deepHashCode with byte array

问题

For some reason, Arrays.deepHashCode() cannot work with byte[].
Is there any other equivalent?

回答1:

First off, no need for "Deep". It's a primitive. You don't need Deep.

Just use Arrays.hashCode(byte[] yourArray)

Edit: To clarify, Deep implies delving into the Objects contained within the array. Given that you are dealing with a primitive, you just need to use the primitive value itself in the calculation. That's why none of the Deep methods revolve around primitives.

回答2:

The accepted answer is correct: using Arrays.hashCode gives identical results for byte[] with the same values. Arrays.deepHashCode is necessary if you have a nested (deep) structure.

import java.util.Arrays;

public class A {

    public static void main(String[] args) {
        byte[] a = {10, 32, -43, 80};
        byte[] b = {13, -40};
        byte[] c = {10, 32, -43, 80};

        System.out.println("NOTE: A and C have identical values, B differs");
        System.out.println("Using byte[].hashCode(): A and C have different hash codes");
        System.out.println("a = " + a.hashCode());
        System.out.println("b = " + b.hashCode());
        System.out.println("c = " + c.hashCode());

        System.out.println("Using Arrays.hashCode(): A and C have identical hash codes");
        System.out.println("a = " + Arrays.hashCode(a));
        System.out.println("b = " + Arrays.hashCode(b));
        System.out.println("c = " + Arrays.hashCode(c));

        System.out.println("Using Arrays.deepHashCode(): A and C have identical hash codes");
        System.out.println("a = " + Arrays.deepHashCode(new Object[]{a}));
        System.out.println("b = " + Arrays.deepHashCode(new Object[]{b}));
        System.out.println("c = " + Arrays.deepHashCode(new Object[]{c}));
    }
}

This results in output:

NOTE: A and C have identical values, B differs
Using byte[].hashCode(): A and C have different hash codes
a = 141847843
b = 329849131
c = 1119051810
Using Arrays.hashCode(): A and C have identical hash codes
a = 1250930
b = 1324
c = 1250930
Using Arrays.deepHashCode(): A and C have identical hash codes
a = 1250961
b = 1355
c = 1250961

Here's an example of when Arrays.deepHashCode is necessary

import java.util.Arrays;

public class B {
    public static void main(String[] args) {
        Object[] d = {"abc", "def", new String[]{"ghi"}};
        Object[] e = {"abc", "def", new String[]{"ghi"}};

        System.out.println("NOTE: D and E have identical nested values");
        System.out.println("Using Object[].hashCode(): different");
        System.out.println("d = " + d.hashCode());
        System.out.println("f = " + e.hashCode());

        System.out.println("Using Arrays.hashCode(): still different");
        System.out.println("d = " + Arrays.hashCode(d));
        System.out.println("e = " + Arrays.hashCode(e));

        System.out.println("Using Arrays.deepHashCode(): identical");
        System.out.println("d = " + Arrays.deepHashCode(d));
        System.out.println("e = " + Arrays.deepHashCode(e));
    }
}

output:

NOTE: D and E have identical nested values
Using Object[].hashCode(): different
d = 241990244
f = 1943487137
Using Arrays.hashCode(): still different
d = 1057745997
e = 709187068
Using Arrays.deepHashCode(): identical
d = 95807651
e = 95807651

回答3:

Using deepHashCode is indeed correct if you want two byte arrays containing the same bytes to have equivalent hash codes, you just need some additional casting for the byte[] array.

import java.utils.Arrays;

public class A {
  public static void main(String[] args) {
    byte[] a = {10,32,-43,80};
    byte[] b = {13,-40};
    byte[] c = {10,32,-43,80};
    // A and C will have different hash codes
    System.out.println(a.hashCode());
    System.out.println(b.hashCode());
    System.out.println(c.hashCode());
    // A and C will now have equivalent hash codes
    System.out.println(Arrays.deepHashCode(new Object[]{a}));
    System.out.println(Arrays.deepHashCode(new Object[]{b}));
    System.out.println(Arrays.deepHashCode(new Object[]{c}));
  }
}

This results in output similar to...

// Hash Codes
a = 16130931
b = 26315233
c = 32716405
// Deep hash codes
a = 1250961
b = 1355
c = 1250961

来源：https://stackoverflow.com/questions/4671858/deephashcode-with-byte-array

标签

java

arrays

hashcode