Java FileNotFoundException

Introduction to Java FileNotFoundException

Java FileNotFoundException is a type of exception that often occurs while working with File APIs in Java where the path specified for a file for reading or writing purposes in constructor of classes FileInputStream, FileOutputStream, and RandomAccessFile, either does not exist or inaccessible due to an existing lock or other technical issues. This is a checked exception is a direct subclass of IOException that has been introduced with JDK 1.0. Also it contains two type of constructors that can be called where one returns an Exception with null message to displayed whereas other prints the specified message in case the exception occurs.

Syntax:

public class FileNotFoundExceptionextends IOException

• public: The keyword public refers that the given class is accessible from any class in the project thus need to be inherit it to throw an exception.

This class is a direct subclass of IOException thus inherits all the methods and variables of this class.

How FileNotFoundException work in Java?

FileNotFoundException is a checked exception is used that occurs when a file path specified for accessing does not exist or is inaccessible. With checked exception it means that java compiler checks at compile time if this exception has been handled or not, otherwise compile-time error occurs. Lets see how exception is thrown at run-time in case it has been handled using try-catch blocks or using throws keyword in its defination at compiler time.

Example:

File fileObj = new File(“C:/JavaPractice.txt”)

Suppose we instantiate a File class object as given above with a path of a file and that file does not exist. In that case when compiler attempts to read or write the file and finds such situation it throws an exception and create an instance of FileNotFoundExceptionclass. In case it is not specified which constructor needs to be called, the constructor with no error message is thrown.

Thus the application fails with below error:

Constructors of Java FileNotFoundException

FileNotFoundException is a subclass of IOException is very useful to trace if the file specified in the file path exist and even accessible. Thus for using this one needs to instantiate it and it is a public class it can be instantiated from any where in the project. And for creating the instance of the class has 2 types of constructors.

Given below are the two types of constructors:

1. Constructor with no error message

This type of constructor is used to create an instance of FileNotFoundException class where it returns null as its error detail message.

Syntax:

public FileNotFoundException()

Example:

FileNotFoundExceptionexcepObj = new FileNotFoundException()

2. Constructor with an error message

This type of constructor is used to create an instance of FileNotFoundException class where it returns specified string as its error detail message.

Syntax:

public FileNotFoundException(String s)

Example:

FileNotFoundExceptionexcepObj = new FileNotFoundException(“This is a FileNotFoundException”)

The error message specified can be easily retrieved using Throwable.getMessage() method, since this is one of the superclass of FileNotFoundException class.

Examples of Java FileNotFoundException

Given below are the examples mentioned:

Example #1

Here we see how exception is thrown by JVM if one file in inaccessible. In this the error message displaying in output is one specified by default by JVM.

Code:

//package Proc;
import java.io.Console;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.PrintWriter;
public class prac1 {
public static void main(String[] args) {
File fileObj = new File("D:/JavaPractice.txt");
FileInputStream fISObj = null;
try{
fISObj = new FileInputStream(fileObj);
while (fISObj.read()!=-1){
System.out.println(fISObj.read());
}
}catch (FileNotFoundException e){
e.printStackTrace();
}catch (IOException e){
e.printStackTrace();
}
}
}

Output:

Example #2

In this example we will use the constructor with specified error message to display the error when file does not exist at the given path. We have used throw keyword to throw the exception.

Code:

//package Proc;
import java.io.Console;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.PrintWriter;
public class prac1 {
public static void main(String[] args) throws FileNotFoundException,IOException{
File fileObj = new File("D:/JavaPractice.txt");
if(!fileObj.exists()){
throw new FileNotFoundException("This file doesnot exist in the path specified "+fileObj.toString());
}
else {
System.out.println("Welcome, we got into file "+fileObj.toString());
}
}
}

Output:

How to avoid FileNotFoundException?

Getting a FileNotFoundException in an application makes an application inefficient. First step to avoid this exception is to check if the specified file exists in at the specified path but still there might occur a situation in real-time applications that the file is missing or if the file to be read is locked by other process to write into it.

Case 1: File is missing

To avoid this one can use java.io.File.exists() method to check if the file one attempts to read exist on the path specified or not. Using this we must make sure if our code is able to handle FileNotFoundException exception.

Case 2: File is inaccessible

To avoid such cases, one needs to take care if the file we are attempting to read is currently locked by other users writing it. For this we can use canRead() or canWrite() methods of java.io. File class that tells if the specified file is available for reading or writing purposes or not.

Using these 2 precautionary measures one can easily avoid an attempt by instance of file class to open a file that can result into a checked exception. Thus improves the efficiency of an application that includes program to access files from a specified paths.

Conclusion

FileNotFoundException is a type of checked exception that occurs once an attempt is made to the file that either does not exist or not accessible at that moment due to some lock. Since it is a checked exception java compiler ensures it has been handled at compile time. But still if one needs to avoid it so they can use exist(), canRead() or canWrite() methods present in File class.

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