Unusual things in Exceptions

I called this post "Unusual things in Exceptions" because you're going to see something that isn't usually used in a real word as well as staffs that you have to memorize for the OCJP certification.

Let's start with the class hierarchy:

	package br.com.ocjp.exception;
	import java.io.IOException;
	public class ExceptionOrder {
	public static void doSomething() {
	try {
	String s = "Exception example";
	} catch (IOException e) { // does not compile
	}
	}
	}

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The code above does not compile you cannot handle an exception if the block try does not "throw" one.

!Import
The block finally always is called, even if there is a return statement in the try/catch block, the finally block will be executed before the return stamement.

	package br.com.ocjp.exception;
	import java.io.File;
	import java.io.FileNotFoundException;
	import java.io.FileOutputStream;
	import java.io.IOException;
	public class ExceptionOrder {
	public static void doSomething() {
	try {
	new FileOutputStream( new File("C:\\file.txt"));
	} catch (IOException e) {
	} catch (FileNotFoundException e) { // does not compile
	}
	}
	}

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In this case the block try "throws" an exception throught the creation of FileOutputStream, but we still getting a compiler error, because the catch that handles FileNotFoundException will never be reached.

	package br.com.ocjp.exception;
	import java.io.IOException;
	public class ExceptionCatchDeclare {
	public void doSomething()/* throws IOException*/ {
	throw new IOException(); // does not compile you have to declare(throws)
	}
	public void doAnything() {
	doSomething(); // here you have to handle(try/catch) or declare(throws)
	}
	}

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That's the rule for checked exceptions if you throw an exception you must declare(throws) from the calling method or catch(try/catch).


Different from checked exception, every exception under RuntimeException/Error are not obligated to catch or declare, but if you did you would be able to catch it by using try/catch block.

	package br.com.ocjp.exception;
	public class ExceptionRunTime {
	public void doIt() throws ArithmeticException {
	throw new ArithmeticException();
	}
	public void doThat() {
	doIt();
	}
	}

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Common Exception

Let's take a look at the common exception, watch out where they come from