What is IOException in Java?

Java is a general-purpose programming language used to develop various applications, websites, and embedded systems. While you write programs or develop applications using Java, you encounter exceptions or errors (in general language).

An exception in computer programming is an event that occurs during the program execution and disrupts the normal flow of the program’s instructions.

In Java, IOException is among the most common exceptions indicating errors in a program’s input and output operations. It can occur while reading or writing into a file, basically accessing the file system (files, directories, and streams).

This blog post aims to explain IOException in Java with examples and its types and common causes.

So, let us get started!

What is IOException in Java?

IOException in Java is a checked exception that represents an error in input and output operations. It serves as the base class for those exceptions occurring when accessing a file, directory, or stream. It is part of the java.io package and the subclass of the Exception class.

Checked exceptions are compile-time exceptions that programmers need to handle explicitly. They can be easily handled with a try-catch block .

Another major category of exceptions in Java is unchecked or runtime exceptions. These exceptions do not need explicit handling. It entirely depends on programmers whether to handle unchecked exceptions explicitly or not.

Some common subclasses of IOException in Java are FileNotFoundException, InterruptedIOException, SocketException, UnsupportedDataTypeException, etc.

Syntax of Java IOException

The following is the basic syntax of how to handle a Java IOException:

try {
//a block od code causing IOException
} catch (IOException e) {
//handling IOException
}

Causes of Java IOException

The following are some common causes of IOException in Java:

• File or directory not found.
• Incorrect file permissions.
• Disk full or write-protected.
• The failure of a network of an I/O device.
• Invalid user input.
• Malfunctioned URL and URI .

Constructors of IOException

Example

When you try to read a file that does not exist, the Java compiler throws an IOException.

Example

import java.io.*;
public class Example
{
public static void main(String[] args) 
{
FileReader f1 = new FileReader("file.txt");
System.out.println(f1.read()); 
f1.close();
}
}

Output

The file name is “Example”. Ensure to use the class name the same as the file name.

Compile the program file using the “javac [filename]”. As IOException is a compile-time error, you get errors immediately after you compile it.

javac Example.java

Example.java:6: error: unreported exception FileNotFoundException; must be caught or declared to be thrown
FileReader f1 = new FileReader("file.txt");
^
Example.java:8: error: unreported exception IOException; must be caught or declared to be thrown
System.out.println(f1.read());
^
Example.java:10: error: unreported exception IOException; must be caught or declared to be thrown
f1.close();
^
3 errors

Explanation

FileReader is a class to read data from a file. When the FileReader object, f1, tries to read data from the file “file.txt”, which does not exist, we get unreported exceptions FileNotFoundException and IOException .

How to Handle IOExceptions in Java?

The best way to handle IOException in Java is to use a try-catch block.

A try-catch block has a separate try block and a catch block.

• The try block is used to enclose the code that might throw an exception. If an exception occurs at any line of code in this block, the remaining code in the block will not be executed.
• The catch block handles exceptions. You need to declare the exception type as a parameter of the catch block. Ensure to write the parent class of the encountered exception. For instance, in the above example, we have encountered FileNotFoundException, whose parent class is IOException.

Syntax

try{ 
//code that can cause an exception 
}catch(Exception_class_name ref)

Remember to use the catch block after the try block. A single try block can have multiple catch blocks.

Here is a solution for the above IOException, FileNotFoundException.

Solution for FileNotFoundException

We will use a simple try-catch block to handle the unreported exceptions we encountered in the above example.

import java.io.*;
public class Example
{
public static void main(String[] args) 
{
try{
FileReader f1 = new FileReader("file.txt");
System.out.println(f1.read()); 
f1.close();
}
catch(IOException ie) { 
System.out.println("An error occurred: " +ie.getMessage());
}
}
}

Output

An error occurred: file.txt (No such file or directory)

We have successfully handled FileNotFoundException and IOException using a try-catch block.

