Question

1) Describe briefly what exceptions are and what their purpose is. What is the try block? What is the catch block? What happens with the flow of control if the catch block does not catch a thrown exception? Give a simple example.

2) Describe briefly what templates are and what their purpose is.

3) Write one line of code for each line below. What is the template type in each case?

- Create a vector to store student IDs
- Create a vector to store student names
- Create a vector to store Point2D points
- Create a set to store unique (no duplicates) student names
- Create a set to store unique (no duplicates) student IDs
- Create a map that maps student IDs to their score
- Create a map that maps student IDs to their name
- Create a map that maps student names to the name of their lab partner

Answer 1

1)Answer:

• Exceptions are events that occur during the execution of programs that disrupt the normal flow of instructions (e.g. divide by zero, array access out of bound, etc.).

• In Java, an exception is an object that wraps an error event that occurred within a method and contains:

  • Information about the error including its type
  • The state of the program when the error occurred
  • Optionally, other custom information
• Exception objects can be thrown andcaught.

Exceptions are used to indicate many different types of error conditions.

• JVM Errors:

  • OutOfMemoryError
  • StackOverflowError
  • LinkageError

System errors:

• FileNotFoundException
• IOException

• SocketTimeoutException

  • Programming errors:
• NullPointerException
• ArrayIndexOutOfBoundsException
• ArithmeticException

• Exceptions separate error handling code from regular code.

  • Benefit: Cleaner algorithms, less clutter

• Exceptions propagate errors up the call stack.

  • Benefit: Nested methods do not have to explicitly catch-and-forward errors (less work, more reliable)

• Exception classes group and differentiate error types.

  • You can group errors by their generalize parent class, or
  • Differentiate errors by their actual class
• Exceptions standardize error handling.

In a traditional programming language like C, an error condition is usually indicated using an integer return value (e.g. -1 for out of memory). However, this practice:

• Requires standardization on error codes (hard for large projects)
• Makes functions hard to use because they must return actual values by reference
• Requires programmers to check return error codes in every nested function call. This leads to cluttered source code and hard-to-follow logic
• Does not allow for the state of the program to be easily captured on an error condition for later examination
• Cannot be enforced, and programmers often ignore error conditions, which leads to security or stability issues in program.

Try block:

The try block contains a block of program statements within which an exception might occur. A try block is always followed by a catch block, which handles the exception that occurs in associated try block. A try block must followed by a Catch block or Finally block or both.

Catch block:

catch block is an exception handler that handles the type of exception indicated by its argument. The argument type, ExceptionType , declares the type of exception that the handler can handle and must be the name of a class that inherits from the Throwable class. The handler can refer to the exception with name.

2)Templates:

template is a form, mold, or pattern used as a guide to making something. Here are some examples:

• A ruler is a template when used to draw a straight line.
• A document in which the standard opening and closing parts are already filled in is a template that you can copy and then fill in the variable parts.
• An overlay that you put on your computer keyboard telling you special key combinations for a particular application is a template for selecting the right keys to press.
• Flowcharting templates (not used much now) help programmers draw flowcharts or logic sequences in preparation for writing the code.