Arrays.hashCode(Object[]) versus Objects.hash(Object...)

Since JDK 1.5, the Arrays class has offered overloaded static methods with the name "hashCode"​. Most of the overloaded methods accept an array of a particular primitive type, but the Arrays.hashCode(Object[]) method can be used to calculate an int hash code for an array of reference types. Since its JDK 1.7 inception, the Objects class has provided a method called hash(Object...) that also returns an int hash code for a provided array of Java objects (the ellipsis [...] representing Java varargs is handled as an array and accepts an array). This post provides a brief comparison between Arrays.hashCode(Object) and Objects.hash(Object...).

We can look at the code in OpenJDK to see how OpenJDK implements the two methods being compared here. It turns out that Arrays.hashCode(Object[]) and Objects.hash(Object...) behave exactly the same way because Objects.hash(Object...) completely delegates to Arrays.hashCode(Object[]). This is shown in the next code listing extracted from the OpenJDK Objects.java class.

public static int hash(Object... values) {
    return Arrays.hashCode(values);
}

So, it turns out that the methods are really the same when one or more individual object references are passed. So, for many cases, whichever you choose is mostly a matter of taste. It may appeal to some to use the Arrays method directly given that's what's going to be called anyway. It is typically preferable to use the Arrays method when passing it a construct that is already known to be a Java array and to use the Objects method for situations where the values are being passed in a comma-separated combination without explicit array syntax being required (such as the case of implementing a custom class's hashCode() method and passing that class's attributes of arbitrary types for the hash code computation). When using an array of primitives of the same type, it is typically better to use the appropriate version of Arrays.hashCode for that particular primitive.

The simple class shown in the next code listing (and available on GitHub) demonstrates the differences and similarities in output between the overloaded versions of Arrays.hashCode and the Objects.hash(Object...) method.

package dustin.examples.hashcodes;

import java.util.Arrays;
import java.util.Objects;

import static java.lang.System.out;

/**
 * Demonstration that writes output to standard output with
 * hash codes generated for the same underlying array data by
 * both {@code Arrays.hashCode(Object[])} and by
 * {@code Objects.hash(Object...)}.
 */
public class HashesComparedDemo
{
   public static void main(final String[] arguments)
   {
      final int[] integers = ArraysCreator.createArrayOfInts();
      out.println("Arrays.hashCode(int[]) for int[]: " + Arrays.hashCode(integers));
      out.println("Objects.hash(Object...) for int[]:   " + Objects.hash(integers));
      out.println("Objects.hashCode(Object) for int[]:  " + Objects.hashCode(integers));

      final Integer[] refIntegers = ArraysCreator.createArrayOfIntegers();
      out.println("Arrays.hashCode(Object[]) for Integer[]: " + Arrays.hashCode(refIntegers));
      out.println("Objects.hash(Object...) for Integer[]:   " + Objects.hash(refIntegers));
      out.println("Objects.hashCode(Object) for Integer[]:  " + Objects.hashCode(refIntegers));

      final String[] strings = ArraysCreator.createArrayOfStrings();
      out.println("Arrays.hashCode(Object[]) for String[]: " + Arrays.hashCode(strings));
      out.println("Objects.hash(Object...) for String[]:   " + Objects.hash(strings));
      out.println("Objects.hashCode(Object) for String[]:  " + Objects.hashCode(strings));
   }
}

The code shown above passes three common data sets (an array of primitive int values, an array of reference Integer values, and an array of String values) to the methods Arrays.hashCode, Objects.hash(Object...), and the Objects.hashCode(Object) method that accepts a single Object (of which an overall array qualifies). The simple example then writes the respective hash code values generated by each method for each data set to standard output. The results of running this code are shown next.

Arrays.hashCode(int[]) for int[]: 1722319241
Objects.hash(Object...) for int[]:   356573628
Objects.hashCode(Object) for int[]:  356573597
Arrays.hashCode(Object[]) for Integer[]: 1722319241
Objects.hash(Object...) for Integer[]:   1722319241
Objects.hashCode(Object) for Integer[]:  1735600054
Arrays.hashCode(Object[]) for String[]: 448603921
Objects.hash(Object...) for String[]:   448603921
Objects.hashCode(Object) for String[]:  21685669

As we would expect, Arrays.hashCode(Object[]) and Objects.hash(Object...) return the same calculated hash code for the reference types Integer and String because they both effectively are the implementation of Arrays.hashCode(Object[]). The array of primitive int values leads to different results from Arrays.hashCode(int[]) than from Objects.hash(Object...) and this is, of course, because the array of primitives is passed to an overloaded Arrays.hashCode(int[]) method specifically implemented for that primitive data type rather than to Arrays.hashCode(Object[]).

If one compares the implementation of Arrays.hashCode(int[]) to the implementation of Objects.hash(Object...) delegated to Arrays.hashCode(Object[]), the implementations look essentially the same in terms of logic. The reason for the different result in the case of an array of primitive int values passed to Arrays.hashCode(int[]) when compared to the result from passing that same array of primitive int values to the Objects.hash(Object...) method is that the entire array of int values is treated as a single Object rather than as an array of individual objects when passed to Objects.hash(Object...). Favor the appropriate overloaded Arrays.hashCode method for an array of primitives instead of using Objects.hash(Object...) and favor Arrays.hashCode(Object[]) for arrays of reference types instead of using Objects.hash(Object...) to avoid compiler warnings and associated ambiguity.