Reading values from sObjects is just as easy as setting them. We can retrieve the value of a field by using the dot notation along with the API name of an existing field.

Say we had a custom sObject in our org named OrderItem__c that has the fields Quantity__c and UnitPrice__c. The total cost of OrderItem__c is determined by multiplying these fields.

The method below accepts a single instance of OrderItem__c as a parameter. It then reads field values from the sObject record and uses it to calculate the total cost.

public Decimal getTotalCost(OrderItem__c orderItem){
     Integer quantity = orderItem.Quantity__c;
     Decimal unitPrice = orderItem.UnitPrice__c;
     
     Decimal totalCost =  unitPrice * quantity; 

     return totalCost;
}

Combine this with setting fields and you got the world on a string, baby!

public void setTotalCost(OrderItem__c orderItem){
     Integer quantity = orderItem.Quantity__c;
     Decimal unitPrice = orderItem.UnitPrice__c;

     Decimal totalCost =  unitPrice * quantity; 
     orderItem.TotalCost__c = totalCost;
}

Sometimes these variables can make our code verbose. If you prefer, you could also write the method like this:

public void setTotalCost(OrderItem__c orderItem){
     orderItem.TotalCost__c = orderItem.Quantity__c * orderItem.UnitPrice__c;
}