Exception Handling Program in Java

Exception Handling Program in Java

Exception means something that is abnormal. In Java, an exception is treated as a problem that disrupts the normal flow of the program. An exception leads to abnormal termination of the Java program. There can be many scenarios where an exception can occur. A few of them are mentioned below.

• When a number is divided by zero, for example, int x = 6 / 0; This statement is syntactically correct, but zero can never divide a number mathematically. Hence, an exception is raised.
• Connecting to a server, but the server is off.
• Opening a file, but the file is already deleted by someone.
• Accessing the index that is not present in the array.
  For example, in the statement int arr[] = {7, 8, 9};, the array contains three elements arr[0], arr[1], arr[2], but if someone tries to access arr[10], then it is not possible as 10^th index is not present in the array leading to the generation of an exception.
• When JVM has run out of memory.
• User is entering some invalid data. For example, someone types 1342 when asked to write his/ her name.

Some of the exception scenarios mentioned above are due to programmer error; some are due to failure of the resources, while others are due to user error.The exception handling program in Java demonstrates how to handle exceptions in order to maintain the natural flow of the program.

Hierarchical Structure of Exception in Java

All classes of exception are derived from the class java.lang.Exception, while the Exception class itself is the child class of the class Throwable. Errors are also the subtypes of the class Throwable.

The need of handling an Exception

Suppose there are 8 statements in a Java program, and an exception has occurred at the 4^th statement. Due to exception, the normal flow of the program is interrupted that leads to abnormal termination of the program. Thus, the statements that are coming after the 4^th statement never get executed. To ensure that statements after the 4^th statement are executed, or to ensure the normal flow of the program, it is required to handle the exception that is raised due to the 4^th statement.

Types of Exception in Java

Exceptions in Java can be categorized into the following three categories:

1) Checked Exception

2) Unchecked Exception

3) Error

Let’s discuss each type of exception in detail.

Checked Exception

Checked exceptions are those exceptions that are notified or handled during compile time. Therefore, these exceptions are also called the compile-time exception. These exceptions can never be ignored as it is generated during the compile-time. For example, if we try to access a file that is not present, it thows FileNotFoundException. Consider the following code.

FileName: FileNotFoundExceptionDemo.Java

 // import class FileReader
import java.io.FileReader;
// import class File
import java.io.File;
public class FileNotFoundExceptionDemo
{
// Driver Method
public static void main(String argvs[])
{                            
  // creating an fileObj using the text file path
  // along with its name
  File fileObj = new File("D:\\file.txt");
  // creating the object fr using the object fileObj
  // This object fr is used to read the file
  FileReader fr = new FileReader(fileObj);
  int rd;  
  // reading the file character by character
  // till the end of file
  while( (rd = fr.read()) != -1)  
  { 
    // printing the characters present in the file
               System.out.print( (char) rd);
  }
  // closing the filereader
  fr.close();   
}
} 

Upon compilation, the following exceptions are raised.

Output:

Explanation: We have seen that we never reached the execution stage because exceptions are raised during the compilation stage.  The methods read(), and close() are the methods of the class FileReader. These methods throw IOException, and the same has been notified by the Java compiler in the output.

Eliminating Checked Exception

We can use a try-catch block to eliminate the exceptions that came in the above-written program. The following code illustrates the same.

FileName: ExceptionHandleDemo.Java

 // import class FileReader
import java.io.FileReader;
// import class File
import java.io.File;
// Importing Exception classes FileNotFoundException & IOException
import java.io.FileNotFoundException;
import java.io.IOException;
public class ExceptionHandleDemo
{
// Driver Method
public static void main(String argvs[])
{             
  // creating an fileObj using the text file path
  // along with its name
  File fileObj = new File("D:\\file.txt");
  FileReader fr = null;
  try
  {
      // creating the object fr using the object fileObj
      // This object fr is used to read the file
      fr = new FileReader(fileObj);
  }
  // handling the FileNotFoundException
  catch(FileNotFoundException fe)
  {
      System.out.println("Inside the catch block of FileNotFoundException");
      fe.printStackTrace();
  }
  int rd;  
  try
  {
      // reading the file character by character
      // till the end of file
      while( (rd = fr.read()) != -1)  
      { 
        // printing the characters present in the file
               System.out.print( (char) rd);
      }
      // closing the filereader
      fr.close();
  }
  // handling the IOException & NullPointerException
  catch(IOException | NullPointerException ex)
  {
      System.out.println("Inside the catch block of NullPointerException & IOException");
      ex.printStackTrace();
  }
}
} 

Output:

Explanation: We have seen because of the statement fr = new FileReader(fileObj); FileNotFoundException is thrown. To handle the exception, the statement fr = new FileReader(fileObj); is put inside the try block, and the raised Exception is handled in the catch block. Similarly, NullPointerException and IOException are also handled. Observe that this time the termination of the program happened normally.

Unchecked Exception

Exceptions that are generated during the execution are called unchecked exception. These exceptions are also known as runtime Exception. These exceptions mainly occur because of some mistakes (logical mistakes, not proper usage of the API) done by the programmer/ developer. For example, if an array is declared of size 7, and the program is trying the access the 8^th element, it generates an exception. The following program illustrates the same.

FileName: ArrayIndexExample.Java

 public class ArrayIndexExample
{
public static void main(String argvs[])
{
// declaring an integer array
int arr[] = {6, 7, 8, 6, 8, 0, 4};
// calculating size of the array
int size = arr.length;
// printing size of the array
System.out.println("Size of the array is " + size );
// displaying the 8th index of the array
System.out.println("The 8th index of the array is " + arr[8] );
}
} 

Output:

Explanation: In the array, the memory is allocated for the seven integers i.e. 0 to 6. Therefore, the 8^th index goes beyond the range of indices of the array arr[]. Hence, we get the exception java.lang.ArrayIndexOutOfBoundsException. If we look at the code syntactically, we find that the statement System.out.println(“The 8th index of the array is ” + arr[8] ); is correct. The Java compiler interprets that the code is accessing the element that is present at the 8^th index. However, the Java compiler has no idea about the memory allocation of the array. This is because memory allocation happens at the run-time. Therefore, we get the exception java.lang.ArrayIndexOutOfBoundsException at the run-time.

Handling Unchecked Exception

Similar to handling a checked exception, a try-catch blocks come in handy in handling unchecked too. The java.lang.ArrayIndexOutOfBoundsException raised in the above program can be handled by the following program.

FileName: ExceptionHandleDemo1.Java

 public class ExceptionHandleDemo1
{
public static void main(String argvs[])
{
// declaring an integer array
int arr[] = {6, 7, 8, 6, 8, 0, 4};
// calculating size of the array
int size = arr.length;
// printing size of the array
System.out.println("Size of the array is " + size );
try
{
    // displaying the 8th index of the array
    System.out.println("The 8th index of the array is " + arr[8] );
}
catch(ArrayIndexOutOfBoundsException excptn)
{
    System.out.println("Inside the catch block of ArrayIndexOutOfBoundsException");
    excptn.printStackTrace();
}
}
} 

Output:

Explanation: This time also the statement that is responsible for raising the ArrayIndexOutOfBoundsException has been put inside a try block. The raised exception is then handled in the catch block.

Errors

Errors are not exceptions. Errors are those problems that are out of the reach of the developers. Errors are not given that much importance because one can hardly do anything about them. Generally, a program can never recover from an error. In other words, errors are not recoverable. Errors occur when there are severe failures. A few of the errors generated by Java environments are VirtualMachineError, OutOfMemoryError, AsserstionError, etc.