Why are my arrays with equal Number or Currency field values (from SObjects) not the same?

Question

I am using various fields, including Number or Currency type ones, in two different object types to form a "key" to allow me to generate an "identity" that I can use in a Map in Apex code, which I then use to store and retrieve details relevant to the one type of object AND the other.

These "keys" are implemented as simple Object arrays (since the field types vary), but the fields used at each index are equivalent from the first object type and the second. When I have a Number or Currency type field, these values are represented by Decimal values in Apex.

For (a very contrived) example, I might use the Account's AnnualRevenue field to match with a custom RevenueBreakdown__c's TotalRevenue__c field, both being Currency fields, in order to access data from my Map<Object[], SomeApexClass>.

However, I'm finding that, while the individual values in the key may be the same, the Map is not correctly retrieving the stored data. Any clues why?

Answer 1

Apex has a quirk that means the scale in a Decimal is sometimes ignored for equality but is sometimes treated as important. Take the following example:

Decimal a = 10;
Decimal b = 10;

// Simulate the number of decimal places being set differently in the
// two fields in the two objects where these values originate
a = a.setScale(2);
b = b.setScale(3);

Decimal[] aArray = new Decimal[] { a };
Decimal[] bArray = new Decimal[] { b };
Decimal[] bFixedArray = new Decimal[] { b.setScale(a.scale()) };

System.debug('a == b is ' + (a == b) );
System.debug('array a == array b is ' + (aArray.equals(bArray)));
System.debug('array a == array b (fixed) is ' + (aArray.equals(bFixedArray)));

The result of running this is:

a == b is true
array a == array b is false
array a == array b (fixed) is true

It looks like numeric equality operates correctly at the individual Decimal level, but when a Decimal is included in an array, the array equality inexplicably considers both the value AND the scale as being relevant. It is likely this reason that causes this issue.

The Currency or Number fields in the object model must have the same scale (number of decimal places) when they are declared to allow for this sort of usage, or you need to explicitly re-align the scales of the values to be used in these keys.

NB: Array equality in Apex is not checking addresses, but is checking CONTENT. The debug above is using Array.equals but you get the same answers using == between two arrays. From the List documentation (NB: arrays are lists are arrays):

Two lists are equal if their elements are equal and are in the same order. The == operator is used to compare the elements of the lists.

The == operator is equivalent to calling the equals method, so you can call list1.equals(list2); instead of list1 == list2;.