List<Integer> cannot be converted to ArrayList<Integer>

java list typeconverter
beepretty picture beepretty · Jul 31, 2015 · Viewed 15k times · Source

At the beginning of my code, there is:

List<List<Integer>> result = new ArrayList();

And then, (here is to reverse a sub-list):

List<Integer> sub = new ArrayList();
re = reverse(result.get(j)); // error occurs here

There is this method:

public ArrayList<Integer> reverse(ArrayList<Integer> list) {
    List<Integer> newList = new ArrayList<>();

    for(int i=list.size()-1; i>=0; i--) {
        newList.add(list.get(i));}
        return newList;
    }
}

The error message is:

List cannot be converted to ArrayList

Why?

Answer

Captain Man picture Captain Man · Jul 31, 2015

Think of it like this, you have a Fruit called re (I use this name because it's the name of the variable you are using).

Fruit re;

You have a method reverse whose input type is Apple.

public Apple reverse(Apple a) {
    // ...
}

We have a variable re that we declared as Fruit which means we're saying it's always going to be some kind of Fruit, perhaps Apple, but maybe Orange -- or even Banana.

When you try to give the Fruit to the method taking Apple the compiler stops you, because it can't be certain that it's 100% an Apple. For example...

Fruit re = new Orange();
reverse(re);

Yikes! We are putting a square peg into a round hole so to speak. reverse takes Apple, not Orange. Bad things could happen!

Side note: Why is it okay then to assign Apple to something declared as Fruit then? (reverse returns an Apple, Fruit f = reverse(re); is legal.) Because an Apple is a Fruit. If it were declared as the more specific Apple and the return type were the more general Fruit, then there would be an issue here. (If reverse returned Fruit, Apple a = reverse(re); would be illegal.)

If you didn't follow the metaphor, replace Fruit with List and Apple with ArrayList and read the above again. List is Fruit, a general way to describe an abstract idea. ArrayList is Apple, a specific implementation of the abstract idea. (LinkedList could be Orange too.)

In general you want to declare things as the most general thing you can to get the functionality you need. Making the below change should fix your problem.

public List<Integer> reverse(List<Integer> list) {

We are taking some kind of List of Integers and returning some kind of List of Integers.

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