Decrease Indentation

And when I talk about indentation, I mean nested if statements, another one of my pet peeves.

This specific refactoring is called: Replace Nested Conditional with Guard Clauses^[1].

Preconditions (GuardClauses) are a great example of where it is okay to return early at the start of the function.

It is very convenient to prevent the ArrowAntiPattern.

Here a bad example:

public boolean getPayment(String a, Double b)
{
    boolean result = false;
    if (a != null && b != null && !a.equals("") && b >= 0.0)
    {
        User user = getUser(a);
        if (user != null && user.getAccountValue() >= b))
        {
            user.setAccountValue(user.getAccountValue() - b);
            return true;
        }
        else
        {
            log.debug("User account not found or insufficient funds.");
            result = false;
        }
    }
    else
    {
        log.debug("Bad values.");
        result = false;
    }
    return result;
}

Here a good example (bad example refactored):

public boolean getPayment(String a, Double b)
{
    // preconditions
    if (a == null || b == null)
    {
        log.debug("Null values");
        return false;
    }
    if ("".equals(a) || b <= 0.0)
    {
        log.debug("Bad values");
        return false;
    }
    User user = getUser(a);
    if (user == null)
    {
        log.debug("User account not found.");
        return false;
    }
    if (user.getAccountValue() < b)
    {
        log.debug("Insufficient funds.");
        return false;
    }
    user.setAccountValue(user.getAccountValue() - b);
    return true;
}

One important skill here is the proper negation of an expression. You'd be surprised how many people lack this skill.

References

[1] Refactoring - Improving the design of existing code

Martin Fowler