Re: Rules for constructors

From: Alex Hunsley (lard_at_tardis.ed.ac.molar.uk)
Date: 03/17/04 

Date: Wed, 17 Mar 2004 16:28:25 +0000

Alex Hunsley wrote:

> I just found quite an annoying thing some code I am maintaining...
>
> Here is the essence of what happened:
> (this is a runnable self-contained example)

[snip actual code]
>
> You'd expect the code to print out memberVariable=7, but it actually
> prints out memberVariable=0.
> This is because the constructor of B calls super(param1, param2), and by
> stepping through the code in eclipse I could see that the java runtime
> processes the call to setUpSomeThings() in super(..) *before* it gets
> around to settings up the member variables in class B! Therefore, once
> the setUpSomeThings in the subclass (B) has been called by the
> constructor for A, the initialisation of the member variables in class B
> s constructor happens, which sets the member variable to 0 and wipes out
> the pervious value of 7.
>
> Anyway, what exactly is the moral of this tale?
>
> I suspect it's either
> a) always check your class is initialised,
> as per
> http://www.javaworld.com/javaworld/jw-12-1998/jw-12-securityrules.html,
> but this is a bit security-paranoid
>
> b) never call non-final methods from your constructor
> (or else they will be overridden causing chaos as I have experienced
> today)
> c) overridden methods should not access member variables that are
> from this class - should only access superclass member variables
>
> any other suggestions?
> What do people take a generally good rules for constructors?
> I already practice the rule that you shouldn't do any populating or,
> worse, realising of GUI items from a constructor.
>
> alex
>

Sorry, I've just realised my code contains non-empty constructors which
aren't used and may distract from the issue at hand. Here is simpler
code that also shows the problem:

public class B extends A {
        private int memberVariable = 0;
        
        public B() {
                super();
                
                System.out.println("memberVariable = "
                        + memberVariable);
        }

        protected void setUpSomeThings() {
                memberVariable = 7;
        }

        
        public static void main(String[] args) {
                B bInstance = new B();
        }
}

class A {

        
        public A() {
                setUpSomeThings();
        }

        protected void setUpSomeThings() {
                // some code in here
        }
}

alex

Relevant Pages

  • Rules for constructors
    ... This is because the constructor of B calls super, and by ... the initialisation of the member variables in class B ... (or else they will be overridden causing chaos as I have experienced ... overridden methods should not access member variables that are ...
    (comp.lang.java.programmer)
  • Re: Initializing form (dialog)
    ... Alex wrote: ... some child form ... So I'm able to initialize controls in the ... someother function in between constructor and dlg.ShowDialog? ...
    (microsoft.public.dotnet.languages.csharp)
  • Re: a question about classes
    ... doesn't calling the constructor result in mD and mS being ... The function iFconst has a return value of X, ... assign values to a new X object's member variables. ... This object is returned to the caller. ...
    (alt.comp.lang.learn.c-cpp)
  • Re: Rules for constructors
    ... > b) never call non-final methods from your constructor ... > c) overridden methods should not access member variables that are ... > from this class - should only access superclass member variables ... access any object state from these methods. ...
    (comp.lang.java.programmer)
  • Re: Can a classs constructor call another constructor "later"?
    ... I find the current constructor behaviour ... with "constructor overrides behave in the opposite way to method ... choose to call the base method wherever you like, ... I should point out that what Alex did in his first example ...
    (microsoft.public.dotnet.csharp.general)