Java IOException Examples

Here are some common examples to explain how IOException occurs.

1. Downloading a File With an Incorrect URL

Example

import java.io.*;
import java.net.*;
public class HelloWorld
{
public static void main(String[] args)
{
URL u1 =new URL("http://helloworld21.com/file.txt"); 
InputStream in = u1.openStream();
}
}

Output

Compile the program file using javac followed by the file name.

HelloWorld.java:8: error: unreported exception MalformedURLException; must be caught or declared to be thrown
URL u1 =new URL("http://helloworld21.com/file.txt");
^
HelloWorld.java:10: error: unreported exception IOException; must be caught or declared to be thrown
InputStream in = u1.openStream();
^
2 errors

Explanation

Here, we encountered unreported exceptions MalformedURLException and IOException.

The program creates the URL object, u1, representing the URL http://helloworld21.com/file.txt . The InputStream in enables reading of the resource located at the specified URL. However, due to an incorrect URL, the Java compiler throws MalformedURLException.

Solution

The solution is the same, i.e., using the try-catch block to catch the exception.

import java.io.*;
import java.net.*;
public class HelloWorld
{
public static void main(String[] args)
{
try{
URL u1 =new URL("http://helloworld21.com/file.txt");
InputStream in = u1.openStream();
}
catch(IOException ie){
System.out.println("An error occurred: " +ie.getMessage());
}
}
}

Output

An error occurred: helloworld21.com

The program with a try-catch block executes successfully without any error or exception.

2. Encoding the String Using an Invalid Format

Example

import java.io.*;
public class Main {
public static void main(String[] args) throws Exception {
String str = "TechGeekBuzz";
byte[] programming;
//We are encoding the string using invalid format. 
//It results in UnsupportedEncodingException. 
programming = str.getBytes("Techies World for Tech Geeks");
}
}

Output

Exception in thread "main" java.io.UnsupportedEncodingException: Techies World for Tech Geeks
at java.base/java.lang.StringCoding.encode(StringCoding.java:427)
at java.base/java.lang.String.getBytes(String.java:941)
at Main.main(Main.java:8)

Explanation

In the above example, we have encoded the given string using an unsupported encoding format, i.e., Bytes. Hence, we get UnsupportedEncodingException.

Solution

import java.io.*;
public class Main {
public static void main(String[] args) throws Exception {
try{
String str = "TechGeekBuzz";
byte[] programming;
//We are encoding the string using invalid format. 
//It results in UnsupportedEncodingException. 
programming = str.getBytes("Techies World for Tech Geeks");
}
catch(IOException ie)
{
System.out.println("An error occurred - " +ie.getMessage());
}
}
}

Output

An error occurred - Techies World for Tech Geeks

Types of IOException in Java

Let us now discuss some common types of IOException in Java.

• FileNotFoundException: It occurs when you access a file that does not exist.

• SocketException: When there is an error in an underlying network protocol, you encounter SocketException.

• EOFException: When the end of the file is reached unexpectedly, you get EOFException.

• InterruptedIOException: You get this exception when your I/O operation is interrupted.

• FileLoadException: When the file you want to access is available but cannot be loaded, you receive FileLoadException.

• DirectoryNotFoundException: When you try to access a directory that does not exist, you receive DirectoryNotFoundException.

• UnsupportedEncodingException: You encounter this exception when working with character encoding. Character encoding is the process of representing information in binary format.

Conclusion

This was all about IOException in Java. It occurs when dealing with input/output operations, such as reading or writing data to files, directories, and streams. It is a checked exception or compile-time exception that a programmer needs to handle explicitly. IOException serves as the base class for many other subclasses, including FileNotFoundException, FileLoadException, and UnsupportedEncodingException.

The best solution to handle this exception is using a try-catch block. Enclose the code block that causes IOException in the try block. The catch block will handle the exception.

If you have any doubts regarding this article, feel free to ask queries in the comments.

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