The exceptions that you catch in a try-catch block must have been raised by a method that you’ve called. You can raise an exception with a statement that consists of the throw keyword, followed by an exception object. This exception object is an instance of any subclass of the Throwable class.

In the example below we have two static methods that throws exception. The first method, throwException() will throw an ArithmethicException when the divider is a zero value integer. The second method, printDate(Date date) will throw a NullPointerException if the date parameter value is null.

package org.kodejava.basic;

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;

public class ThrowExample {
    public static void main(String[] args) {
        try {
            ThrowExample.throwException();
        } catch (Exception e) {
            e.printStackTrace();
        }

        try {
            ThrowExample.printDate(null);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static void throwException() {
        int x = 6;
        int[] numbers = {3, 2, 1, 0};

        for (int y : numbers) {
            if (y == 0) {
                // Throws an ArithmeticException when about to
                // divide by zero.
                String message = String.format(
                        "x = %d; y = %d; a division by zero.",
                        x, y);
                throw new ArithmeticException(message);
            } else {
                int z = x / y;
                System.out.println("z = " + z);
            }
        }
    }

    public static void printDate(Date date) {
        if (date == null) {
            throw new NullPointerException("Date cannot be null.");
        }
        DateFormat df = new SimpleDateFormat("dd/MM/yyyy");
        System.out.println("Date: " + df.format(date));
    }
}

The output of our code:

z = 2
z = 3
z = 6
java.lang.ArithmeticException: x = 6; y = 0; a division by zero.
    at org.kodejava.basic.ThrowExample.throwException(ThrowExample.java:33)
    at org.kodejava.basic.ThrowExample.main(ThrowExample.java:10)
java.lang.NullPointerException: Date cannot be null.
    at org.kodejava.basic.ThrowExample.printDate(ThrowExample.java:43)
    at org.kodejava.basic.ThrowExample.main(ThrowExample.java:16)