Throwing custom exceptions in Java versus built in exceptions

Question

I'm a little confused because I want to be able to throw a custom exception in Java. To do this inside a function, I have to add throws MyException to the header of the function.

And then anything that calls that function in turn must add throws MyException or have a try-catch block. But why is this?

For example, when creating a Stack in java and calling the pop function, I don't have to have a try-catch and yet the pop method in Java throws a NoSuchElementException (or w/e it is) if there isn't an element on the stack.

You absolutely, totally, completely want to throw your own, "checked" exception. Checked exceptions are Good. "Advertising" the exceptions your method can throw as a standard part of the method signature is Good. "Overloading" system, unchecked exceptions for your own use is Bad. Don't do it :) — paulsm4, Dec 23 '12 at 08:04
http://docs.oracle.com/javase/tutorial/essential/exceptions/index.html and more specifically, http://docs.oracle.com/javase/tutorial/essential/exceptions/catchOrDeclare.html — Brian, Dec 23 '12 at 08:13
@paulsm4 "The price of checked exceptions is an Open/Closed Principle violation. If you throw a checked exception from a method in your code and the catch is three levels above, you must declare that exception in the signature of each method between you and the catch. This means that a change at a low level of the software can force signature changes on many higher levels." —Robert C. Martin, «Clean Code», page 107 — bowmore, Dec 23 '12 at 08:29

score 4 · Accepted Answer · edited Jun 20 '20 at 09:12

4

NoSuchElementException is a RuntimeException (un-checked exception) and we don't need to handle or declare RuntimeExceptionS, thus the compiler wont complain, but instead throw it at runtime.

In the case of checked exceptions i.e., all the exceptions which are not subtypes of RuntimeException, the compiler will check for a suitable catch clause, enabling the program to continue its course of execution, after performing the operations within the catch; to do this, you need to handle them using try/catch blocks or declare them using the throws clause - delegating the responsibility of handling the exception higher up the call chain. If your custom exception is not a RuntimeException rules of checked exceptions apply to your custom exception as well.

Checked exceptions ---> Forced by the compiler to handle or propagate

Un-checked exceptions ---> Not checked by the compiler thus appears at runtime

edited Jun 20 '20 at 09:12

Community

1
1

answered Dec 23 '12 at 08:04

PermGenError

45,977
8
87
106

so what should I extend to make my own exceptions that don't reuire me to add this 'throws exceptoin' stuff – CodeGuy Dec 23 '12 at 08:05
@CodeGuy just extend your custom exception with RunTimeException. – PermGenError Dec 23 '12 at 08:07
@GanGnaMStYleOverFlowErroR, You mean `RuntimeException`. – wulfgarpro Dec 23 '12 at 09:16

score 3 · Answer 2 · answered Dec 23 '12 at 08:05

Java has so called checked exceptions and unchecked exceptions. A checked exception must be declared in method signature, an unchecked exception does not need to be declared.

If you want to define an unchecked exception yourself derive it from RuntimeException insted of Exception.

score 0 · Answer 3 · answered Dec 23 '12 at 08:04

0

you are supposed to throw or try-catch all checked Exceptions , its not required for RunTimeException and exception that you are talking about is a RuntimeException

answered Dec 23 '12 at 08:04

Subin Sebastian

10,870
3
37
42

So what do I extend in order to make an exceptoin that I dont have to use try-catch for? – CodeGuy Dec 23 '12 at 08:05

score 0 · Answer 4 · edited May 23 '17 at 12:18

See my answer to this question.

As a general rule of thumb, when deciding whether or not to create a custom exception class, I try and use the built-in exception classes as much as possible, where it makes sense to do so, e.g. IllegalArgumentException, etc.

Here is an example of how to create a custom RuntimeException, which, as you have discovered, will not require a try/catch/finally block, or a throws clause.

public class MyRuntimeException extends RuntimeException {
    public MyRuntimeException() {
        super();
    }

    public MyRuntimeException(String message) {
        super(message);
    }
}

public class MyClass {
    public void go(int val) {
        if(val <= 0) throw new MyRuntimeException("Val must be greater than 0.");
    }
}

Throwing custom exceptions in Java versus built in exceptions

4 Answers4