Remove object from Arraylist Java, only working in specific scenarios

Question

I'm trying to remove an object from an arraylist in java using android studio.

public class ToDoListManager {

    private List<ToDoItem> items;

    public ToDoListManager() {
        items = new ArrayList<ToDoItem>();

        items.add(new ToDoItem("Get Milk", false));
        items.add(new ToDoItem("Walk the dog", true));
        items.add(new ToDoItem("Go to the gym", false));
    }

    public List<ToDoItem> getItems() {
        return items;
    }

    public void addItem(ToDoItem item) {
        items.add(item);
    }

    public void removeItem(ToDoItem item) {
       items.remove(item);
    }
}

I've been calling the removeItem function from a keypress

When I add, let's say "test" to the array, it successfully adds it using items.add(item), however when I try items.remove(item) given the same string, it won't work. It works if I do items.remove(1), but not if I do items.remove("test")

How could I fix this? I've tried many different ways. Thanks.

You could not possibly call either items.remove(1) or items.remove("test") with this code, so you're not providing accurate information — Sean F
– Sean F, Commented Dec 4, 2021 at 4:02

Harry · Accepted Answer · 2021-12-06 22:29:48Z

The "remove" methods implemented in the various interfaces that make up ArrayList take different arguments and do very different things.

If you look at the "List interface" there are two methods

public interface List<E> extends Collection<E> {
    ...

    // It removes from the position in the list so it's 
    // going to work if the list has more than "index" 
    // items. So when you call it with an integer this 
    // gets used and works.

    public E remove(int index) 

    ...
    // There is a second remove method in the interface

    // This Removes an object from the list if the object 
    // equals an object in the list. So when you pass it a 
    // string ie "test" this cannot work because "test" is 
    // not a ToDoItem so the equals comparison fails.

    boolean remove(Object o);
    ...
}

If you want to use the boolean remove(Object o); method ie remove an Object you need to make sure the "equals" method is going to work in "ToDoItem". So what does your equals method look like in ToDoItem?

Gaurav Kukade · Accepted Answer · 2021-12-04 08:27:20Z

0

Your items only accepts the ToDoItem objects.

So, items.remove("test") will not work. Here "test" is a String object.

But items.remove(1) will work because here you are passing an index value as a parameter to remove() method. So the object from items at the specified index will be removed.

To remove the specified object from items list, you need to pass the ToDoItem object as a parameter.

Read more: How To Remove An Element From An ArrayList?

Note: If you want to compare two ToDoItem objects by its data members values, override the equals method in ToDoItem.

answered Dec 4, 2021 at 8:27

Gaurav Kukade

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3 Comments

Hypo Over a year ago

I tried passing it as an object instead but it still didn't work. I ended up changing how my program works in the end, thanks tho!

Gaurav Kukade Over a year ago

Are you creating a new object and passing it to remove() method? If yes, Have you overridden the equals() method in your ToDoItem class? If you are creating a new object (even with same param values) and passing it to arraylist remove method, it will not work. The default equals() method compare object reference for equality.

Gaurav Kukade Over a year ago

ArrayList remove method uses this condition to remove the object. (o==null ? get(i)==null : o.equals(get(i))) You need to override the equals() method ( and then hashcode() method to maintain the contract) to compare object.

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