Builder pattern with inheritance support generics issue

Question

I implemented pattern based on this answer I have the following asbtract config:

public abstract class AbstractConfig {

    public static abstract class Builder<B extends Builder<B>> {

        private int calories = 0;

        public Builder() {

        }

        public B setCalories(int calories) {
            this.calories = calories;
            return (B) this;
        }

        public abstract AbstractConfig build();
    }

    private int calories = 0;

    protected AbstractConfig(final Builder builder) {
        calories = builder.calories;
    }
}

And I have the following concrete config:

public class DialogConfig extends AbstractConfig {

    public static class DialogConfigBuilder<B extends DialogConfigBuilder<B>> extends Builder<B> {

        private double width;

        private double height;

        public DialogConfigBuilder() {
            //does nothing.
        }

        public B setWidth(final double value) {
            width = value;
            return (B) this;
        }

        public B setHeight(final double value) {
            height = value;
            return (B) this;
        }
        public DialogConfig build() {
            return new DialogConfig(this);
        }
    }

    private final double width;

    private final double height;

    protected DialogConfig(final DialogConfigBuilder builder) {
        super(builder);
        width = builder.width;
        height = builder.height;
    }

    public double getWidth() {
        return width;
    }

    public double getHeight() {
        return height;
    }
}

And this is how I use it

DialogConfig config = new DialogConfig.DialogConfigBuilder()
                .setWidth(0)
                .setCalories(0)
                .setHeight(0) //X LINE
                .build();

At X line I get - Can't find symbol method setHeight. What is my mistake?

EDIT - I will have and a ExtendedDialogConfig that must extend DialogConfig and etc. I mean there will be other subclasses.

You're using a raw type which means `B` resolves to `Builder`. Change your class declaration to `class DialogConfigBuilder extends Builder`. — shmosel, Aug 15 '17 at 06:48
Note that you should be wary of copying that solution as it also [uses raw types](https://stackoverflow.com/questions/17164375/subclassing-a-java-builder-class/17165079#comment59589591_17165079). — shmosel, Aug 15 '17 at 06:54
@shmosel Than you for your comment. But if I change to `class DialogConfigBuilder extends Builder` can I extend DialogConfig in SuperDialogConfig? I think that if I follow your advise then setCalories() will return DialogConfig but not SuperDialogConfig. — Pavel_K, Aug 15 '17 at 07:10
Hmm... what if you do `new DialogConfig.DialogConfigBuilder<>()`, does that compile? Do you need to be to instantiate both builder classes, or just the subclass? Btw, `SuperDialogConfig` is a Super confusing name for a subclass. — shmosel, Aug 15 '17 at 07:22
@shmosel I changed SuperDialogConfig to ExtendedDialogConfig. Answering to your question - I need DialogConfig, ExtendedDialogConfig and other sub classes where each implements builder pattern. — Pavel_K, Aug 15 '17 at 07:27
All right then. And just for the record - I assume you want to wait some time with the accept - in any case, don't forget about that step in the long run ;-) — GhostCat, Aug 15 '17 at 08:41

GhostCat · Answer 1 · 2017-08-15T08:40:10.900

You would first change setCalories() to:

public Builder<B> setCalories(int calories) {
  this.calories = calories;
  return this;
}

to get rid of that cast and the warning. And now look at this closely. You return a Builder. This code doesn't know about future subclasses. It only returns an instance of that base builder.

As a consequence, when you have that chained call:

 .setHeight(0) .build();

that would return that base builder. To then call build() - which would build an abstract configuration. But you want to assign that to a more specific DialogConfig. Thus the error.

A (ugly) workaround:

DialogConfig.DialogConfigBuilder<?> builder = new DialogConfig.DialogConfigBuilder<>().setHeight(0);
builder.setCalories(0);

...config = builder.build();

And a solution - by again reworking setCalories():

@SuppressWarnings("unchecked")
public <T extends B> T setCalories(int calories) {
  this.calories = calories;
  return (T) this;
}

Fixes the compile error; and allows chaining the setCalories() call as well. Final exercise of getting rid of the cast/suppress is left as exercise to the reader.

And for the record - the "complete" solution, including all adaptions to get rid of raw types and other warnings:

abstract class AbstractConfig {
    public static abstract class Builder<B extends Builder<B>> {
        private int calories = 0;

        @SuppressWarnings("unchecked")
        public <T extends B> T setCalories(int calories) {
            this.calories = calories;
            return (T) this;
        }

        public abstract AbstractConfig build();
    }

    private int calories = 0;
    public int getCalories() { return calories; }

    protected <B extends Builder<B>> AbstractConfig(final Builder<B> builder) {
        calories = builder.calories;
    }
}

final class DialogConfig extends AbstractConfig {
    public static class DialogConfigBuilder<B extends DialogConfigBuilder<B>> extends Builder<B> {

        private double width;    
        private double height;

        public DialogConfigBuilder<B> setWidth(final double value) {
            width = value;
            return this;
        }

        public DialogConfigBuilder<B> setHeight(final double value) {
            height = value;
            return this;
        }

        public DialogConfig build() {
            return new DialogConfig(this);
        }
    }

    private final double width;
    private final double height;

    protected <B extends DialogConfigBuilder<B>> DialogConfig(final DialogConfigBuilder<B> builder) {
        super(builder);
        width = builder.width;
        height = builder.height;
    }

    public double getWidth() { return width; }
    public double getHeight() { return height; }
}

public class Builders {
    public static void main(String[] args) {
        DialogConfig config = new DialogConfig.DialogConfigBuilder<>().setHeight(0).setCalories(0).build();
        System.out.println(config);
    }
}

In your solution you make DialogConfig final and besides all setters in DialogConfigBuilder return DialogConfigBuilder. This is not what I need. I did write that I need to extend DialogConfig in other subclasses and each subclass must implement builder pattern. — Pavel_K, Aug 15 '17 at 08:46
@Pavel_K That is not **my** code. That part I copied from your input. And again - it is just an attempt to "complete" the input for the given task. Nothing changes when you remove that `final` from that class. And the changed return type is about **getting** rid of casts and warnings! — GhostCat, Aug 15 '17 at 08:49

score 1 · Accepted Answer · answered Aug 15 '17 at 09:19

I found my mistake. This is how I used DialogConfigBuilder

DialogConfig config = new DialogConfig.DialogConfigBuilder()
                .setWidth(0)
                .setCalories(0)
                .setHeight(0) //X LINE
                .build();

This is how I should use DialogConfigBuilder

DialogConfig config = new DialogConfig.DialogConfigBuilder<>()
                .setWidth(0)
                .setCalories(0)
                .setHeight(0) //X LINE
                .build();

Pay attention to <> in the second case.

You should accept your self-answer then. And maybe put a thick first line in there "my mistake was to use **raw** types and now I understand why everybody says *do not use **raw** types*" ;-) — GhostCat, Aug 15 '17 at 10:23

Builder pattern with inheritance support generics issue

2 Answers2