Elegant way to implement abstract class

Question

I have an abstract class with a single abstract method; and a number of implementing classes (about 6).

The method returns an object that "needs" two parameters.

However, in some cases, only one of the two parameters is required.

Is there an elegant way to implement this case? (instead of return this parameter as empty)

public class NormResult {
    protected List<String> normWords;
    protected  List<String> unNormWords;

    public NormResult(List<String> normWords,List<String> unNormWords) {
        this.normWords = normWords;
        this.unNormWords = unNormWords;
    }

    public NormResult(List<String> normWords) {
        this.normWords = normWords;
        this.unNormWords =  Collections.emptyList();
    }
}

public abstract class AbstractNormalizer {
    protected abstract List<NormResult> doNorm();
}

public class FirstNormImpl extends AbstractNormalizer {
    protected List<NormResult> doNorm() {
        List<String> normWords = new ArrayList<>(5);
        List<String> unNormWords = new ArrayList<>(7);

        NormResult result = new NormResult(normWords, unNormWords);
        return result;      
    }
}

public class SecondNormImpl extends AbstractNormalizer {
    protected List<NormResult> doNorm() {
        List<String> normWords = new ArrayList<>(8);    
        NormResult result = new NormResult(normWords);
        return result;
    }
}

Personally, I'd replace `Collections.emptyList()` with `null`, but apart from that, I don't see any issue in that approach. — domsson, Jun 20 '17 at 11:30
why do you do **return result;** when you should return a List?? — ΦXocę 웃 Пepeúpa ツ, Jun 20 '17 at 11:36
I thought maybe add another abstract class and only classes which have the second parameter will implements it. — userit1985, Jun 21 '17 at 08:19

ΦXocę 웃 Пepeúpa ツ · Accepted Answer · 2017-06-20T11:38:30.300

2

if you do this to members final:

 protected final List<String> normWords;
 protected final List<String> unNormWords;

then in the constructor you have to initialize them both... then you can set to an empty collection or a null reference the one you dont have/need

and your overloaded constructor can look like:

public NormResult(List<String> normWords, List<String> unNormWords) {
    this.normWords = normWords;
    this.unNormWords = unNormWords;
}

public NormResult(List<String> normWords) {
    this(normWords, Collections.emptyList());
}

edited Jun 20 '17 at 11:38

answered Jun 20 '17 at 11:34

ΦXocę 웃 Пepeúpa ツ

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I thought maybe add another abstract class and only classes which have the second parameter will implements it. – userit1985 Jun 21 '17 at 08:19

score 2 · Answer 2 · answered Jun 20 '17 at 11:37

2

The two changes I would make:

Make the fields final
Use constructor telescoping

as in:

public NormResult(List<String> normWords) {
  this(normWords(), Collections.emptyList()); 
}

to avoid even that simple "code duplication" of assigning values twice.

Beyond that; I agree with the comments; this approach looks reasonable.

answered Jun 20 '17 at 11:37

GhostCat

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[OT]: Ü-krass!!, Danke! – ΦXocę 웃 Пepeúpa ツ Jun 20 '17 at 11:39
I thought maybe add another abstract class and only classes which have the second parameter will implements it. – userit1985 Jun 21 '17 at 08:19
That might be an option. Thing is: such decisions are hard to make without knowing all the details. We can't tell you the optimal model that best works for all your requirements. We can only give hints; and that is what we tried ;-) – GhostCat Jun 21 '17 at 08:24

Elegant way to implement abstract class

2 Answers2