Re: Clean up after exception thrown in constructor

From:
Daniel Pitts <newsgroup.spamfilter@virtualinfinity.net>
Newsgroups:
comp.lang.java.programmer
Date:
Sun, 20 Apr 2008 11:12:27 -0700
Message-ID:
<480b8784$0$29100$7836cce5@newsrazor.net>
Philipp wrote:

Hello

I have a class hierarchy where the constructor of a child class ("Ford")
can throw an exception, while the constructor of the parent class
("Car") has some side-effects.
If construction fails due to the Exception on the child class being
thrown, the side-effect of the parent constructor cannot be cleaned up.

What would be a better approach to this problem, so that if construction
fails for whatever reason, the system is left in a pristine state?

Thanks for your advice
Phil

Example SSCCE:
package exceptionInCtor;

import java.util.ArrayList;
import java.util.List;

public class Car {
  private static List<String> nameList = new ArrayList<String>();
  private String name;

  public Car(String name){
    nameList.add(name); // name is added to list as side-effect
    this.name = name;
  }

  public void remove(){ // called when car is no longer used
    nameList.remove(name);
  }

  public static class Ford extends Car{
    public Ford(String name, int wheels){
      super(name);
      if(wheels > 4){
        throw new IllegalArgumentException("Too many wheels");
      }
    }
  }

  public static void main(String[] args) {
    Car c = null;
    try{
      c = new Ford("myFord", 5);
    } catch (Exception e) {
      // cannot call c.remove() here!
      e.printStackTrace();
    }

    System.out.println(c); // prints null
    System.out.println(nameList); // prints [myFord], should be empty
  }
}


The answer is... Constructors should not cause side effects! Ever! For
just this reason.

The better approach is to use either a two-stage approach, or a factory
approach that encapsulates that two-stage approach

public abstract class Car {
   final String name;
   protected Car(String name) { this.name = name; }
   public void init() { nameList.add(name); }

}
Car myCar = new Ford("Blah", 4);
myCar.init();

--
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