Apa iku design by contract?

Question

Accepted Answer

**Design by Contract (DbC)** nggawé hubungan antara sawijining metode lan panggilan-e minangka perjanjian formal, sing ditetapke kanthi telu bab: **prekondisi** (apa sing kudu dijamin panggilan), **potkondisi** (apa sing dijamin metode buké), lan **invariant** (apa sing tetep bener kanggo obyek ing kabeh wektu).

## Telu obligasi

```text
PRECONDITION  → caller's duty: inputs/state the method requires to run correctly
POSTCONDITION → method's duty: what it promises on return (if precondition held)
INVARIANT     → always-true property of the object, before and after every method
```

## Ing kode

```python
class Account:
    def __init__(self, balance):
        self._balance = balance
        self._check_invariant()

def withdraw(self, amount):
        assert amount > 0, "PRE: amount must be positive"        # precondition
        assert amount <= self._balance, "PRE: sufficient funds"  # precondition
        old = self._balance
        self._balance -= amount
        assert self._balance == old - amount, "POST: balance reduced"  # postcond
        self._check_invariant()

def _check_invariant(self):
        assert self._balance >= 0, "INVARIANT: balance never negative"
```

Yen *prekondisi* gagal, **panggilan** sing duwe bug. Yen *potkondisi* utawa *invariant* gagal, **metode** sing duwe bug — kontrak nuduhake kasengsaraan kanthi presisi.

## Hubungan karo LSP

DbC nggawé Liskov presisi: subtype bisa **ngeslabke prekondisi** lan **ngetegke potkondisi**, nanging ora bisa sing kebalik. Iku persis aturan substitusibilitas, sing disebutke minangka kontrak.

## Pertukaran

```text
✓ Bugs surface at the boundary, with clear blame   ✓ Contracts double as documentation
⚠️ Runtime checks cost performance (often disabled in prod via assertions)
⚠️ Not all languages enforce it natively (Eiffel does; most use asserts/validation)
```

## Mengapa iku penting

Kontrak ngowahi asumsi sing ora jelas ("jumlah kudu positif, mengkene") dadi aturan sing eksplisit lan bisa dicek — lan dheweke nentoake salah kanthi presisi nalika ana sing rusak.

Sawetara ing penggantian Liskov lan desain API sing matěg: kontrak metode punika antarmuko sejatine, luwih jujur tinimbang tanda tangan jinis piyambak.