Adding elements to Arraylist of int arrays in java

Question

I have the following class for arraylist with int array.

public class MyClass extends ArrayList<MyClass> {

public int[] numAsc = {};
public int[] numDsc = {};

public MyClass(int[] numAsc , int[] numDsc){
    this.numAsc = numAsc ;
    this.numDsc = numDsc;

}
}

When I am trying add elements to arraylist, I am just getting empty arrays.

ArrayList<MyClass> numberSeries = new ArrayList<MyClass>();
int[] orders  = {1,2,3};
int[] request = {3,2,1};
numberSeries.add(new MyClass(orders,request));

Log.d(TAG, "numberSeries: " + numberSeries);

RESULT FROM LOGCAT: numberseries: [[]]

What am I missing here?

You're adding to `numberSeries_info` but printing `numberSeries` — Eran, Nov 18 '19 at 13:03
And your MyClass class must NOT extend ArrayList. It's not an ArrayList. — JB Nizet, Nov 18 '19 at 13:04
@JBNizet Could be please explain little bit more ? Sorry, I am not that experienced with java, yet — cantona_7, Nov 18 '19 at 13:07
Well, remove `extends ArrayList`. MyClass is a class that has two arrays. It's not a list of other MyClass objects. So it has absolutely no reason to extend ArrayList. You can put cars in a queue. But that doesn't mean that a car is a queue. — JB Nizet, Nov 18 '19 at 13:10

score 3 · Answer 1 · edited Nov 18 '19 at 13:35

I don't see why you extend ArrayList. I think you are overthinking it, if you just want to build a list of MyClass it does not need to extend ArrayList

Remove the extend,
make a sensible toString method on your MyClass object so you can see the values in your arrays

class MyClass {

    public int[] numAsc;
    public int[] numDsc;

    public MyClass(int[] numAsc , int[] numDsc){
        this.numAsc = numAsc ;
        this.numDsc = numDsc;
    }

    @Override
    public String toString() {
        return "numAsc: " + Arrays.toString(numAsc) + " | " +
                "numDsc: " +  Arrays.toString(numDsc);
    }
}

Then run your main, it outputs:

numberSeries = [numAsc: [1, 2, 3] | numDsc: [3, 2, 1]]

score 2 · Accepted Answer · edited Nov 18 '19 at 13:34

Please consider the following points:

MyClass does not need to extend ArrayList.
Use Generics to instantiate the concrete classes e.g. List<MyClass> numberSeries = new ArrayList<MyClass>();
Override the toString() method in MyClass so that it becomes easier for you to print it.

Given below is the complete program incorporating the above points:

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

public class MyClass{

    public int[] numAsc = {};
    public int[] numDsc = {};

    public MyClass(int[] numAsc, int[] numDsc) {
        this.numAsc = numAsc;
        this.numDsc = numDsc;
    }
    public String toString() {
        return "numAsc: "+Arrays.toString(numAsc)+", numDsc: "+Arrays.toString(numDsc);
    }
}

public class TestMyClass{
    public static void main(String[] args) {
        List<MyClass> numberSeries = new ArrayList<MyClass>();
        int[] orders  = {1,2,3};
        int[] request = {3,2,1};    

        numberSeries.add(new MyClass(orders,request));

        System.out.println(numberSeries);
    }
}

Output:

[numAsc: [1, 2, 3], numDsc: [3, 2, 1]]

Adding elements to Arraylist of int arrays in java

2 Answers2