equals vs Arrays.equals in Java

Question

When comparing arrays in Java, are there any differences between the following 2 statements?

array1.equals(array2);
Arrays.equals(array1, array2);

And if so, what are they?

Answer 1

The Arrays.equals(array1, array2) :

check if both arrays contain the same number of elements, and all corresponding pairs of elements in the two arrays are equal.

The array1.equals(array2) :

compare the object to another object and return true only if the reference of the two object are equal as in the Object.equals()

Answer 2

Sigh. Back in the 70s I was the "system programmer" (sysadmin) for an IBM 370 system, and my employer was a member of the IBM users group SHARE. It would sometimes happen thatsomebody submitted an APAR (bug report) on some unexpected behavior of some CMS command, and IBM would respond NOTABUG: the command does what it was designed to do (and what the documentation says).

SHARE came up with a counter to this: BAD -- Broken As Designed. I think this might apply to this implementation of equals for arrays.

There's nothing wrong with the implementation of Object.equals. Object has no data members, so there is nothing to compare. Two "Object"s are equal if and only if they are, in fact, the same Object (internally, the same address and length).

But that logic doesn't apply to arrays. Arrays have data, and you expect comparison (via equals) to compare the data. Ideally, the way Arrays.deepEquals does, but at least the way Arrays.equals does (shallow comparison of the elements).

So the problem is that array (as a built-in object) does not override Object.equals. String (as a named class) does override Object.equals and give the result you expect.

Other answers given are correct: [...].equals([....]) simply compares the pointers and not the contents. Maybe someday somebody will correct this. Or maybe not: how many existing programs would break if [...].equals actually compared the elements? Not many, I suspect, but more than zero.