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I was thinking of using a Double as the key to a HashMap but I know floating point comparisons are unsafe, that got me thinking. Is the equals method on the Double class also unsafe? If it is then that would mean the hashCode method is probably also incorrect. This would mean that using Double as the key to a HashMap would lead to unpredictable behavior.
Can anyone confirm any of my speculation here?
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The Map interface stores the elements as key-value pairs. It does not allow duplicate keys but allows duplicate values. HashMap and LinkedHashMap classes are the widely used implementations of the Map interface. But the limitation of the Map interface is that multiple values cannot be stored against a single key.
Yes, we can use any object as key in a Map in java but we need to override the equals() and hashCode() methods of that object class. Please refer an example below, in which I am storing an object of Pair class as key in a hashMap with value type as string in map.
If you want to make a mutable object as a key in the hashmap, then you have to make sure that the state change for the key object does not change the hashcode of the object. This can be done by overriding the hashCode() method. But, you must make sure you are honoring the contract with equals() also.
Short answer: Don't do it
Long answer: Here is how the key is going to be computed:
The actual key will be a java.lang.Double object, since keys must be objects. Here is its hashCode() method:
public int hashCode() {
long bits = doubleToLongBits(value);
return (int)(bits ^ (bits >>> 32));
}
The doubleToLongBits() method basically takes the 8 bytes and represent them as long. So it means that small changes in the computation of double can mean a great deal and you will have key misses.
If you can settle for a given number of points after the dot - multiply by 10^(number of digits after the dot) and convert to int (for example - for 2 digits multiply by 100).
It will be much safer.
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