Java – Introduction to Exceptions

Introduction: While Compiling and Executing Java application, chance of getting 3 types of errors.

• Compile time errors: Compiler raises error when we are not following language rules to develop the code.
  • Every Method should have return type.
  • Statement ends with semi-colon;
  • Invoking variable when it is not present

• Logical Errors: If we get the output of code instead of Expected contains Logical error.

Expected			Result
1				12345
12				1234
123				123
1234				12
12345				1

• Runtime Errors: Runtime Error is called Exception which terminates the normal flow of program execution.
  • Handling invalid input by user
  • Opening a file which is not present.
  • Connect to database with invalid user name and password.

InputMismatchException: It occurs if user enter invalid input while reading through Scanner.

import java.util.Scanner; class Code {             public static void main(String[] args) {                         Scanner sc = new Scanner(System.in);                         System.out.println(“Enter 2 numbers : “);                         int a = sc.nextInt();                         int b = sc.nextInt();                         int c = a+b;                         System.out.println(“Sum is : ” + c);             } }

Handling Exception using try-catch:

try block:

• It is used to place the code that may raise exception.
• When error occurs an exception object will be raised.

catch block:

• Exception object raised in try block can be collected in catch block to handle.

import java.util.Scanner; import java.util.InputMismatchException; class Code {             public static void main(String[] args) {                         Scanner sc = new Scanner(System.in);                         try{                                     System.out.println(“Enter 2 numbers : “);                                     int a = sc.nextInt();                                     int b = sc.nextInt();                                     int c = a+b;                                     System.out.println(“Sum is : ” + c);                         }                         catch (InputMismatchException e){                                     System.out.println(“Exception : Invalid input given”);                         }             } }

Note : Catch block executes only if exception raises in try block.

ArrayIndexOutOfBoundsException: Occurs when we try to access array elements out of index.

void main() {             int[] arr = {10,20,30,40,50};             System.out.println(arr[5]); }

NumberFormatException: Occurs when we try to perform invalid data conversions.

void main() {             String s = “abc”;             int x = Integer.parseInt(s); }

ArithmeticException: Occurs when try to divide any number with zero.

void main() {             int a=10, b=0;             int c=a/b; }

NullPointerException: Occurs when we access variable or method .

void main() {             Stirng s = null;             System.out.println(s.length()); }