Checked vs unchecked

Definition

A checked exception must be caught or declared with throws, so the compiler makes you deal with it. An unchecked exception (a RuntimeException) needs no declaration and usually means a bug in the code.

Example code

Reading a missing file (checked) and bad input (unchecked)

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;

class Demo {
    // IOException is checked: the method must declare it
    static String readFile(String name) throws IOException {
        return Files.readString(Path.of(name));
    }

    // IllegalArgumentException is unchecked: no declaration needed
    static int percent(int part, int whole) {
        if (whole == 0) throw new IllegalArgumentException("whole must not be zero");
        return part * 100 / whole;
    }

    public static void main(String[] args) {
        try {
            readFile("missing.txt");
        } catch (IOException e) {
            System.out.println("Could not read: " + e.getClass().getSimpleName());
        }
        try {
            percent(1, 0);
        } catch (IllegalArgumentException e) {
            System.out.println("Bad input: " + e.getMessage());
        }
    }
}

// Output:
// Could not read: NoSuchFileException
// Bad input: whole must not be zero