Question

I have a class called Scraper with a static inner class Builder, and also a non-static inner class called StringPair, I’m trying to get one of the builder methods to construct a new StringPair(a,b) and add it to an ArrayList<StringPair>, however it doesn’t compile, the compiler says Error: non-static variable this cannot be referenced from a static context and specifically highlights the new StringPair(a,b) as the source of the problem.

I don’t understand what’s wrong here, any help would be appreciated!

class Scraper {

    private final ArrayList<StringPair> stringPairs;

    Scraper(Builder builder) {

        stringPairs = builder.builderStringPairs;


    }

    public static class Builder {

        private static ArrayList<StringPair> builderStringPairs = new ArrayList<StringPair>();

        public Builder addStringPairs(String a, String b) {

            builderStringPairs.add(new StringPair(a, b));
            return this;
        }

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

    }

    class StringPair {
        String a,b;

        StringPair(String a, String b) {
            this.a = a;
            this.b = b;

        }



        String getA() {
            return a;
        }
        String getB() {
            return b;
        }


    }

}

Answer 1

Builder isn’t an inner class – it’s just a nested class, with no implicit reference to an instance of the containing class (Scraper).

StringPair, however, is an inner class. In order to construct an instance of StringPair, you need to have a reference to an instance of Scraper. So, options for making Builder work:

• Make it an inner class (non-static)
• Make StringPair a nested class (static)

• Give Builder an instance somewhere, at which point you could use:

  builderStringPairs.add(instance.new StringPair(a, b));
  

I suspect the second option is the most appropriate one here – I can’t see any reason why StringPair needs a reference to an instance of Scraper.

See section 8.1.3 of the JLS for more information about nested and inner classes.