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Lilian Blot VARIABLE SCOPE EXCEPTIONS FINAL WORD Final Lecture Spring 2014 TPOP 1.

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Presentation on theme: "Lilian Blot VARIABLE SCOPE EXCEPTIONS FINAL WORD Final Lecture Spring 2014 TPOP 1."— Presentation transcript:

1 Lilian Blot VARIABLE SCOPE EXCEPTIONS FINAL WORD Final Lecture Spring 2014 TPOP 1

2 Lilian Blot Overview Variable Scope and Life time Handling Errors Java Collections API Final Words TPOP 2 Spring 2014

3 Lilian Blot Variable Types In a class definition, there are three kinds of variables.  instance variables: Any method in the class definition can access these variables  parameter variables: Only the method where the parameter appears can access these variables. This is how information is passed to the object.  local variables: Only the method where the parameter appears can access these variables. These variables are used to store intermediate results. TPOP 3 Spring 2014

4 Lilian Blot Scope Related to the idea of ‘block structures’ The scope of a variable (field) is defined as the area where the variable can be seen ‘inner’ blocks can see variables declared in ‘outer’ blocks TPOP 4 Spring 2014

5 Lilian Blot Scope TPOP 5 Spring 2014 public class TwoSides { int side1, side2 ; public boolean testRightTriangle( int hypoteneuse ) { int side1Squared = side1 * side1 ; int side2Squared = side2 * side2 ; int hypSquared = hypoteneuse * hypoteneuse ; return side1Squared + side2Squared == hypSquared ; } Code Parameter variable Instance variable locale variable

6 Lilian Blot Scope We say the scope of instance variables is the entire class definition. The scope refers to the section of code where a variable can be accessed. The scope for a parameter is simply the method body in which the parameter is located. TPOP 6 Spring 2014

7 Lilian Blot Local Variables Have No Memory When you exit a method, both the local variables and the parameter variables disappear  although, if you pass an object as a parameter, you usually have access to the object when you exit the method. TPOP 7 Spring 2014

8 Lilian Blot Lifetime Different kinds of variables have different lifetimes. Parameter variables exist when the method call is made, and lasts until the method call is complete. Local variables also have a similar lifetime. They are created when the declaration appears, and lasts until you exit the method. Instance variables have a much longer lifetime. They are created when the object is constructed, and goes away when the object disappears.  The object disappears when it is no longer being used, and when that happens, the garbage collector gets rid of the object. The garbage collector is a program that runs when your program runs, and looks for objects with no variables that have handles to it. TPOP 8 Spring 2014

9 Lilian Blot Scope vs. Lifetime Scope refers to the range of code where you can access a variable. We can divide scope into  method body scope Variable is accessible in method body only (local variables, parameters)  class definition scope Variable is accessible in class definition (instance variables) Lifetime refers to the amount of time a variable (or object) exists. We can divide lifetime into categories too:  method body lifetime Exists on method body entry, disappears on method body exit. (local variables, parameters)  class definition lifetime Exists as long as object is around (instance variables) TPOP 9 Spring 2014

10 Lilian Blot Scope TPOP 10 Spring 2014 public class TwoSides { int side1, side2 ; public boolean testRightTriangle( int hypoteneuse ) { if(hypotenuse >= 0){ int side1Squared = side1 * side1 ; int side2Squared = side2 * side2 ; int hypSquared = hypoteneuse * hypoteneuse ; } else: return false; return side1Squared + side2Squared == hypSquared ; } Code Parameter variable Instance variable locale variable Block scope ERROR

11 Lilian Blot Be careful What is the outcome of calling print() ? TPOP 11 Spring 2014 public class TwoSides { int side1 = 1, side2 = 2; public void print() { int side1 = 4; System.out.println(side1 * side2); } Code The result is 8! Local variable side1 is hiding instance variable side1.

