Magic Strings

Magic strings are String values that are specified directly within application code that have an impact on the application’s behaviour. Frequently, such strings will end up being duplicated within the code.

Avoid hard-coded Strings

// bad 

function Boolean isClosed(Opportunity deal) {
    if (deal.StageName == 'Closed') {
        return true;
    }
    return false;
}

Best practice

Create a class which contains all the String values for a given pick list

public class OpportunityStageNames {
    public static final String New_x = 'New';
    public static final String Discovery = 'Discovery';
    public static final String Proposal = 'Proposal';
    public static final String Closed = 'Closed';
}

And then used this class to access the single instance of the String value.

// good

function Boolean isClosed(Opportunity deal) {
    if (deal.StageName == OpportunityStageNames.Closed) {
        return true;
    }
    return false;
}