Exceptions and Design by Contract

Making failures part of the design

A well-designed class makes its contract clear: preconditions (what callers must ensure, such as a positive amount), postconditions (what the method guarantees afterwards) and class invariants (what is always true for every object, such as "balance ≥ 0"). This idea, design by contract, comes from Bertrand Meyer's Eiffel language. In Java and similar languages, check preconditions at the start of public methods and fail fast with clear exceptions: IllegalArgumentException for bad arguments, IllegalStateException when the object is in the wrong state for the call, NullPointerException (often via Objects.requireNonNull) for missing values. Create custom exceptions for domain failures callers may handle (InsufficientFundsException). Java distinguishes checked exceptions (must be declared or handled, for recoverable conditions) from unchecked ones (programming errors); many modern codebases prefer unchecked exceptions or result types. Never swallow exceptions silently, and preserve the original cause when wrapping one exception in another.

Preconditions, invariants and a domain exception

Invalid calls fail immediately with a clear message.

public class InsufficientFundsException extends RuntimeException {
    public InsufficientFundsException(long requested, long available) {
        super("requested " + requested + " but only " + available + " available");
    }
}

public class Wallet {
    private long balancePaise;                                  // invariant: >= 0
    private boolean frozen;

    public void pay(long paise) {
        if (paise <= 0) throw new IllegalArgumentException("amount must be positive");   // precondition
        if (frozen) throw new IllegalStateException("wallet is frozen");
        if (paise > balancePaise) throw new InsufficientFundsException(paise, balancePaise);
        balancePaise -= paise;
        assert balancePaise >= 0 : "invariant broken";            // postcondition / invariant
    }
}

Fail at the boundary, not deep inside

A negative amount accepted by a constructor might cause a confusing error much later in a report. Validating at the public boundary points straight to the caller that broke the contract.

त्वरित जाँच: Which exception best signals that a method was called while the object is in the wrong state?

• IllegalArgumentException
• ArithmeticException
• IllegalStateException
• ClassNotFoundException