12 Lilian Blot Handling Errors Spring 2014 TPOP 12

13 Lilian Blot Main concepts to be covered Defensive programming.  Anticipating that things could go wrong. Exception handling and throwing. Error reporting. TPOP 13 Spring 2014

14 Lilian Blot Not always programmer error Errors often arise from the environment:  Incorrect URL entered.  Network interruption. File processing is particular error-prone:  Missing files.  Lack of appropriate permissions. TPOP 14 Spring 2014

15 Lilian Blot Error reporting How to report illegal arguments? To the user?  Is there a human user?  Can they solve the problem? To the client object?  Return a diagnostic value.  Throw an exception. TPOP 15 Spring 2014

16 Lilian Blot Client responses Test the return value.  Attempt recovery on error.  Avoid program failure. Ignore the return value.  Cannot be prevented.  Likely to lead to program failure. Exceptions are preferable. TPOP 16 Spring 2014

17 Lilian Blot Throwing an exception TPOP 17 Spring 2014 /** * Look up a name or phone number and return the * corresponding contact details. key The name or number to be looked up. The details corresponding to the key, * or null if there are none matching. NullPointerException if the key is null. */ public ContactDetails getDetails(String key) { if(key == null) { throw new NullPointerException( "null key in getDetails"); } return book.get(key); } Code

18 Lilian Blot Throwing an exception An exception object is constructed:  new ExceptionType("...") The exception object is thrown:  throw... Javadoc documentation: ExceptionType reason TPOP 18 Spring 2014

19 Lilian Blot The exception class hierarchy TPOP 19 Spring 2014

20 Lilian Blot Exception categories Checked exceptions  Subclass of Exception  Use for anticipated failures.  Where recovery may be possible. Unchecked exceptions  Subclass of RuntimeException  Use for unanticipated failures.  Where recovery is unlikely. TPOP 20 Spring 2014

21 Lilian Blot The effect of an exception The throwing method finishes prematurely. No return value is returned. Control does not return to the client’s point of call.  So the client cannot carry on regardless. A client may ‘catch’ an exception. TPOP 21 Spring 2014

22 Lilian Blot Unchecked exceptions Use of these is ‘unchecked’ by the compiler. Cause program termination if not caught. This is the normal practice. IllegalArgumentException is a typical example. TPOP 22 Spring 2014

23 Lilian Blot Exception handling Checked exceptions are meant to be caught. The compiler ensures that their use is tightly controlled.  In both server and client. Used properly, failures may be recoverable. TPOP 23 Spring 2014

24 Lilian Blot The throws clause Methods throwing a checked exception must include a throws clause: TPOP 24 Spring 2014 public void saveToFile(String destinationFile) throws IOException {... } Code

25 Lilian Blot The try statement Clients catching an exception must protect the call with a try statement: TPOP 25 Spring 2014 try { Protect one or more statements here. } catch(Exception e) { Report and recover from the exception here. } Code

26 Lilian Blot Catching multiple exceptions Clients catching an exception must protect the call with a try statement: TPOP 26 Spring 2014 try {... ref.process();... } catch(EOFException e) { // Take action on an end-of-file exception.... } catch(FileNotFoundException e) { // Take action on a file-not-found exception.... } Code

27 Lilian Blot The finally clause Clients catching an exception must protect the call with a try statement: TPOP 27 Spring 2014 catch(Exception e) { Report and recover from the exception here. } finally { Perform any actions here common to whether or not an exception is thrown. } catch(FileNotFoundException e) { // Take action on a file-not-found exception.... } Code

28 Lilian Blot The finally clause A finally clause is executed even if a return statement is executed in the try or catch clauses. A uncaught or propagated exception still exits via the finally clause. TPOP 28 Spring 2014

29 Lilian Blot Defining new exceptions Extend RuntimeException for an unchecked or Exception for a checked exception. Define new types to give better diagnostic information.  Include reporting and/or recovery information. TPOP 29 Spring 2014

30 Lilian Blot Defining new exceptions TPOP 30 Spring 2014 public class NoMatchingDetailsException extends Exception { private String key; public NoMatchingDetailsException(String key) { this.key = key; } public String getKey() { return key; } public String toString() { return "No details matching '" + key + "' were found."; } Code

31 Lilian Blot Summary Runtime errors arise for many reasons.  An inappropriate client call to a server object.  A server unable to fulfill a request.  Programming error in client and/or server. Runtime errors often lead to program failure. Defensive programming anticipates errors – in both client and server. Exceptions provide a reporting and recovery mechanism. TPOP 31 Spring 2014

32 Lilian Blot Final Words TPOP 32 Spring 2014


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