Java Exception Handling

What is Exception Handling?

• Exception handling is a process to handle the execution when exception arises in the java code.
• Exception is an abnormal condition that arises in a java code sequence at run-time.
• All exception types are the subclasses of the predefined Throwable class.
• Two branches in Throwable class: Exception (is also called RuntimeException) and Error.
• Exception can be generated by Java run-time system or manually generated by java code.
• Java exception handling is managed via five keywords: try, catch, throw, throws and finally.
• Java code with raising Exception:

  class ExceptionTest {                                                           
   static int division() {                                                        
    int divider = 0;                                                              
    int result = 100/divider;                                                     
    return result;                                                                
   }                                                                              
   public static void main(String args[]) {                                       
    int val = ExceptionTest.division();                                           
    System.out.println(val);                                                      
   }                                                                              
  }  
  

  Java code raises ArithmeticException here.

  $ javac ExceptionTest.java
  $ java ExceptionTest
  Exception in thread "main" java.lang.ArithmeticException: / by zero
  	at ExceptionTest.division(ExceptionTest.java:4)
  	at ExceptionTest.main(ExceptionTest.java:8)
  

How to define try catch block in Java?

try block is used to monitor the java code and transfers execution to catch section if any exception raises.
catch block is used to execute alternate java codes if particular exception occurs.
finally block executes always at the end of try catch block's even exception is occurred or not. Mostly finally block is used for resource clean up codes.

try {
 //statements to monitor exception's
}
catch(ExceptionType1 exceptionObject) {
 //exception handler statements for ExceptionType1
}
catch(ExceptionType2 exceptionObject) {
 //exception handler statements for ExceptionType2
}
//...
finally {
 //Statements to be executed before try statements end
}

Example try catch:

class ExceptionHandlingTest {                                                   
 static int division() {                                                        
  int divider = 0;                                                              
  int result = 100/divider;                                                     
  return result;                                                                
 }                                                                              
 public static void main(String args[]) {                                       
  int val = 0;                                                                  
  try {                                                                         
    val = ExceptionTest.division();                                             
    System.out.println(val);                                                    
  } catch(ArithmeticException ex) {                                             
    System.out.println("Division by zero error is occurred.");                   
  }                                                                             
 }                                                                              
}  

Output:

$ javac ExceptionHandlingTest.java
$ java ExceptionHandlingTest
Division by zero error is occurred.

How to define try with Multiple catch and finally blocks?

when try block finds any exception, checks catch block in sequence and catch block that matched triggered exception will be executed.
finally block executes always at the end of try block even exception is occurred or not.

//Multiple catch with finally blocks
class MultipleCatchFinallyTest {                                                
 public static void main(String args[]) {                                       
  int val = 0;                                                                  
  try {                                                                         
    int d =0;                                                                   
    val = 100/d;                                                                
    int a[] = new int[2];                                                       
    a[3] = 100;                                                                 
    System.out.println(val);                                                    
  } catch(ArithmeticException ex) {                                             
    System.out.println("Division by zero error is occurred.");                   
  } catch(ArrayIndexOutOfBoundsException ex) {                                  
    System.out.println("Array index out of bound error is occurred.");           
  } finally {                                                                   
    System.out.println("After try and catch block!");                           
  }                                                                             
 }                                                                              
}  

Output:

Division by zero error is occurred.
After try and catch block!

How to define nested try with catch blocks?

try statement can be nested in java program.
try block can be placed inside the another try block.
Nested try example:

class NestedTryCatchTest {                                                      
 public static void main(String args[]) {                                       
  int val = 0;                                                                  
  try {                                                                         
    int d =0;                                                                   
    try {                                                                       
     val = 100/d;                                                               
    }  catch(ArithmeticException ex) {                                          
    System.out.println("Division by zero error is occurred.");                   
    }                                                                           
    int a[] = new int[2];                                                       
    a[3] = 100;                                                                 
    System.out.println(val);                                                    
  } catch(ArrayIndexOutOfBoundsException ex) {                                  
    System.out.println("Array index out of bound error is occurred.");           
  } finally {                                                                   
    System.out.println("After try and catch block!");                           
  }                                                                             
 }                                                                              
}  

Output:

$ javac NestedTryCatchTest.java 
$ java NestedTryCatchTest 
Division by zero error is occurred.
Array index out of bound error is occurred.
After try and catch block!

What is the use of throw statement?

throw statement is used to throw the exception explicitly.

class ThrowTest {                                                               
 static int division() {                                                        
  int divider = 0, result;                                                      
  try {                                                                         
   result = 100/divider;                                                        
  } catch(ArithmeticException ex) {                                             
    System.out.println("division method: Division by zero error.");             
    // rethrow the same exception to calling method                             
    throw ex;                                                                   
  }                                                                             
  return result;                                                                
 }                                                                              
 public static void main(String args[]) {                                       
  int val = 0;                                                                  
  try {                                                                         
    val = ThrowTest.division();                                                 
    System.out.println(val);                                                    
  } catch(ArithmeticException ex) {                                             
    System.out.println("Recaught: " + ex);                                      
  }                                                                             
 }                                                                              
}

Output:

$ javac ThrowTest.java 
$ java ThrowTest
division method: Division by zero error.
Recaught: java.lang.ArithmeticException: / by zero

What is the use of throws statement?

throws class is used to lists the types of exceptions that a method might throw.

type method-name(parameter-list) throws exception-list {
// method definition
}

throws example:

class ThrowsTest {                                                              
 static int division() throws ArithmeticException {                             
  int divider = 0, result;                                                      
  result = 100/divider;                                                         
  return result;                                                                
 }                                                                              
 public static void main(String args[]) {                                       
  int val = 0;                                                                  
  try {                                                                         
    val = ThrowTest.division();                                                 
    System.out.println(val);                                                    
  } catch(ArithmeticException ex) {                                             
    System.out.println("Exception: " + ex);                                     
  }                                                                             
 }                                                                              
}    

Output:

$ javac ThrowsTest.java 
$ java ThrowsTest
Exception: java.lang.ArithmeticException: / by zero

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