Re: exception inside constructors and finalize

Ross Bamford wrote:

.... 
An exception handled within a constructor is a regular exception, but
one thrown by a constructor *should* result in the object not being
allocated, or deallocated immediately. It *always* leaves the object in
an undefined state (Null in my experience but don't rely on it)

For example the following code:

public class Test
{
public String tester;
public Test() throws Exception
{
tester = "Tester object";
throw(new Exception()); }
public static void main(String args[])
{
Test t = null;
try {
t = new Test();
} catch (Exception e) { /* ignore */ }
String s = (t == null) ? "Null object" : t.tester;
System.out.println(s);
}
}

produces the output "Null object", unless we remove the throws clause
from the constructor (when it's "Tester object" obviously).


I agree generally with your comments, but I don't think you
got this one quite right.

t is null because you initialized it to null, and nothing
changed it between initialization and the null test. You can
count on an assignment not happening if evaluation of its
right hand side completes abruptly.

Memory for the object is allocated, and any initialization
that happens before the exception gets done. Whether the
object is immediately eligible for garbage collection
depends, as always, on its reachability.

Here's a variation of your program that demonstrates this.

public class ConstructorTest
{
  static ConstructorTest lastConstructed = null;
  public String tester;

  public ConstructorTest() throws Exception
  {
    lastConstructed = this;
    tester = "ConstructorTest object";
    throw(new Exception());
  }

  public String toString()
  {
    return tester;
  }

  public static void main(String args[])
  {
    Object t = "Not a ConstructorTest object";
    try {
      t = new ConstructorTest();
    } catch (Exception e) { /* ignore */ }

    String s = (t == null) ? "Null object" : t.toString();
    System.out.println(s);
    if(lastConstructed != null)
    {
      System.out.println(lastConstructed);
    }
  }
}

It prints:

Not a ConstructorTest object
ConstructorTest object

Patricia







.