how to override methods with generic return type in java in case of multi level inheritance

bushra Published at Dev

bushra

I have an abstract class containing an abstract method which returns a generic type. lets say

abstract class AbstractSample{
  //class members

  //this method needs to be overriden
  public abstract SampleBuilder<? extends AbstractSample,?> copyBuilder ();

  //other methods
  //builder
  public static abstract class SampleBuilder<T extends AbstractSample, B extends SampleBuilder<T,B>> {
    //properties
    private String as;

    public SampleBuilder(AbstractSample object) {
      this.as = object.as;
    }

    public B as(String as) {
      this.as = as;
      return (B)this;
    }

    public abstract T build(); 
  }
}

Now I have a class Sample1 which extends this class and implements the method copyBuilder()

public class Sample1 extends AbstractSample {
  //class members

  @Override
  public AbstractSample.SampleBuilder<Sample1,Sample1.Builder> copyBuilder()      {
    return new Sample1.Builder(this);
  }

  //other methods
  //builder
  public static class Builder extends SampleBuilder<Sample1,Builder> {
    //implementation
    //containing members and methods
  }
}

There is another class Sample2 which is a subclass of Sample1. When I try to override the method copyBuilder() in this class, I am getting issues in the return type.

class Sample2 extends Sample1 {
  //class members

  // I am not allowed to implement this as below
  @Override
  public AbstractSample.SampleBuilder<Sample2,Sample2.Builder> copyBuilder() { 
    return new Sample2.Builder(this) ; 
  }

  //other methods
  //builder
  public static class Builder extends SampleBuilder<Sample2,Builder> {
    //implementation
    //containing members and methods
  }
}

IntelliJ says the method clashes with the one in the superclass , attempting to use incompatible return type.

Could anyone suggest how to implement the method copyBuilder() in Sample2 so that it returns Sample2.Builder .

Rafał Spryszyński

I don't like a part B extends SampleBuilder<T, B>. Why is it even useful to reference itself?

Anyway You can make it work if You change

SampleBuilder<Sample1,Sample1.Builder> copyBuilder()

SampleBuilder<? extends Sample1, ?> copyBuilder()

A complete example is below

public class Test {
abstract static class AbstractSample {

    private String as;

    //this method needs to be overriden
    public abstract SampleBuilder<? extends AbstractSample, ?> copyBuilder();

    public static abstract class SampleBuilder<T extends AbstractSample, B extends SampleBuilder<T, B>> {

        private String as;

        public SampleBuilder(AbstractSample object) {
            this.as = object.as;
        }

        public B as(String as) {
            this.as = as;
            return (B) this;
        }

        public abstract T build();
    }
}

static class Sample1 extends AbstractSample {

    @Override
    public SampleBuilder<? extends Sample1, ?> copyBuilder() {
        return new Builder(this);
    }

    public static class Builder extends SampleBuilder<Sample1, Builder> {

        public Builder(Sample1 sample1) {
            super(sample1);
        }

        @Override
        public Sample1 build() {
            return null;
        }
    }
}

static class Sample2 extends Sample1 {

    @Override
    public SampleBuilder<? extends Sample2, ?> copyBuilder() {
        return new Builder(this);
    }

    public static class Builder extends SampleBuilder<Sample2, Builder> {

        public Builder(Sample2 sample2) {
            super(sample2);
        }

        @Override
        public Sample2 build() {
            return null;
        }
    }
}
}

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