Custom exceptions in Java


You can create your own exceptions in Java and they are known as user-defined exceptions or custom exceptions.

To create a user-defined exception extend one of the above-mentioned classes. To display the message override the toString() method or, call the superclass parameterized constructor bypassing the message in String format.

MyException(String msg){
   super(msg);
}
Or,
public String toString(){
   return " MyException [Message of your exception]";
}

Then, in other classes wherever you need this exception to be raised, create an object of the created custom exception class and, throw the exception using the throw keyword.

MyException ex = new MyException ();
If(condition……….){
   throw ex;
}

Custom Checked and Custom Unchecked

  • All exceptions must be a child of Throwable.

  • If you want to write a checked exception that is automatically enforced by the Handle or Declare Rule, you need to extend the Exception class.

  • If you want to write a runtime exception, you need to extend the RuntimeException class.

Example: Custom Checked exception

The following Java program demonstrates how to create a Custom checked exception.

 Live Demo

import java.util.Scanner;
class NotProperNameException extends Exception {
   NotProperNameException(String msg){
      super(msg);
   }
}
public class CustomCheckedException {
   private String name;
   private int age;
   public static boolean containsAlphabet(String name) {
      for (int i = 0; i < name.length(); i++) {
         char ch = name.charAt(i);
         if (!(ch >= 'a' && ch <= 'z')) {
            return false;
         }
      }
      return true;
   }
   public CustomCheckedException(String name, int age){
      if(!containsAlphabet(name)&&name!=null) {
         String msg = "Improper name (Should contain only characters between a to z (all small))";
         NotProperNameException exName = new NotProperNameException(msg);
         throw exName;
      }
      this.name = name;
      this.age = age;
   }
   public void display(){
      System.out.println("Name of the Student: "+this.name );
      System.out.println("Age of the Student: "+this.age );
   }
   public static void main(String args[]) {
      Scanner sc= new Scanner(System.in);
      System.out.println("Enter the name of the person: ");
      String name = sc.next();
      System.out.println("Enter the age of the person: ");
      int age = sc.nextInt();
      CustomCheckedException obj = new CustomCheckedException(name, age);
      obj.display();
   }
}

Compile-time exception

On compiling, the above program generates the following exception.

CustomCheckedException.java:24: error: unreported exception NotProperNameException; must be caught or declared to be thrown
   throw exName;
   ^
1 error

Example: Custom unChecked exception

If you simply change the class that your custom exception inherits to RuntimeException it will be thrown at run time

class NotProperNameException extends RuntimeException {
   NotProperNameException(String msg){
      super(msg);
   }
}

If you run the previous program By replacing the NotProperNameException class with the above piece of code and run it, it generates the following runtime exception.

Runtime exception

Enter the name of the person:
Krishna1234
Enter the age of the person:
20
Exception in thread "main" july_set3.NotProperNameException: Improper name (Should contain only characters between a to z (all small))
   at july_set3.CustomCheckedException.<init>(CustomCheckedException.java:25)
   at july_set3.CustomCheckedException.main(CustomCheckedException.java:41)
raja
Published on 15-Oct-2019 10:54:11
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