Adding double array values to an arraylist using a for loop

Question

I apologize in advance if I am making any mistakes asking this question. I am new to stackoverflow and java.

My problem is that I want to be able to convert an ordinary array of doubles to an arraylist, I need to operate on this arraylist element wise to change values from x to -x (using an interface)

I am trying to just get array doubles to convert to an arraylist using a for loop element by element first (thought I should get is working first) but the .add does not seem to work which is the solution that seems to appear when I research it. currently it saying "cannot instantiate the type Num" I've tried removing abstract still no good.

Am I going about this completely wrong? Thanks for help in advance.

Here are my codes.

public interface Num {
    public void neg(); // negation
    public void sqrt(); // square root 
    public String asString(); // number represented as a string
}

public class NumDouble implements Num {

    Double d;
    public NumDouble(Double d) {
        this.d = d;
    }

    @Override
    public void neg() {
        d = -d;
    }

    @Override
    public void sqrt() {
        d = Math.sqrt(d);
    }

    @Override
    public String asString() {
        return d.toString();
    }

}

THIS IS THE CODE I HAVE BEEN TRYING

import java.util.ArrayList;
import java.util.Arrays;

public abstract class NumList implements Num {

    ArrayList<Num> nums = new ArrayList<Num>();
    double[] doubles = {2.3, 2.2, 2.4};

    public void neg() {
        for (int i = 0; i < doubles.length; i++) {
            double value = doubles[i];
            nums.add(new Num(value));    <------this is highlighting as an error
        }
        return;
    }

    public void sqrt() {

    }
}

It should be new NumDouble(value) -- you cannot instantiate interfaces — fge
– fge, Commented Mar 13, 2014 at 23:59

Adam Arold · Accepted Answer · 2014-03-14 00:19:41Z

1

You can either use NumDouble to do so or create an anonymous instance of Num.

The latter would look like something like this:

Num num = new  Num() {

        Double d;

        @Override
        public void neg() {
            d = -d;
        }

        @Override
        public void sqrt() {
            d = Math.sqrt(d);
        }

        @Override
        public String asString() {
            return d.toString();
        }

    };

As you can see this does not make sense since you can't add constructors to an anonymous instance and your interface does not contain a setter.

Just a note: NumList is not really a Num since it does not fit in the Num abstraction you create.

So my suggestion is to either use NumDouble like this:

nums.add(new NumDouble(value));

or create a new class extending Num.

answered Mar 14, 2014 at 0:19

Adam Arold

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