Why does Arrays.sort take Object[] rather than Comparable[]?

Question

I was wondering why the sort method of the Arrays class is asking for a parameter of type Object[]. Why the parameter is not of type Comparable[]. If you don't pass a Comparable[] it's generating a ClassCastException.

Why ... public static void sort(Object[] a) and not public static void sort(Comparable[] a) ? Thanks

You have same situation at other places in the Java api, e.g. the ObjectOutputStream which expects an Object implementing Serializable. I guess, the developers tried to prevent us from doing unecessary casts. — ZeissS
– ZeissS, Commented Feb 11, 2010 at 18:24
Back in the day, there were multiple people implementing JDK's, not just Sun. Implementations of the class may have desired the Comparable, but allowed any deterministic, stable sort. (Hypothetically speaking) — Kylar
– Kylar, Commented Feb 11, 2010 at 18:38

Alexander Torstling · Accepted Answer · 2010-02-11 18:38:52Z

9

Because the second form would require a reallocation of the array. Even if you know that your array contains only comparables, you cannot just cast it to Comparable[] if the original type was Object[], since the array type does not match.

You can do:

Object[] arr = new String[0];
String[] sarr = (String[]) arr;

But you can't do:

Object[] arr = new Object[0];
String[] sarr = (String[]) arr;

So it's premature optimization :)

answered Feb 11, 2010 at 18:38

Alexander Torstling

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1 Comment

Karl Knechtel Over a year ago

"you cannot just cast it to Comparable[] if the original type was Object[]" - but why should I expect to be able to call it with an Object[]? If it's an intentional design constraint of the program that the array should only contain Comparables, why wouldn't I create and use a Comparable[] in the first place?

BalusC · Accepted Answer · 2010-02-11 18:29:46Z

3

Otherwise you can't pass Object[] in.

answered Feb 11, 2010 at 18:29

BalusC

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3 Comments

Jonathon Faust Over a year ago

@BalusC Is there ever a situation where you would pass to sort an array of Objects that you didn't know were all implementing Comparable? Any use of the sort method will be by objects that are Comparable. Seems like the only reason for accepting Object[] is that Object is used more frequently and is more familiar, and like ZeissS said, otherwise we would have to cast.

BalusC Over a year ago

There are still a lot of toArray() like methods in Java API which returns Object[].

chama Over a year ago

They could be implementing Comparator<T>, which is similar, but not the same as Comparable<T>

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