Java generic class extending comparable cannot be cast to java.base/[Ljava.lang.Comparable

Question

I am trying to get Object[] array casted to generic sorted I've implemented this part of the code

public class SortedArraySet<T extends Comparable<T>> implements Set<T>, Comparator<T> {

    T[] arr;
    int size = 5, index = 0;

    @SuppressWarnings("unchecked")
    SortedArraySet() {
        arr = (T[]) new Object[5];
        System.out.println("New set was initiated");
    }

    @Override
    public int compare(T a, T b) {

        if (a.compareTo(b) > 0)
            return 1;
        else

            return 0;
    }

When ever I run it I get the following compilation error

Exception in thread "main" java.lang.ClassCastException: java.base/[Ljava.lang.Object; cannot be cast to java.base/[Ljava.lang.Comparable; at q3.SortedArraySet.(SortedArraySet.java:12) at q3.q3main.main(q3main.java:6)

At line 6 it is stated as follows

SortedArraySet<Integer> sa = new SortedArraySet<Integer>();

The code was used to work fine before I added extends Comparable (and so compareTo) in order to sort the set

And is it possible to use Collections.sort?? I have tried but it doesn't seem to work with an array like that!

This is 100% runtime. The compiler put its trust in you when you declared those casts. — Makoto
– Makoto, Commented Jun 26, 2018 at 16:01
Yes I am aware about every other data structure but I am allowed to use array only :| , I would have solved that in 10 minutes with LinkedList , the point here is to implement a generic array sorted set — Adam Ma
– Adam Ma, Commented Jun 26, 2018 at 16:02

Amit Bera · Accepted Answer · 2018-06-26 16:46:53Z

2

You need to use :

arr = (T[]) new Comparable[5];

Instead of

arr = (T[]) new Object[5];

As after Type erasing T[] arr will be Comparable[] arr.So

T[] arr = (T[]) new Object[5];

Will become

Comparable[] arr = (Comparable[]) new Object[5];

And obviously, it will throw java.lang.ClassCastException.

edited Jun 26, 2018 at 16:46

answered Jun 26, 2018 at 16:01

Amit Bera

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4 Comments

Adam Ma Over a year ago

And can you explain why I have to use comparable as far is it is also extending Object

Amit Bera Over a year ago

@Athl1n3 If it solves your current issue. Then you should accept it and post a new question as too much editing may mislead the question and answer. My suggestion will be posting a new question with the new problem. I would like to help you.

Amit Bera Over a year ago

@Athl1n3 as you can't write a code like Comparable[] arr = (Comparable[]) new Object[5]; . It will gives you the same excpetion.

Adam Ma Over a year ago

Thank you, I appreciate that and I have solved the previous mentioned issue

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