Refactoring a complicated if-condition

Question

Can anyone suggest best way to avoid most if conditions? I have below code, I want avoid most of cases if conditions, how to do it ? any solution is great help;

if (adjustment.adjustmentAccount.isIncrease) {
    if (adjustment.increaseVATLine) {
        if (adjustment.vatItem.isSalesType) {
            entry2.setDebit(adjustment.total);
            entry2.setCredit(0d);
        } else {
            entry2.setCredit(adjustment.total);
            entry2.setDebit(0d);
        }
    } else {
        if (adjustment.vatItem.isSalesType) {
            entry2.setCredit(adjustment.total);
            entry2.setDebit(0d);
        } else {
            entry2.setDebit(adjustment.total);
            entry2.setCredit(0d);
        }
    }
} else {
    if (adjustment.increaseVATLine) {
        if (adjustment.vatItem.isSalesType) {
            entry2.setCredit(adjustment.total);
            entry2.setDebit(0d);
        } else {
            entry2.setDebit(adjustment.total);
            entry2.setCredit(0d);
        }
    } else {
        if (adjustment.vatItem.isSalesType) {
            entry2.setDebit(adjustment.total);
            entry2.setCredit(0d);
        } else {
            entry2.setCredit(adjustment.total);
            entry2.setDebit(0d);
        }
    }
}

Answer 1

How to go about this... Let's extract a couple of methods, so we can better see the logic.

private void a() {
    entry2.setDebit(adjustment.total);
    entry2.setCredit(0d);
}
private void b() {
    entry2.setCredit(adjustment.total);
    entry2.setDebit(0d);
}

if (adjustment.adjustmentAccount.isIncrease) {
    if (adjustment.increaseVATLine) {
        if (adjustment.vatItem.isSalesType) {
            a();
        } else {
            b();
        }
    } else {
        if (adjustment.vatItem.isSalesType) {
            b();
        } else {
            a();
        }
    }
} else {
    if (adjustment.increaseVATLine) {
        if (adjustment.vatItem.isSalesType) {
            b();
        } else {
            a();
    }
} else {
    if (adjustment.vatItem.isSalesType) {
        a();
    } else {
        b();
    }
}

So now, looking at it, that first block

if (adjustment.increaseVATLine) {
    if (adjustment.vatItem.isSalesType) {
        a();
    } else {
        b();
    }
} else {
    if (adjustment.vatItem.isSalesType) {
        b();
    } else {
        a();
    }
}

just amounts to doing a() if adjustment.increaseVATLine has the same value as adjustment.vatItem.isSalesType, b() otherwise. So we can reduce it:

if (adjustment.adjustmentAccount.isIncrease) {
    if (adjustment.increaseVATLine == adjustment.vatItem.isSalesType) {
        a();
    } else {
        b();
    }
} else {
    if (adjustment.increaseVATLine) {
        if (adjustment.vatItem.isSalesType) {
            b();
        } else {
            a();
        }
    } else {
        if (adjustment.vatItem.isSalesType) {
            a();
        } else {
            b();
        }
    }
}

And the remaining block is the same, just reversing a() and b():

if (adjustment.adjustmentAccount.isIncrease) {
    if (adjustment.increaseVATLine == adjustment.vatItem.isSalesType) {
        a();
    } else {
        b();
    }
} else {
    if (adjustment.increaseVATLine == adjustment.vatItem.isSalesType) {
        b();
    } else {
        a();
    }
}

So we begin to see the logic. If it's an increase, and the increaseVATLine matches the isSalesType, then we debit, otherwise credit, but if it's a decrease, then we credit only if they don't match. What's a good way of expressing this? Well, for one, name a() and b() smarter - now that we can see what they're doing

if (adjustment.adjustmentAccount.isIncrease) {
    if (adjustment.increaseVATLine == adjustment.vatItem.isSalesType) {
        debitEntry();
    } else {
        creditEntry();
    }
} else {
    if (adjustment.increaseVATLine == adjustment.vatItem.isSalesType) {
        creditEntry();
    } else {
        debitEntry();
    }
}

And now it's a little clearer still. Debit the account when it's an increase account and an increase VAT line, and a sales type, or when it's a decrease and either it's a decrease VAT line OR it's a sales type, but not both. Does this truth table help? First column is adjustmentAmount.isIncrease; second is adjustment.increaseVATLine; third is adjustment.vatItem.isSalesType. Fourth column is D for debit, C for credit; in parentheses are the number of TRUE values among the flags.

TTT -> D (3) 
TFF -> D (1) 
TTF -> C (2)
TFT -> C (2) 
FTT -> C (2) 
FFF -> C (0)
FTF -> D (1) 
FFT -> D (1)

Now you can see why @Xavier Ho's solution works; the odd totals are all debits, the even ones all credits.

This is just one exploratory path; I hope it's helpful.

Answer 2

I haven't thoroughly verified the logic, but this is the basic idea:

amt = adjustment.total
if (adjustment.adjustmentAccount.isIncrease
    ^ adjustment.increaseVATLine
    ^ adjustment.vatItem.isSalesType)
{
    amt = -amt;
}

entry2.setCredit(amt > 0 ? amt : 0);
entry2.setDebit(amt < 0 ? -amt : 0);

I should note that this logic is slightly different in that it correctly handles a negative value of adjustment.total, whereas the original seems to assume (perhaps correctly) that the value will always be non-negative.