How To Calculate Average (Mean) Of Session Variables

My knowledge of programming is very limited. I would like to calculate the average of session variables in Interpreted Java. However, some of session variables may not always be available. The first thing I do is remove the non numerical portion of the session variables:

String [] variables = {"A",'B','C','D','E'};
i = 0;
Iterate through each variable in the array above
while (i < variables.length){
Get the variables to be fixed
value = session.getVariable(variables[i]);
Log the UNFIXED values
session.log("UNFIXED: " + variables[i] + " = " + value);
if(value != null){
Remove non-numerical elements
value = value.replaceAll("\\D", "");
Set variables with new values
dataRecord.put(variables[i], value);
session.setVariable(variables[i], value);
Log the FIXED values
session.log("FIXED " + variables[i] + " = " + session.getVariable(variables[i]));
} i++;
}

Then I make sure the session variables can be treated as numbers:
A = Integer.parseInt(session.getVariable("A")).intValue();
B = Integer.parseInt(session.getVariable("B")).intValue();
C = Integer.parseInt(session.getVariable("C")).intValue();
D = Integer.parseInt(session.getVariable("D")).intValue();
E = Integer.parseInt(session.getVariable("E")).intValue();
Are they still alive?
session.log("A**" + A + "**");
session.log("B**" + B + "**");
session.log("C**" + C + "**");
session.log("D**" + D + "**");
session.log("E**" + E + "**");

I would like to calculate the average of A and B and save it as session variable AB, calculate the average of C and D and save it as session variable CD, and then the average of AB, CD, and E. Here's the weird part; AB, CD, and or E will not always be available. For example, AB will not be available so I need to calculate the average of CD and E or E will not be available so I need to calculate the average of AB and CD. As I said, my knowledge of programming is very limited so if you could provide code snippet with explanation of each step and where to plug in my session variables.