Runtime errors occur while a program is running if the JVM detects an operation that is impossible to carry out. For example, if you access an array using an index that is out of bounds, you will get a runtime error with an ArrayIndexOutOfBoundsException. If you enter a double value when your program expects an integer, you will get a runtime error with an InputMismatchException.

In Java, runtime errors are thrown as exceptions. An exception is an object that represents an error or a condition that prevents execution from proceeding normally. If the exception is not handled, the program will terminate abnormally. How can you handle the exception so the program can continue to run or else terminate gracefully? This chapter introduces this subject, and text input and output.

To demonstrate exception handling, including how an exception object is created and thrown, let’s begin with the example in listing below which reads in two integers and displays their quotient.

import java.util.Scanner; 

public class Quotient {
  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    
    // Prompt the user to enter two integers
    System.out.print("Enter two integers: ");
    int number1 = input.nextInt();
    int number2 = input.nextInt();
    
    System.out.println(number1 + " / " + number2 + " is " +
      (number1 / number2));
  }
}