How to make an effective hashCode?

Question

I have three hashCode methods, as described below, I have determined their priority by their effectiveness. I am wondering if there is another way to make the hashCode method more efficient.

1) public int hashCode() { //terrible
     return 5; 
   }
2) public int hashCode() { //a bit less terrible
     return name.length; 
   }
3) public int hashCode() { //better
     final int prime = 31;
     int result = 1;
     result = prime * result + ((name == null) ? 0 : name.hashCode());
     return result;
   }

Answer 1

There is no reliable way to ensure that your function is hashcodeoptimal, as it is measured by two different metrics.

• Efficiency . How to quickly calculate.
• Clashes . What is the probability of a collision.

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• Finds a place somewhere in the middle - but still not good.
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Answer 2

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Answer 3

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Answer 4

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public int hashCode() {
     return Objects.hashCode(name);
}

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Map<Key, Value> map;
map.put(key, value);
Value value = map.get(key);

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Map<String, Value> map;
map.put(key.getName(), value);
Value value = map.get(key.getName());

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public final class String
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    /** Cache the hash code for the string */
    private int hash; // Default to 0

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class Key 
{
    private final String name;
    ... // Other fields...

    private final int hashCode;

    Key(String name, ...)
    {
        this.name = name;
        ... // Other fields

        // Pre-compute and store the hash code:
        this.hashCode = computeHashCode();
    }


    private int computeHashCode()
    {
        int result = 31;
        result = 31 * result + Objects.hashCode(name);
        result = 31 * result + ... // Other fields
        return result;
    }
}

Answer 5

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