Constructor implementation inside an abstract class (Java)

Question

I'm trying to create a constructor inside an abstract class and then make two other classes extend it. But - I get this yellow line under some variable saying it never read locally, but if I delete it, I get compilation errors.

It looks like this:

public abstract class SizeFilters {

private double value; // this one never read locally
private int MIN_VALUE = 0;

public SizeFilters(double value) {
    if (value >= MIN_VALUE) {
        this.value = value; 
    } else {
        throw new IllegalArgumentException(
                "Value must be a positive number.");
    }
}

And the class that extends:

public class GreaterThanFilter extends SizeFilters {

private double value;

public GreaterThanFilter(double value) {
    super(value);
}

Should I ignore that warning or is there something I do wrong?

Thanks a lot!!!

@PeterRader So do you down-vote everyone who makes a mistake? A +1 for me for a clear question and a code example that demonstrates the problem. — Duncan Jones
– Duncan Jones, Commented May 28, 2014 at 9:57
@Duncan, no i downvote questions only, not persons. I downvote questions that are founded in unrealistic situations like: When i balance a chair in magma, why hurts my neck? Give me a realistic situation for the code, and the yellow line will be disappear as well as the question disappears! — Grim
– Grim, Commented May 28, 2014 at 10:05
@PeterRader I think the OP has genuinely written this code and doesn't understand the warning. I don't think this is an unrealistic or hypothetical question. — Duncan Jones
– Duncan Jones, Commented May 28, 2014 at 10:14
@PeterRader I believe most answers below explain the core problem behind the warning. My biased view is that my own answer provides a few additional pointers about variable hiding. Anyway, let's leave the conversation here - it's quite fine if we disagree on down-votes. — Duncan Jones
– Duncan Jones, Commented May 28, 2014 at 10:26

Duncan Jones · Accepted Answer · 2014-05-28 09:58:13Z

1

In your subclass, you are hiding the variable defined in the superclass. This is because you've declared a field with the same name: value:

public abstract class SizeFilters {
  private double value; // this one never read locally


public class GreaterThanFilter extends SizeFilters {
  private double value; // hides the field in the parent class

As a result, even if you use the value field elsewhere in your subclass, the superclass field will always have a warning since the field is never read.

I suspect you need to remove your value field from your subclass and provide a getValue() method in your superclass to provide access to it. Or just make the field in your superclass protected rather than private.

edited May 28, 2014 at 9:58

answered May 28, 2014 at 9:53

Duncan Jones

69.7k32 gold badges204 silver badges262 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

JB Nizet · Accepted Answer · 2014-05-28 09:50:32Z

0

The warning basically tells you that your class has a field, which is initialized, but is never used by any method of the class (and can't be used by subclasses since it's private).

So it's a hint that either your class lacks methods using the field, or that the field is useless and could thus be removed.

answered May 28, 2014 at 9:50

JB Nizet

694k94 gold badges1.3k silver badges1.3k bronze badges

Comments

Savv · Accepted Answer · 2014-05-28 09:51:41Z

0

The reason you're getting the warning is most probably because it's never actually used apart from the constructor. If you're going to use it within the class later on (with a getter method or other way), ignore the warning.

answered May 28, 2014 at 9:51

Savv

4312 silver badges7 bronze badges

Comments

Lukas Warsitz · Accepted Answer · 2014-05-28 09:52:02Z

0

You are never read this variable. The only thing you do with it is giving it a value but you never read it and it's value.

answered May 28, 2014 at 9:52

Lukas Warsitz

1,2391 gold badge11 silver badges20 bronze badges

Comments

Laabidi Raissi · Accepted Answer · 2014-05-28 09:52:56Z

0

I think you should make the double value in SizeFilters as protected, and then delete the one defined in subclasses like GreaterThanFilter. So you use the inherited one from your base abstract class.
As for your question, and as all other answers mentioned, it's because you never read the value attribute, you just set it. And the compiler, tells you, that it's never used

answered May 28, 2014 at 9:52

Laabidi Raissi

3,3411 gold badge25 silver badges28 bronze badges

Comments

j3ny4 · Accepted Answer · 2014-05-28 12:18:48Z

0

You shouldn't ignore the warning. Private member isn't visible to the sub-classes, so if you don't access it in the abstract class it won't be accessed at all. If you plan accessing it in the sub-classes, add a getter or make it protected.

On other hand, if the only purpose of the abstract class is sharing the member, I'd suggest you to avoid doing that. You should share functionality (methods), but not members alone.

edited May 28, 2014 at 12:18

answered May 28, 2014 at 9:56

j3ny4

4412 silver badges9 bronze badges

Collectives™ on Stack Overflow

Constructor implementation inside an abstract class (Java)

6 Answers 6

Comments

Comments

Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

6 Answers 6

Comments

Comments

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related