Can Decimals be used as Map keys? [duplicate]

Question

I have seen at least one other question where a user has had issues with using Decimals as map keys (his issue was interchangeably using ints vs decimals), but it isn't the same issue I'm having, where Decimal keys appear to be treated as Strings for comparison:

Map<Decimal,String> m = new Map<Decimal,String>();
m.put(3.10,'Decimal key with trailing zero');
List<Decimal> keys = new List<Decimal>(m.keySet());
Decimal d = keys.get(0);

//Both of these statements should be true. If not, at the very least,
//      they should both be false
System.debug(d+' = 3.1? '+(d == 3.1));//3.10 = 3.1? true
System.debug('CONTAINS KEY 3.1? '+m.containsKey(3.1));//CONTAINS KEY 3.1? false

//I can successfully add a Decimal key equivalent to one already in the Collection
m.put(3.1,'Equivalent Decimal key without trailing zero');
System.debug('SIZE = '+m.keySet().size());//SIZE = 2

//Again, the Decimal objects are equal
Decimal d1 = 3.1;
Decimal d2 = 3.10;
System.debug('Values are equal? '+(d1 == d2));//Values are equal? true

//But even if I remove the shorter value....
String removeValue = m.remove(3.1);
System.debug('removeValue = '+removeValue);//removeValue = Equivalent Decimal key without trailing zero

//...It will not match in the get statement either
String res = m.get(d1);
System.debug('Res == null? '+(res == null));//Res == null? true

Am I missing something?

Answer 1

Check out Using Custom Types in Map Keys and Sets to get a better understanding of how the uniqueness properties of objects are determined.

Map keys use the hashCode and equals methods to determine if two map/set keys are the same. The values 3.1 and 3.10 are "equal". However, the system hashes them differently, so they end up being considered unique.

system.assertEquals(3.1, 3.10);
system.assertNotEquals(system.hashCode(3.1), system.hashCode(3.10));

You can see a more complete answer here.

Answer 2

in the map, the keys are specific values. So having 2 decimals 3.10 and 3.1 with difference Scale are different Keys in the map. If you compare 3.10 and 3.1 as decimals they will return the same because salesforce does the scaling automatically for ease of use in most situations.