[java] How to create a custom exception type in Java?

I would like to create a custom exception in Java, how do I do it?

...

try{

...

String word=reader.readLine();

if(word.contains(" "))
  /*create custom exception*/

}
catch(){

When I create my custom exception with throw new..., I obtain the error unreported exception...must be caught or declared to be thrown

As a careful programmer will often throw an exception for a special occurrence, it worth mentioning some general purpose exceptions like IllegalArgumentException and IllegalStateException and UnsupportedOperationException. IllegalArgumentException is my favorite:

throw new IllegalArgumentException("Word contains blank: " + word);

An exception is a class like any other class, except that it extends from Exception. So if you create your own class

public class MyCustomException extends Exception

you can throw such an instance with

   throw new MyCustomException( ... );
   //using whatever constructor params you decide to use

And this might be an interesting read

You can create you own exception by inheriting from java.lang.Exception Here is an example that can help you do that.

You have to define your exception elsewhere as a new class

public class YourCustomException extends Exception{

//Required inherited methods here
}

Then you can throw and catch YourCustomException as much as you'd like.

You need to create a class that extends from Exception. It should look like this:

public class MyOwnException extends Exception {
    public MyOwnException () {

    }

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

    public MyOwnException (Throwable cause) {
        super (cause);
    }

    public MyOwnException (String message, Throwable cause) {
        super (message, cause);
    }
}

Your question does not specify if this new exception should be checked or unchecked.

As you can see here, the two types are different:

• Checked exceptions are meant to flag a problematic situation that should be handled by the developer who calls your method. It should be possible to recover from such an exception. A good example of this is a FileNotFoundException. Those exceptions are subclasses of Exception.

• Unchecked exceptions are meant to represent a bug in your code, an unexpected situation that you might not be able to recover from. A NullPointerException is a classical example. Those exceptions are subclasses of RuntimeException

Checked exception must be handled by the calling method, either by catching it and acting accordingly, or by throwing it to the calling method. Unchecked exceptions are not meant to be caught, even though it is possible to do so.

Since you can just create and throw exceptions it could be as easy as

if ( word.contains(" ") )
{
     throw new RuntimeException ( "Word contains one or more spaces" ) ;
}

If you would like to be more formal, you can create an Exception class

class SpaceyWordException extends RuntimeException
{

}

Either way, if you use RuntimeException, your new Exception will be unchecked.

Great answers about creating custom exception classes. If you intend to reuse the exception in question then I would follow their answers/advice. However, If you only need a quick exception thrown with a message then you can use the base exception class on the spot

String word=reader.readLine();

if(word.contains(" "))
  /*create custom exeception*/
  throw new Exception("My one time exception with some message!");
}

You just need to create a class which extends Exception (for a checked exception) or any subclass of Exception, or RuntimeException (for a runtime exception) or any subclass of RuntimeException.

Then, in your code, just use

if (word.contains(" "))
    throw new MyException("some message");
}

Read the Java tutorial. This is basic stuff that every Java developer should know: http://docs.oracle.com/javase/tutorial/essential/exceptions/