User defined Exception subclass in Java

Java provides rich set of built-in exception classes like: ArithmeticException, IOException, NullPointerException etc. all are available in the java.lang package and used in exception handling. These exceptions are already set to trigger on pre-defined conditions such as when you divide a number by zero it triggers ArithmeticException.

Apart from these classes, Java allows us to create our own exception class to provide own exception implementation. These type of exceptions are called user-defined exceptions or custom exceptions.

You can create your own exception simply by extending java Exception class. You can define a constructor for your Exception (not compulsory) and you can override the toString() function to display your customized message on catch. Lets see an example.

Example: Custom Exception

In this example, we are creating an exception class MyException that extends the Java Exception class and

class MyException extends Exception
{
  private int ex;
  MyException(int a)
  {
    ex = a;
  }
  public String toString()
  {
    return "MyException[" + ex +"] is less than zero";
  }
}

class Demo
{
  static void sum(int a,int b) throws MyException
  {
    if(a<0)
    {
      throw new MyException(a); //calling constructor of user-defined exception class
    }
    else
    {
      System.out.println(a+b);
    }
  }

  public static void main(String[] args)
  {
    try
    {
      sum(-10, 10);
    }
    catch(MyException me)
    {
      System.out.println(me); //it calls the toString() method of user-defined Exception
    }
  }
}

MyException[-10] is less than zero

Example: Custom Exception

Lets take one more example to understand the custom exception. Here we created a class ItemNotFound that extends the Exception class and helps to generate our own exception implementation.

class ItemNotFound extends Exception
{
  public ItemNotFound(String s) {
    super(s);
  }

}

class Demo
{
  static void find(int arr[], int item) throws ItemNotFound
  {
    boolean flag = false;
    for (int i = 0; i < arr.length; i++) {
      if(item == arr[i])
        flag = true;
    }
    if(!flag)
    {
      throw new ItemNotFound("Item Not Found"); //calling constructor of user-defined exception class
    }
    else
    {
      System.out.println("Item Found");
    }
  }

  public static void main(String[] args)
  {
    try
    {
      find(new int[]{12,25,45}, 10);
    }
    catch(ItemNotFound i)
    {
      System.out.println(i);
    }
  }
}

ItemNotFound: Item Not Found

Points to Remember

1. Extend the Exception class to create your own exception class.
2. You don't have to implement anything inside it, no methods are required.
3. You can have a Constructor if you want.
4. You can override the toString() function, to display customized message.