How to Effectively Implement hashCode() for Collections?

Implementing hashCode() for Collections

The optimal implementation of hashCode() for a collection depends on its usage pattern. However, one widely-accepted approach proposed by Josh Bloch in his book "Effective Java" is as follows:

Algorithm:

1. Assign a non-zero value to an integer variable result.

2. For each field f used in the equals() method:

   • For boolean fields, calculate (f ? 0 : 1).
   • For numeric fields (byte, char, short, int), calculate (int)f.
   • For long fields, calculate (int)(f ^ (f >>> 32)).
   • For float fields, calculate Float.floatToIntBits(f).
   • For double fields, calculate Double.doubleToLongBits(f) and treat the result as a long value.
   • For object fields, use the hashCode() method of the object or 0 if f is null.
   • For array fields, recursively calculate the hash values of each element and combine them.
3. Combine each hash value c with result: result = 37 * result c.
4. Return result.

Advantages:

• Provides a reasonable distribution of hash values for most use cases.
• Methodic approach ensures consistent behavior across different data types.
• Sensitive to changes that affect object equality.

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