Is there a way in Java to hand over HashSet arguments to a method so that the method will accept HashSets of subclasses of Class?

Question

Question & Explanation

Is there a way in Java to hand over HashSet<Class> arguments to a method so that the method will accept HashSets of subclasses of Class?

This would be useful if there are several subclasses of a class the differences of which are not relevant to the method in question. If this is not possible, the only options to handle this seem to be

to write as many methods as there are subclasses
to loop through the HashSet in the calling function and call a similar method with the individual class instances as argument instead of the HashSets

... both are not very practical.

The version is Java 8 (openjdk 1.8.0).

Example

All files are assumed to be in the same directory so that we do not have to worry about packages and files importing each other. Consider a classes NNB and a subclass NNC (subclass of NNB).

NNB.java:

import java.util.HashMap;
import java.util.Map;


public class NNB {

    public final Map<Long, Double> dict;

    public NNB () {
        this.dict = new HashMap<Long, Double>();
        int c = 0;
        while (c<10) {
            Long XX = (long)10;
            Double YY = 2.0;
            this.dict.merge(XX, YY, Double::sum); 
            System.out.println(dict);
            c++;
        }
    }
}

NNC.java:

import java.util.HashMap;
import java.util.Map;


public class NNC extends NNB {

    private Map<Long, Double> dict2;

    public NNC () {
        super();
        this.dict2 = new HashMap<Long, Double>();
    }
}

Now assume, we want to call a method over a list of instances of NNB and NNC (say, to print NNB.dict and NNC.dict).

This does not work: NNA2.java:

import java.util.HashSet;
import java.util.ArrayList;
import java.util.List;
import java.util.Arrays;

public class NNA2 {

    public void printDict(HashSet<NNB> nnbs) {
        for (NNB nnb : nnbs) {
            System.out.println(nnb.dict);
        }
    }

    public static void main(String args[]) {
        NNA2 inst = new NNA2();
        HashSet<NNB> x = new HashSet<NNB>(Arrays.asList(new NNB()));
        HashSet<NNC> y = new HashSet<NNC>(Arrays.asList(new NNC()));

        inst.printDict(x);
        inst.printDict(y); //here it fails
        System.exit(0);
    }
}

Error:

NNA2.java:26: error: incompatible types: HashSet<NNC> cannot be converted to HashSet<NNB>
        inst.printDict(y);

This works (but is not quite what we wanted and would be unpractical if we wanted to perform more complex operations): NNA1.java:

import java.util.HashSet;
import java.util.ArrayList;
import java.util.List;
import java.util.Arrays;

public class NNA1 {

    public void printOneDict(NNB nnb) {
        System.out.println(nnb.dict);
    }

    public static void main(String args[]) {
        NNA1 inst = new NNA1();
        HashSet<NNB> x = new HashSet<NNB>(Arrays.asList(new NNB()));
        HashSet<NNC> y = new HashSet<NNC>(Arrays.asList(new NNC()));

        for (NNB nnb : x) {
            inst.printOneDict(nnb);
        }

        for (NNC nnb : y) {
            inst.printOneDict(nnb);
        }

        System.exit(0);
    }
}

See also PECS: http://stackoverflow.com/questions/2723397/what-is-pecs-producer-extends-consumer-super — JB Nizet, May 08 '17 at 20:57
Don't use mutable `public` fields. Make the field `final` and/or `private`, and make a getter. That way you can utilize polymorphism if you decide to override the getter. — 4castle, May 08 '17 at 20:57
@4castle Thanks; I agree, I should have done that. I was trying to keep the code samples short as they are quite long already. But I agree, it is bad style. — 0range, May 08 '17 at 21:06

score 5 · Accepted Answer · edited May 23 '17 at 11:47

5

If I understand you correctly, you're looking for this:

public void printDict(HashSet<? extends NNB> nnbs) {
    // ...
}

See also Is List<Dog> a subclass of List<Animal>? Why aren't Java's generics implicitly polymorphic? and What is PECS (Producer Extends Consumer Super)?

edited May 23 '17 at 11:47

Community

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answered May 08 '17 at 20:54

shmosel

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Is there a way in Java to hand over HashSet arguments to a method so that the method will accept HashSets of subclasses of Class?

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