How can you compare arrays for equality?

An array is itself an object in Java, so it inherits Object.equals, which compares the two references. In that sense, an array is only ever equal to itself — two distinct arrays with the exact same elements are still considered different objects.

Most of the time, what you actually want is to compare the content of your arrays: two arrays are equal if they contain the same elements, in the same order. That's what Arrays.equals gives you. It takes two arrays and returns whether their contents match — there are also overloads that take index ranges, so you can compare only a portion of the first array to a portion of the second.

You may think you're done, but you're not — an array can itself contain subarrays. If you compare a two-dimensional array with Arrays.equals, each subarray is compared with Object.equals, i.e. by reference, which is almost never what you want.

For that case, you need Arrays.deepEquals, which recurses into subarrays and compares their content as well.

java
int[] a1 = {1, 2, 3};
int[] a2 = {1, 2, 3};

a1.equals(a2);          // false — reference comparison
Arrays.equals(a1, a2);  // true  — content comparison

int[][] m1 = {{1, 2}, {3, 4}};
int[][] m2 = {{1, 2}, {3, 4}};

Arrays.equals(m1, m2);      // false — subarrays compared by reference
Arrays.deepEquals(m1, m2);  // true  — subarrays compared by content

Rule of thumb: use Arrays.equals for simple, single-dimensional arrays, and Arrays.deepEquals for multi-dimensional (or otherwise nested) arrays.

One last detail: both methods support null elements. The convention is that two null values are considered equal, so they won't fail if your array contains nulls. That said — don't put null values in your arrays if you can avoid it.