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Christian Neverdal
Christian Neverdal

Reputation: 5375

In Java, how come I can get away with not handling some kinds of exceptions but not others?

How come I can have

public void someMethod() {
    throw new UnsupportedOperationException();
}

but not

public void someMethod() {
    throw new throw new IOException();
}

The first is fine, the second generates the compile error "unhandled exception type IOException".

Upvotes: 1

Views: 132

Answers (7)

atmaish
atmaish

Reputation: 2613

IOException is a "checked" Exception and must be handled.

Upvotes: 0

Bohemian
Bohemian

Reputation: 424983

All things that can be thrown are Throwable. There are two types of Throwable:

One subclass of Exception is RuntimeException, which is "unchecked" - meaning you don't have to declare or catch them. These are generally for "programming errors", like NullPointerException or ArrayOutOfBoundsException.

Errors are also "unchecked" and are for "unrecoverable" situations, such as OutOfMemoryError etc.

Any Throwable not a subclass of Error or RuntimeException is "checked" and must be declared as thrown or caught.

Upvotes: 5

Paul
Paul

Reputation: 20061

UnsupportedOperationException is a RuntimeException which is a type of unchecked exception. These are exceptions that arise during the execution of a program and indicate a state that is unlikely to be recovered from.

Checked exceptions, the type you must handle in some way, are exceptions that you can probably write code to handle gracefully.

The Java Language Specification, section 11.2.5, states it very well:

The runtime exception classes (RuntimeException and its subclasses) are exempted from compile-time checking because, in the judgment of the designers of the Java programming language, having to declare such exceptions would not aid significantly in establishing the correctness of programs.

There are some good explanations of the idea behind the two types. The Java Tutorial has a page titled Unchecked Exceptions — The Controversy, and if you want a more lengthy discussion see Checked or Unchecked Exceptions?

Upvotes: 1

DaveH
DaveH

Reputation: 7335

Yo need to look at the difference between check and unchecked exceptions.

Upvotes: 1

Peter Svensson
Peter Svensson

Reputation: 6173

IOException is a checked exception while UnsupportedOperationException is a RuntimeException (unchecked). Checked exceptions must be caught or rethrown. http://java.sun.com/docs/books/jls/second_edition/html/exceptions.doc.html

Upvotes: 1

Dave Newton
Dave Newton

Reputation: 160170

Because RuntimeExceptions and its subclasses are "unchecked" exceptions and do not need to be declared thrown, or caught. Checked exceptions do.

JLS 11, Exceptions covers exceptions in exhaustive detail, JLS 11.2, Compile-Time Checking of Exceptions discusses the differences at a high level.

Upvotes: 1

Brian Roach
Brian Roach

Reputation: 76888

There are two types of exceptions in java; checked and unchecked.

You have to catch or specify that your method throws checked exceptions. That's the case in your second example with IOException.

http://docs.oracle.com/javase/tutorial/essential/exceptions/runtime.html

Upvotes: 1

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