What are the rules of exception handling with respect to method overriding in java?

While a superclass method throws an exception while overriding it you need to follow the certain rules.

Should throw Same exception or, sub type

If the super-class method throws certain exception, the method in the sub-class should throw the same exception or its sub type.

Example

In the following example, the readFile() method of the super-class throws an IOEXception and, the readFile() method of the sub-class throws FileNotFoundException exception.

Since the FileNotFoundException exception is the sub type of the IOException this program gets compiled and executed without any errors.

import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.Scanner;
abstract class Super {
   public String readFile(String path) throws IOException {
      throw new IOException();
   }
}
public class ExceptionsExample extends Super {
   @Override
   public String readFile(String path) throws FileNotFoundException {
      Scanner sc = new Scanner(new File("E://test//sample.txt"));
      String input;
      StringBuffer sb = new StringBuffer();
      while (sc.hasNextLine()) {
         input = sc.nextLine();
         sb.append(" "+input);
      }
      return sb.toString();
   }
   public static void main(String args[]) {
      String path = "E://test//sample.txt";
      ExceptionsExample obj = new ExceptionsExample();
      try {
         System.out.println(obj.readFile(path));
      }catch(FileNotFoundException e) {
         System.out.println("Make sure the specified file exists");
      }
   }
}

Output

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Example

In the same way if the sub-class throws the same exception as the super-class the program gets compiled and executed successfully.

import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.Scanner;
abstract class Super {
   public void sampleMethod()throws FileNotFoundException {
      System.out.println("Method of superclass");
   }
}
public class ExceptionsExample extends Super {
   public void sampleMethod()throws FileNotFoundException {
      System.out.println("Method of Subclass");
   }
   public static void main(String args[]) {
      ExceptionsExample obj = new ExceptionsExample();
      obj.sampleMethod();
   }
}

Output

Method of Subclass

Should not throw an exception of super type

If the super-class method throws certain exception, the method in the sub-class should not throw its super type.

Example

In the following example the readFile() method of the super-class throws FileNotFoundException exception and, the readFile() method of the sub-class throws an IOException, which is the super type of the FileNotFoundException.

import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.Scanner;
abstract class Super {
   public String readFile(String path)throws FileNotFoundException {
      throw new FileNotFoundException();
   }
}
public class ExceptionsExample extends Super {
   @Override
   public String readFile(String path)throws IOException {
      //method body ......
   }
}

Compile time error

On compiling, the above program gives you the following output −

ExceptionsExample.java:13: error: readFile(String) in ExceptionsExample cannot override readFile(String) in Sup
   public String readFile(String path)throws IOException {
                 ^
   overridden method does not throw IOException
1 error

Without throwing any exception

If the super-class method throws certain exception, you can override it without throwing any exception.

Example

In the following example the sampleMethod() method of the super-class throws FileNotFoundException exception and, the sampleMethod() method does not throw any exception at all. Still this program gets compiled and executed without any errors.

import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.Scanner;
abstract class Super {
   public void sampleMethod()throws FileNotFoundException {
      System.out.println("Method of superclass");
   }
}
public class ExceptionsExample extends Super {
   public void sampleMethod() {
      System.out.println("Method of Subclass");
   }
   public static void main(String args[]) {
      ExceptionsExample obj = new ExceptionsExample();
      obj.sampleMethod();
   }
}

Output

Method of Subclass