How to store RGB colors to a hashmap?

问题:

I want to create a HashMap with RGB colors as keys. How should I store them to have best performance? I mean, how does the "speed" of a hashmap refer to the object type of the key?

Should I use Integer (018052175) where each triplet would be one of RGB, String (1234AF) as HEX, or own Color class with int r, g, b? What might be the fastest implementation?

回答1:

The speed of hash map is constrained by several essential properties of the hashCode and equals functions:

• How easy it is to calculate,
• How well is hashCode at distributing values into buckets, and
• How easy it is to compare values for equality

The hashCode function of a String is very good, and String caches its result to improve performance. However, equality check may be longer than with Integer.

The Integer class has a very hard-to-beat implementation of hashCode in terms of speed, but since the hash codes of similar colors would be close to each other, you may get more collisions with Integers.

Color is as fast as the Integer, but it is also the most descriptive. I seriously doubt that choosing one of these three representations would hamper performance so significantly as to make a visible difference, so I would suggest going for the most descriptive choice, and then profile if necessary.

回答2:

The only difference is going to be the speed of the hashcode/equals functions, and an Integer should be the fastest, though it is unlikely to be a bottleneck in any case. You should probably use whatever is going to be most convenient elsewhere in the code.

回答3:

HashMap is based on hashCode() and Integer.hashCode() is as fast as you can get (it's an identity function):

a hash code value for this object, equal to the primitive int value represented by this Integer object.

Thus go for Integer representing 24-bit RGB value. However it turns out Color.hashCode() is as fast, and since Color class is much more expressive and readable than Integer, use Color class instead.

回答4:

Java has a built in class for storing color called Color.

I would go and make optimizations (using integer will not be much faster I guess) unless you actually see its a bottleneck

回答5:

Use Integers, and get them with new Color(r, g, b).getRGB().

To get the color from an integer, just use new Color(hashmap.get(index)).

回答6:

Integer.hashCode() is no faster than Color.hashCode(). java.awt.Color's hashCode implementation (Java 1.6):

/**
 * Computes the hash code for this <code>Color</code>.
 * @return     a hash code value for this object.
 * @since      JDK1.0
 */
public int hashCode() {
    return value;
}

equals for both are also equivalent. So in your case I'd use Color as it better conveys the semantics.