Need some references for Constructor with Exceptions

[Davie Lin]

Hi people, I was trying to learn the iText api and was noticing creating the pdf document object's constructor throws exceptions.
I have not read any code before that constructors throws exceptions and I am sure that pdf type of object must be risky to create so it throws exceptions to let you know the program didn't make it.

At this point, I am guessing that this is legal to do in Java and it follow most of the exception rule with methods which means it must be wrap in a try/catch block when you instantiate this object. I been reading SCJP from K & B but didn't see anything talking about my subject of interest. So if anyone have reference about constructor with exceptions and can post them would be much appreciated.

Thanks

[David Newton]

I'm not sure what you want to know--constructors can throw exceptions just like any other method; there's nothing special about it. For example, the java.io.FileWriter constructor throws an exception if the file can't be opened.

[Muhammad Khojaye]

As per JLS

A compiler for the Java programming language checks, at compile time, that a program contains handlers for checked exceptions, by analyzing which checked exceptions can result from execution of a method or constructor. For each checked exception which is a possible result, the throws clause for the method (§8.4.6) or constructor (§8.8.5) must mention the class of that exception or one of the superclasses of the class of that exception

[Davie Lin]

Cool, so the exception for constructor follows the same rule as exception for methods, that's all I wanted to know. I raise this question because I have not seen any code from real life nor any of my books, so I wasn't sure if exception for constructor follows the same rule as methods or it has a different set of rules it follows or what not.

Anyways, thanks for clarify for me

[Ernest Friedman-Hill]

The one thing to realize -- and it's an important thing! -- is that if a constructor throws an exception, then the "new" expression doesn't return, and therefore most of the time, the object effectively is not created. This is a good thing -- it lets you abort object creation if you don't have enough data to construct a valid object, for example.

Now, I said "most of the time" and "as if" because the object actually is created, and you can arrange to actually use it after a constructor aborts. Look at this (BAD!) code:

public class Foo {
    private static ArrayList foos = new ArrayList();
    public Foo() throws Exception {
        foos.add(this);
        throw new Exception();
    }
    public static void main(String[] argv){
        try {
            new Foo();
        } catch (Exception e) {
            System.out.println(foos);
        }
    }
}

The constructor stores a reference to the object, then aborts with an exception. But that stored reference remains: the partially constructed object lives on, and the println() shows it. Make sure you never, ever write code like this!

[David Newton]

I had no idea--that's pretty twisted.