Constructor, subclass method called from base and instance member order initialization problem

Question

So, I'm trying to design a small base abstract class:

public abstract class BaseClass
{
    abstract void reload();


    protected BaseClass()
    {
        reload();
        // schedule reload for every X minutes
    }
}

Now, subclass:

class SubClass extends BaseClass
{
    private int member = 5;

    @Override
    void reload()
    {
        member = 20;
    }
}

Now, the problem I'm facing is that reload() method is called before the member is initialized. Thus, member is assigned 20 and afterwards, assigned with the value 5. (this is only an example of course, the actual code is different, but the same idea). What is the best design for what I'm trying to achieve? I want the order of the initialization to be - member assigned 5, and if reload() fails for some reason i want it to stay with the initial value. However in this code, 5 overrides the value of reload(). If I don't assign an initial value for the instance member, it works of course. Is it possible what I'm asking?

If you are going to set member to 20 anyways, why set it to 5 first? — Sweeper, Dec 06 '18 at 10:37
Don't call non-final methods in a constructor. "Effective Java" contains a good explanation. — daniu, Dec 06 '18 at 10:59
@sweeper im giving an initial value in case the reload fails (for example the reload fetch data from DB and might fail). In this case, i want the default value to remain — paranoidAndroid, Dec 06 '18 at 11:02
I accepted as duplicate. So what I'm trying to do is basically, impossible? — paranoidAndroid, Dec 06 '18 at 11:03

minus · Answer 1 · 2018-12-06T10:58:24.203

You can use a builder to achieve that.

This way you can achieve full control.

public abstract class Foo {

    public abstract void postConstruct();

    public static void main(String[] args) {
        Bar bar = Foo.build(Bar.class);
        System.out.println(bar);

    }

    public static <T extends Foo> T build(Class<T> clazz) {
        T obj;
        try {
            obj = clazz.newInstance();
            obj.postConstruct();
            return obj;
        } catch (Exception e) {
            throw new IllegalArgumentException(
                  "Class "+clazz.getName()+" is not a valid class",e);
        }
    }

}


public class Bar extends Foo {

    int value = 10;

    protected Bar() {
        super();
    }

    public void postConstruct() {
        value = 7;
    }


    @Override
    public String toString() {
        return "Bar [value=" + value + "]";
    }

}

This is nice but a bit overkill for my use case, but thanks – paranoidAndroid Dec 06 '18 at 11:03 — paranoidAndroid, Dec 06 '18 at 11:03

score 0 · Answer 2 · answered Dec 06 '18 at 10:53

0

Override the constructor and call reload() there but do not call super(), this might not be acceptable for other reasons :)

class SubClass { 
    private int member = 5;

    public SubClass() {
        reload();
    }
    ...
}

answered Dec 06 '18 at 10:53

Joakim Danielson

43,251
5
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No that doesnt work, base constructor is always called explicitly – paranoidAndroid Dec 06 '18 at 10:56
@paranoidAndroid Not sure what you mean, this works for me. And it also works when calling super() in the constructor. – Joakim Danielson Dec 06 '18 at 11:47

Constructor, subclass method called from base and instance member order initialization problem

2 Answers2