try and catch

What is an exception?

When something goes wrong while a program runs – dividing by zero, reading a number that isn't a number – Java throws an exception. If nobody catches it, the program crashes:

public class Main {
    public static void main(String[] args) {
        int a = 10;
        int b = 0;
        System.out.println(a / b);
        System.out.println("This never prints");
    }
}

Output (Java stops with an error)

Exception in thread "main" java.lang.ArithmeticException: / by zero

Catching it

Put the risky code in a try block. If an exception happens, Java jumps to the catch block instead of crashing:

public class Main {
    public static void main(String[] args) {
        try {
            int result = 10 / 0;
            System.out.println(result);
        } catch (ArithmeticException e) {
            System.out.println("Oops: " + e.getMessage());
        }
        System.out.println("The program keeps going");
    }
}

Output

Oops: / by zero
The program keeps going

Think of it like this: A trapeze artist with a safety net. try is the risky jump; catch is the net. If they fall, the show goes on.

Checking user input

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String text = sc.nextLine();
        try {
            int age = Integer.parseInt(text);
            System.out.println("Next year you'll be " + (age + 1));
        } catch (NumberFormatException e) {
            System.out.println("'" + text + "' is not a number");
        }
    }
}

Input (what the user types)

twenty

Output

'twenty' is not a number

Common exceptions