Extending ArrayList and using/creating a similar constructor

Question

I implemented ArrayList here

public class JsonList<T> extends ArrayList<T>{
//extended functions

}

When creating an ArrayList, you can do something List<?> arrayList = new ArrayList<>(alreadyCreatedList);

I want to be able to do this with my JsonList. But currently, JsonList only has the default constructor JsonList<>(). I tried copying the ArrayList constructor like this

public JsonList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // defend against c.toArray (incorrectly) not returning Object[]
            // (see e.g. https://bugs.openjdk.java.net/browse/JDK-6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

And when creating a JsonList instance I do

JsonList<?> jsonList = new JsonList<>(alreadyCreatedList);

However, the elements are not saved. It returns an empty array. In addition, I can no longer create an empty instance JsonList<?> jsonList = new JsonList<>();

Solution: I don't know why I didn't think of it but for those out there here it is

public JsonList(Collection<? extends E> c) {
    super(c); // <-- invoke the appropriate super constructor
}

super(c) is all you need.

ArrayList had elementdata, size, and EMPTY_ELEMENTDATA declared as variables in that class so I just simply redeclared them in my class. — Rei Brown
– Rei Brown, Commented Jan 31, 2018 at 2:10

Elliott Frisch · Accepted Answer · 2018-01-31 01:57:37Z

1

Make the first line super(c); without that, the compiler will insert super() which won't invoke the one you want (taking the Collection<? extends E> c).

public JsonList(Collection<? extends E> c) {
    super(c); // <-- invoke the appropriate super constructor
    elementData = c.toArray();
    // ...
}

answered Jan 31, 2018 at 1:57

Elliott Frisch

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3 Comments

Rei Brown Over a year ago

I tried this. But I'm getting errors on my instances declared like this JsonList<?> filteredList = new JsonList<>();

Rei Brown Over a year ago

The error says JsonList(Collection<? extends E> c) cannot be applied to ()

Elliott Frisch Over a year ago

@ReiBrown If you also want an empty constructor, then provide one. If you add a constructor, the compiler no longer inserts the default constructor.

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