Exceptions and the Throwable hierarchy

Errors as part of a contract

A successful result is only one possible program behavior. A user may enter an invalid string, a file may be unavailable, or a domain operation may be invalid in the current state. A correct program must not silently turn such a situation into an ordinary number that looks plausible.

Distinguish compilation errors, runtime exceptions, and logic errors. A missing parenthesis cannot be fixed with catch, and an incorrect denominator in an average may not cause any exception. Each kind requires its own checks.

An exception is an object that signals abnormal completion of an operation. After throw, the normal execution path is interrupted. The JVM searches for a compatible handler in the current call and then in calls higher up the stack. Local variable values are not rolled back automatically.

Material on exceptions: https://dev.java/learn/exceptions/. Throwable API: https://docs.oracle.com/en/java/javase/26/docs/api/java.base/java/lang/Throwable.html. All complete examples in this topic are tested on JDK 27.

The Throwable hierarchy

Throwable is the base of Exception and Error. RuntimeException belongs to the Exception branch. RuntimeException and its subtypes, along with Error and its subtypes, are unchecked by the compiler. Other exceptions are checked.

Figure 4.1. Checked and unchecked exceptions

A method that can propagate a checked exception must declare it in throws or catch it. This is a compiler rule, not a measure of how serious a situation is. NumberFormatException is unchecked, but an invalid string is entirely expected in a CLI. IOException is checked, but is not always recoverable within a particular helper method.

Do not use catch (Throwable error) as a universal input check. Such a handler also catches serious environment errors after which continuing may be incorrect. Choose a type that matches the known contract.

NullPointerException often signals a programming error or a violated null contract. IllegalArgumentException indicates an invalid argument. IllegalStateException indicates a state in which an operation is not allowed. Choosing the right type helps callers distinguish these situations without searching for words in a message.

try and catch order

try surrounds an operation, and catch describes the response to a compatible exception. The first matching handler runs once. Place a subtype before a supertype; otherwise, the narrower handler becomes unreachable. Java checks many such errors at compile time.

multi-catch lets you combine several independent types using |. You cannot combine a type and its subtype, such as Exception and NumberFormatException: the broader type already covers the narrower one.

Example 1. Integer division

Contract: two arguments, each a valid int. Print the quotient and remainder. Invalid format and a zero divisor have different causes but the same CLI failure code. The pair Integer.MIN_VALUE and -1 is also prohibited: the mathematical quotient does not fit in int.

public class SafeDivide {
    static int run(String[] args) {
        if (args.length != 2) {
            System.err.println("Usage: SafeDivide a b");
            return 2;
        }
        try {
            int a = Integer.parseInt(args[0]);
            int b = Integer.parseInt(args[1]);
            if (a == Integer.MIN_VALUE && b == -1) {
                throw new ArithmeticException("Quotient too large");
            }
            System.out.println("Quotient: " + a / b);
            System.out.println("Remainder: " + a % b);
            return 0;
        } catch (NumberFormatException | ArithmeticException e) {
            System.err.println("Error: " + e.getMessage());
            return 2;
        } finally {
            System.err.println("Attempt completed");
        }
    }

    public static void main(String[] args) {
        System.exit(run(args));
    }
}

For 17 5, stdout contains a quotient of 3 and a remainder of 2, with exit code 0. For 17 0 or abc 5, no successful result is printed, and the code is 2. The system's cause message may depend on the JDK; focus on checking your own contract, streams, and code.

In this example, division is the first operation used to form the result. In a more complex report, calculate all required values first and then print the summary so that an error halfway through does not leave a misleading partial success.