Declaring an Exception in Java

Language

Version: SE 8

Object Oriented Programming

Apart from the built in exceptions Java provides, exception classes can be written by the programmer. These can then be thrown like the built-in Java exceptions.

Syntax

modifier exceptionName extends Exception {
    //fields and methods
}

modifier exceptionName extends RuntimeException { //for runtime exceptions
    //fields and methods
}

Notes

If the exception thrown is handled or declared, the class declared must extend the Exception class.

If the exception thrown is a runtime exception, the class declared must extend the RuntimeException class.

Example

//exception for negative numbers
public negativeNumberException extends Exception { //class header
    private int errorNumber;
    public negativeNumberException(int number){ //exception class constructor
        this.errorNumber = number;
    }
    public int getNumber() { //method within class
        return this.errorNumber;
    }
}

//a class that only takes positive numbers
public class positiveNumbers {
     private int number;
     //method within class throws negative number exception
     public positiveNumber(int number) throws negativeNumberException{
         if(number >= 0) this.number = number;
         else {
             throw new negativeNumberException(number);
         }
     }
}

//trying the exception, elsewhere in the code
try {
    positiveNumbers negativeOne = new positiveNumbers(-1);
} catch (negativeNumberException e) {
    System.out.println("ERROR: This number is not positive: " + e.getNumber());
}