What is the difference between handled and un-handled exceptions

Question

I'd like to know the difference between handled and unhandeled Exceptions in Java. which one i should use a try/catch block and when I have to extend throwables.

also, if i extend throwables, do i always have to handle the exception in a separate class?

If exception is to be handled in a separate class, can i create custom functionality (e.g. Invoke another method or break a loop) instead of overriding the super constructor with just a custom message?

Deaconry

The question is a bit unclear. Do you have a code example to discuss this? – Allene

Am actually looking for a more generic definition, I have no particular example. – Deaconry

Answer 1

I presume by 'handled and unhandled' exceptions you mean 'checked and unchecked'.

Unchecked exceptions - All classes which extend RuntimeException are called unchecked. They usually indicate programming bugs, such as logic errors or improper use of an API.

Example of unchecked exceptions:

● ArithmeticException
● NullPointerException
● IndexOutOfBoundsException
● IllegalArgumentException
● ClassCastException

For example if you try to access the 10th cell of an array with 5 cells only, it would cause an ArrayIndexOutOfBoundsException. This is a programmer bug and a programmer`s fault, so it should be treated as such. This exception should not be handled with try/catch, but instead should be checked with an if statement for the size of the array.

Checked exceptions - These are exceptional conditions that a well-written application should anticipate and recover from. Checked exceptions in Java extend the Exception class but do not extend RuntimeException class. Checked exceptions are subject to the “Catch or Specify Requirement“:

When in method's body some code may throws checked exception, the method must either handle this exception or specify that it may throws this exception

Examples of checked exceptions:

● FileNotFoundException
● IOException
● SQLException

For example you may have a perfectly well-written code that reads or writes data into a file, but the file may be suddenly deleted from the file system by another user. This is not a programmer`s error, but it CAN happen, so you MUST anticipate and handle this situation.

So to summarize: Unchecked exceptions SHOULD NOT BE handled with try/catch - they should be treated as a bug and should be fixed (or avoided with if statements). Ofcourse you can use a try/catch block to handle a RuntimeException (for example NullPointerException), but it is NOT a good practice.

Checked exceptions MUST BE either handled with try/catch blocks or when the method does not know how to handle them, they should be declared to be thrown by the method itself. Thus the responsibility for handling the exception is transferred to the methods that would invoke this very method. This is what the Catch or Specify requirement says.

Lagunas

I believe you are on the right track by referring to the 'checked and unchecked' exceptions. i just need more clarification on one point, in your last paragraph, you said that "when the method does not know how to handle them, they should be declared to be thrown by the method itself.". as far as i know you can use the throws keyword to throw that exception. which, is basically ganna terminate the process. So, how you can handle the throwable to a custom error message and a possibility for a retry with different inputs? – Deaconry

Yes, you can use the throws keyword to specify that the method can throw this exception. You can create your own exception by creating a class that extends Exception. You can override the constructor that requires a String message and thus provide your own description. However, when you want to retry with different inputs, you have to handle this in the catch block rather than in the exception object itself – Lagunas