SatyaCodes

Java- try, catch, finally

try, catch, finally

  • We have to place the risky code inside the try block and the corresponding exception handling code inside catch block. 
    --------------------------------
Without try-catch
--------------------------------
class Test {
 public static void main(String arg[]) {
   System.out.println("Statement 1");
   System.out.println(10 / 0);
   System.out.println("Statement 2");
   }
 }
 Statement 1
 Exception in thread "main" java.lang.ArithmeticException: / by zero

---------------------------------
With try-catch
--------------------------------
class Test {
  public static void main(String arg[]) {
    System.out.println("Statement 1");
        
    try {
      System.out.println(10 / 0);
    }
    catch(ArithmeticException e) {
      System.out.println(10 / 2);
    }
        
    System.out.println("Statement 2");
  }
}
Statement 1
5
Statement 2

  • In the case of try with multiple catch blocks the order of catch blocks is important. And it should be from child to parent, otherwise Compiler Error. Saying Exception xxx has already been caught.

  • If there is no chance of raising an exception in try statement, then we are not allowed to maintain catch block for that exception. If we do so, violation leads to compile time error. but this rule is applicable only for fully checked exceptions. 
    class Test {
  public static void main(String arg[]) {
    try {
      System.out.println("Hi");
    } catch(IOException e) {}
  }
}
Test.java:7: error: exception IOException is never thrown in body of
corresponding try statement
                } catch (IOException e) {

  • It is not recommended to maintain cleanup code with in the catch block. because there is no guaranty of execution of particular catch block.

  • finally block should always execute irrespective of whether the exception is raised or not and handled or not handled.

  • The finally block won’t be executed ,if the system itself exists(JVM shutdown) i.e in the case of System.exit() finally block won’t be executed.


Possible combinations of try, catch, finally trycases

class Demo {
  public static void main(String arg[]) {

    try {
      statement1;
      statement2;
      statement3;
    } 
    catch(Exception e) {
      statement4;
    } 
    finally {
      statement5;
    }
    
    statement6;
  }
}

  • if there is no exception, then the statements 1, 2, 3, 5, 6 will execute with normal termination.

  • if an exception raised at statement-2 and the corresponding catch block matched, then the statements 1, 4, 5, 6 will execute with normal termination.

  • if an exception raised at statement-2 but the corresponding catch block not matched then the statements 1, 5 will execute with abnormal termination.

  • if an exception raised at statement-2 and while executing the corresponding catch block at statement–4 an exception raised then the statements 1, 5 will execute with abnormal termination.

  • if an exception raised at statement-5 or at statement-6 then it is always abnormal condition.


What happens if we put return statement on try/catch? Will finally block execute?
Yes, finally block will execute even if you put a return statement in the try block or catch block.

try {
    //try block
    ...
    return success;
}
catch (Exception ex) {
    //catch block
    .....
    return failure;
}
finally {
    System.out.println("Inside finally");
}

The answer is yes. finally block will execute. The only case where it will not execute is when it encounters System.exit().


What happens when a finally block has a return statement?
Finally block overrides the value returned by try and catch blocks.

public static int myTestingFuncn(){
  try{
     ....
     return 5;
  } 
  finally {
     ....
     return 19;
   }
}

This program would return value 19 since the value returned by try has been overridden by finally.

Throws

If our code may be a chance of raising checked exception then compulsory we
should handle that checked exception either by using try, catch or we have to
delegate that responsibility to the caller using throws keyword otherwise `C.E
must be caught or declared to be thrown`

Throws will give an indication to the calling function to keep the called function under try and catch blocks.

It gives an information to the programmer that there may occur an exception so it is better for the programmer to provide the exception handling code so that normal flow can be maintained

class Cal {
  public void div(String a, String b) throws ArithmeticException,
  NumberFormatException {
    int c = Integer.parseInt(a) / Integer.parseInt(b);
  }
}

public class A {
  public static void main(String[] args) {
    Cal ob = new Cal();
    try {
      ob.div("a", "b");
    } catch(ArithmeticException e) {
      System.out.println("Divide By Zero");
    } catch(NumberFormatException e) {
      System.out.println("Enter Only INT's");
    } catch(Exception e) {
      System.out.println(" Some Other " + e);
    }
  }
}

.................................
Enter Only INT 's'

In above throws ArithmeticException, NumberFormatException Indicates it may throws these exceptions so please put ob.div(str,str) method in try,catch block

Throw

Throw keyword is used to explicitly throw an exception.

In above we didn’t create any Exception class Object in throws because JVM automatically creates Objects. If you want to create Exception class object manually and throw exception using throw keyword

public class Marks {
  public void pass(int marks) {
    if (marks < 35) {
      throw new ArithmeticException("You are Failed");
    } else {
      System.out.println(" You are Pass : " + marks);
    }
  }

  public static void main(String[] args) {
    Marks m = new Marks();
    m.pass(26);
  }
}
_____________________________________
Exception in thread "main"
java.lang.ArithmeticException: You are Failed
at excep.Marks.pass(Marks.java: 9)
at excep.Marks.main(Marks.java: 18)
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