Exceptions: when things go wrong

Exceptions: when things go wrong • Topics to be covered: • What is an exception • Throwing an exception • Catching an exception

Expecting the unexpected • When you design a program, you develop an algorithm for what’s supposed to happen; but, as we all know, life doesn’t always work that way. Example: • The user enters a floating point number instead of a integer • You try to evaluate X / Y, when Y has the value 0 • You try to access position 10 in an array with 10 elements

Example public class Exception1 { public static void main(String [] args) { int x = 1, y = 0; int z = x/y; System.out.println("Finished!"); } } Screen Output: Exception in thread “main” java.lang.ArithmeticException: / by zero at Exception1.main(Exception1.java:7)

Exceptions are objects • When something goes wrong we need to something about it • Exceptions are objects that contain information about what went wrong and where it happened • The getMessage() method returns a string explaining the exception Exception in thread “main” java.lang.ArithmeticException: / by zero • The printStackTrace() method prints the call stack trace at Exception1.main(Exception1.java:7)

Exception class hierarchy Object Throwable Error Exception RunTimeException ArithmeticException IndexOutOfBoundsException You can define your own exceptions

Creating your own exception public class MyException extends Exception { public MyException(String message) { super(message); } }

Another (silly) example public class Exception2 { public static void main(String [] args) { int x = 1, y = 0; System.out.println("Starting the calculations"); int ans = calculations(x,y); System.out.println("The answer is: " + ans); } public static int calculations(int x, int y) { int ans = division(x,y); return ans; } public static int division(int x, int y) { int z = x/y; return z; } }

Normal control flow main() calculations() division() Execution begins in main() which invokes calculations() which then invokes division(). When division() finishes control returns to calculations() and then from there to main().

When an exception is generated main() calculations() division() OH NO! When a method has a problem, it generates an exception object. It then throws the exception back along the call path. The exception travels along the call path until it is caught by a compatible catch statement. The catch statement may exist in the method where the exception was first generated or in any other method on the call path.

The throw clause • A method must declare that it may throw an exception • This information is as much a part of its normal header as its return type or its parameters public static int division(int x, int y) throws MyException • The information lets anyone calling the method know that they must be prepared for it to throw this exception • A throw statement is a little like a return statement except that a method may throw more than one exception and each exception may be of a different type

Throwing an exception import MyException; public class Exception2 { public static void main(String [] args) throws MyException { int x = 1, y = 0; System.out.println("Starting the calculations"); int ans = calculations(x,y); System.out.println("The answer is: " + ans); } public static int calculations(int x, int y) throws MyException { int ans = division(x,y); return ans; } public static int division(int x, int y) throws MyException { int z = 0; if(y==0) { MyException exceptionObj = new MyException("Division would be undefined"); throw exceptionObj; } else z = x/y; return z; } }

The screen output Starting the calculations Exception in thread "main" MyException: Division would be undefined at Exception2.division(Exception2.java:19) at Exception2.calculations(Exception2.java:12) at Exception2.main(Exception2.java:7) Press any key to continue...

Catching an exception • If a throw statement is like return statement, then a try/catch statement is a little like an if/else statement. • Statements that may throw an exception must be enclosed in a try block---this is just a regular block of code preceded by the key word try • The try block is followed by a catch block, which gives the instructions to execute if an exception of the specified type is caught • The catch clause always has a single parameter specifying the exception to be caught • The catch body is executed only if an exception of the appropriate type is thrown

A try/catch example public class Exception2 { … public static int calculations(int x, int y) { … } public static int division(int x, int y) { int z = 0; try { if(y==0) { MyException eObj = new MyException("Division would be undefined"); throw eObj; } else z = x/y; } catch(MyException e) { System.out.println("The exception message is: " + e.getMessage()); } return z; } }

The screen output Starting the calculations The exception message is: Division would be undefined The answer is: 0 Press any key to continue...

Points to remember • The try body contains statements that may throw an exception; at least one statement in the block must potentially throw an exception • Each catch block has a parameter that defines the type of exception it can catch • If a method catches an exception, it stops the propagation of that exception up the call path. Methods with appropriate try/catch blocks do have throw declarations in their headers. public static int division(int x, int y) throws MyException NO

Exceptions: when things go wrong