Exception Handling and Managing Input and Output

What Exception Handling and Managing Input and Output covers

Exception Handling and Managing Input and Output is examined in Unit AS 1, Introduction to Object Oriented Development — a written examination. It covers why errors have to be trapped, types of error and how an exception is raised, the try and catch block, catching more than one kind of exception, the finally block and releasing resources, throwing exceptions and writing an exception class, checked and unchecked exceptions, reading and validating input from the command line, controlling screen output, text files, binary files and object files and serialisation.

The 12 subtopics

Each one has its own page.

• Why errors have to be trappedGraceful failure and run-time condition the program itself cannot control.Open →
• Types of error and how an exception is raisedSyntax error, run-time error and logic error.Open →
• The try and catch blockEnclosing the statements that may fail, what happens to the rest of the block once an exception is thrown, catching by type, and what a handler should do besides reporting.Open →
• Catching more than one kind of exceptionSeveral catch blocks against one try, why the order matters, why a general handler placed first makes the ones after it unreachable, and the finally block that runs either way.Open →
• The finally block and releasing resourcesFinally block and resource leak.Open →
• Throwing exceptions and writing an exception classCustom exception class, exception propagation and rethrowing an exception.Open →
• Checked and unchecked exceptionsThe distinction the compiler enforces, which faults must be caught or declared, which are left to the programmer, and why an unchecked exception usually signals a fault in the code rather than in the input.Open →
• Reading and validating input from the command lineCommand line input, range check and presence check.Open →
• Controlling screen outputStandard output stream and difference between print and println.Open →
• Text filesText file and comma separated values file.Open →
• Binary filesBinary file, fixed-length record and direct access.Open →
• Object files and serialisationObject file and version problem in serialisation.Open →

What you need to know

Answer each one before you reveal it.

Take Exception Handling and Managing Input and Output further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

• Checked exception vs Unchecked exception

  Checked exception: a fault the compiler insists is caught or declared. Unchecked exception: a fault the compiler lets pass without comment.

Questions from this topic

Each one with the model answer underneath.

1. 1

   What does it mean to trap an error in a program?

   Model answer

   Trapping an error means detecting a fault at the moment it happens and dealing with it inside the program, so execution continues in a controlled way instead of the program stopping.

2. 2

   What happens to a Java program when an exception is never caught?

   Model answer

   An uncaught exception makes the program stop immediately and print a stack trace to the console, and any work not already saved is lost.

3. 3

   Why is an unhandled crash unacceptable in a program updating a customer's records?

   Model answer

   A crash part way through an update can leave the records half written, so the stored data no longer agrees with itself and the customer's account is left wrong.

4. 4

   Give two costs of a program crashing in front of the person using it.

   Model answer

   The user loses everything they had entered and has to begin again, and confidence in the software falls even where the underlying fault is a small one.

5. 5

   Why can a programmer not remove the need for error trapping simply by checking the inputs?

   Model answer

   Some faults arise outside the program altogether, such as a missing file, a full disk or a broken network connection, and no amount of checking what the user types can prevent them.

6. 6

   What is a graceful failure?

   Model answer

   A graceful failure is one in which the program reports the problem in plain language, protects or saves what it can, and either recovers or shuts down in an orderly way.

What is inside Levo

Six ways to work the same material.

Exception Handling and Managing Input and Output revision questions

What does Exception Handling and Managing Input and Output cover in CCEA A-Level Software Systems Development?

12 subtopics: why errors have to be trapped, types of error and how an exception is raised, the try and catch block, catching more than one kind of exception, the finally block and releasing resources, throwing exceptions and writing an exception class, checked and unchecked exceptions, reading and validating input from the command line, controlling screen output, text files, binary files and object files and serialisation. Each one has its own page with questions and model answers.

How is Exception Handling and Managing Input and Output organised?

By the CCEA CL4 specification, subtopic by subtopic, so it matches the order you are taught it in.