Java IO - Input/Output in Java with Examples

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Java provides various Streams with its I/O package that helps the user to perform all the input-output operations. These streams support all the types of objects, data-types, characters, files, etc., to fully execute the I/O operations.

The image below demonstrates the flow of data from a source to a destination.

Standard or Default Streams in Java

Before exploring various input and output streams, let's look at 3 standard or default streams that Java has to provide, which are also most commonly used:

• System.in: This is the standard input stream (System.in) that is used to read characters from the keyboard or any other standard input device.
• System.out: This is the standard output stream (System.out) that is used to produce the result of a program on an output device like the computer screen.

Here is a list of the various print functions that we use to output statements:

Java Print Functions Used with System.out

Now, we are going to discuss the main print function used with System.out which are listed below:

1. print(): This method in Java is used to display a text on the console. This text is passed as the parameter to this method in the form of String. This method prints the text on the console and the cursor remains at the end of the text at the console. The next printing takes place from just here.

Syntax:

System.out.print(parameter);

Example: Java program to illustrate print()

import java.io.*;

class Geeks {
    public static void main(String[] args)
    {
        // using print() all are printed in the same line
        System.out.print("GfG! ");
        System.out.print("GfG! ");
        System.out.print("GfG! ");
    }
}

GfG! GfG! GfG! 

2. println(): This method in Java is also used to display a text on the console. It prints the text on the console and the cursor moves to the start of the next line at the console. The next printing takes place from the next line.

Syntax:

System.out.println(parameter);

Example: Java program to illustrate println()

import java.io.*;

class Geeks {
    public static void main(String[] args)
    {
        // using println() all are printed in the different line
        System.out.println("GfG! ");
        System.out.println("GfG! ");
        System.out.println("GfG! ");
    }
}

GfG! 
GfG! 
GfG! 

3. printf(): This is the easiest of all methods as this is similar to printf in C. Note that System.out.print() and System.out.println() take a single argument, but printf() may take multiple arguments. This is used to format the output in Java.

Example:

class Geeks {
  
    public static void main(String[] args) {
        int x = 100;
      
        // Printing a simple integer
        System.out.printf("Printing simple integer: x = %d%n", x);

        // Printing a floating-point number with precision
        System.out.printf("Formatted with precision: PI = %.2f%n", Math.PI);

        float n = 5.2f;

        // Formatting a float to 4 decimal places
        System.out.printf("Formatted to specific width: n = %.4f%n", n);

        n = 2324435.3f;

        // Right-aligning and formatting a float to 20-character width
        System.out.printf("Formatted to right margin: n = %20.4f%n", n);
    }
}

Printing simple integer: x = 100
Formatted with precision: PI = 3.14
Formatted to specific width: n = 5.2000
Formatted to right margin: n =         2324435.2500

System.err Example

It is used to display the error messages. It works similarly to System.out with print(), println(), and printf() methods.

Example: Java Program demonstrating System.err

public class Geeks {
  
    public static void main(String[] args) {
      
        // Using print()
        System.err.print("This is an error message using print().\n");

        // Using println()
        System.err.println("This is another error message using println().");

        //Using printf()
        System.err.printf("Error code: %d, Message: %s%n", 404, "Not Found");
    }
}

Output:

This is an error message using print().
This is another error message using println().
Error code: 404, Message: Not Found

Types of Streams

Depending on the type of operations, streams can be divided into two primary classes:

1. Input Stream: These streams are used to read data that must be taken as an input from a source array or file or any peripheral device. For eg., FileInputStream, BufferedInputStream, ByteArrayInputStream etc.

2. Output Stream: These streams are used to write data as outputs into an array or file or any output peripheral device. For eg., FileOutputStream, BufferedOutputStream, ByteArrayOutputStream etc.

Types of Streams Depending on the Types of File

Depending on the types of file, Streams can be divided into two primary classes which can be further divided into other classes as can be seen through the diagram below followed by the explanations.

1. ByteStream: This is used to process data byte by byte (8 bits). Though it has many classes, the FileInputStream and the FileOutputStream are the most popular ones. The FileInputStream is used to read from the source and FileOutputStream is used to write to the destination.

Here is the list of various ByteStream Classes:

Example: Java Program illustrating the Byte Stream to copy contents of one file to another file.

import java.io.*;
public class Geeks {
    public static void main(
        String[] args) throws IOException
    {

        FileInputStream sourceStream = null;
        FileOutputStream targetStream = null;

        try {
            sourceStream
                = new FileInputStream("sorcefile.txt");
            targetStream
                = new FileOutputStream("targetfile.txt");

            // Reading source file and writing content to target file byte by byte
            int temp;
            while ((
                       temp = sourceStream.read())
                   != -1)
                targetStream.write((byte)temp);
        }
        finally {
            if (sourceStream != null)
                sourceStream.close();
            if (targetStream != null)
                targetStream.close();
        }
    }
}

Output:

Shows contents of file test.txt

2. CharacterStream: In Java, characters are stored using Unicode conventions (Refer this for details). Character stream automatically allows us to read/write data character by character. Though it has many classes, the FileReader and the FileWriter are the most popular ones. FileReader and FileWriter are character streams used to read from the source and write to the destination respectively.

Here is the list of various CharacterStream Classes:

Example:

import java.io.*;

public class Geeks 
{
    public static void main(String[] args) throws IOException
    {
        FileReader sourceStream = null;
        
      	try {
            sourceStream = new FileReader("test.txt");

            // Reading sourcefile and writing content to target file character by character.
            int temp;
          
            while (( temp = sourceStream.read())!= -1 )
                System.out.println((char)temp);
        }
        finally {
          
            // Closing stream as no longer in use
            if (sourceStream != null)
                sourceStream.close();
        }
    }
}

Note: Please make sure that the file test.txt is in the same directory, this file contains the text which we want to read.

Output in Java

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