is :

550 

Explanation:

• FileInputStream: Used to read bytes from the file count.txt.
• InputStreamReader: Converts the bytes read by FileInputStream into characters.
• BufferedReader: Provides the readLine() method to read the file line by line.
• Numbers Array: An integer array to store the numbers read from the file.
• Sum Calculation: A loop to sum up all the numbers stored in the array.
• FileOutputStream: Used to write bytes to the file.
• OutputStreamWriter: Converts the character stream to a byte stream.
• BufferedWriter: Provides buffering for writing characters, arrays, and lines efficiently.

Summary

Java's I/O streams provide a powerful way to handle file operations. By using InputStream and OutputStream, and their buffered counterparts, you can efficiently read from and write to files.