Checked vs unchecked exceptions

This is the most-asked exception question in Java interviews, and the reason is simple: the rule takes ten seconds to state, but the design question behind it ("which one would you use, and why?") has no single right answer. Java is almost alone among popular languages in having checked exceptions, and even the JDK uses them inconsistently. An interviewer wants to see that you know the compiler rules cold and that you have a defensible opinion.

For this lesson, we'll focus on the compiler rules, the long-running debate, a practical way to choose, wrapping exceptions without losing the cause, the pain of checked exceptions inside lambdas, the "sneaky throw" trick that proves checked exceptions only exist in javac, and the rules for throws clauses when you override a method.

01Picture it first: a signed delivery versus a dropped parcel

Think of a checked exception as a parcel that needs a signature. The courier (the compiler) won't leave it at your door: somebody in the call chain has to sign for it, either by handling it (catch) or by explicitly forwarding it (throws). An unchecked exception is a parcel thrown over the fence. Nobody was forced to sign, so it lands wherever it lands, usually at the request handler at the top of the stack.

Neither one is "better". A signature is useful when the receiver can do something with the parcel, and tedious when everyone in the chain forwards it without opening it, which is exactly the complaint people have about checked exceptions.

The diagram below is the decision path I use when I design a method and have to choose.

Fig. 1

Decision tree: if the failure is a programming error such as a null argument or illegal state, throw an unchecked exception like IllegalArgumentException; otherwise, if the immediate caller can realistically recover by retrying, falling back or asking the user, use a checked exception or a result type; if not, use an unchecked exception that wraps the cause and let a boundary handler deal with it

02Under the hood: the compiler rules

String loadInvoice(Path path) {
    return Files.readString(path);               // 1
}

void validate(int quantity) {
    if (quantity <= 0) throw new IllegalArgumentException("quantity must be > 0");  // 2
}

void ping() {
    try {
        System.out.println("ping");
    } catch (IOException e) {                    // 3
        System.out.println("never happens");
    }
}