Generics - ClassCastException when extending Number [duplicate]

Question

I'm learning to work with generics, I can't understand why this is throwing an exception:

My 'GenericList' class:

package com.company.generics;

public class GenericList<T extends Number>{

    private T[] items = (T[]) new Object[10];
    private int count;

    public void add(T item){
        items[count++] = item;
    }

}

My main class:

package com.company;

import com.company.generics.GenericList;

public class Main {

    public static void main(String[] args) {

        GenericList<Integer> g = new GenericList<>();

    }
}

Exception thrown:

Exception in thread "main" java.lang.ClassCastException: class [Ljava.lang.Object; cannot be cast to class [Ljava.lang.Number; ([Ljava.lang.Object; and [Ljava.lang.Number; are in module java.base of loader 'bootstrap')
    at com.company.generics.GenericList.<init>(GenericList.java:4)
    at com.company.Main.main(Main.java:9)

Because you can't do it. You can't cast Number[] to Integer[], for example. You have to cast the array element everywhere required. — user207421
– user207421, Commented Mar 14, 2021 at 4:46
Does this answer your question? Why "java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to" error for Bounded type parameters and not for Formal Type parameters? — Savior
– Savior, Commented Mar 14, 2021 at 5:04

markspace · Accepted Answer · 2021-03-14 05:09:51Z

2

Since T has an upper bound, the cast has an upper bound and is actually resolving to Number rather than Object. You'll have to declare your array of that type instead of Object.

private T[] items = (T[]) new Number[10];

And according to Effective Java, you may have to add a @SupressWarnings("unchecked") to that assignment. I didn't actually try it however.

edited Mar 14, 2021 at 5:09

answered Mar 14, 2021 at 4:48

markspace

11.1k3 gold badges28 silver badges44 bronze badges

Sign up to request clarification or add additional context in comments.

8 Comments

M. Justin Over a year ago

A change from when?

markspace Over a year ago

Well I don't recall it working that way before. I've always just used Object[] like the op has. But I might be misremembering.

M. Justin Over a year ago

I feel like it's always resolved to the most specific type that can be inferred at compile time, though that doesn't come up too often.

markspace Over a year ago

Because the type is correctly declared as T[], so you want to match it. It's safer to do it that way because a return items[x] will return the correct type. C.f. Effective Java by Joshua Bloch, Item 26 "Favor Generic Types."

markspace Over a year ago

I'm sure the OP's example has been reduced to the minimum for the sake of the question. :) I don't see any practical use for a class with only an add() method but no way to retrieve the values.

|

Collectives™ on Stack Overflow

Generics - ClassCastException when extending Number [duplicate]

1 Answer 1

8 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

8 Comments

Linked

Related