Exceptions: IllegalStateException signals bad state, IllegalArgumentException a bad argument

IllegalArgumentException means the caller passed a value the method cannot accept, while IllegalStateException means the arguments were fine but the object itself is not currently in a state where the call is allowed.

Code

class Counter {
    private int value;
    private boolean started;
    void start() {
        started = true;
    }
    void increment(int amount) {
        if (amount < 0) {
            throw new IllegalArgumentException("amount cannot be negative: " + amount);
        }
        if (!started) {
            throw new IllegalStateException("counter has not been started");
        }
        value += amount;
    }
}
Counter counter = new Counter();
try {
    counter.increment(5);
} catch (IllegalStateException e) {
    System.out.println("State: " + e.getClass().getSimpleName() + ": " + e.getMessage());
}
counter.start();
try {
    counter.increment(-1);
} catch (IllegalArgumentException e) {
    System.out.println("Argument: " + e.getClass().getSimpleName() + ": " + e.getMessage());
}

Output

State: IllegalStateException: counter has not been started
Argument: IllegalArgumentException: amount cannot be negative: -1