Chapter 15 - Exception Handling

Question 1

What can some API methods lead to because they are dangerous?

Answer 1

A runtime error

Question 2

What is a technique for handling runtime errors?

Answer 2

Use exception handling. More specifically, surround the method call with a try block and insert a catch block immediately after the try block.

Question 3

Try/Catch sytax and code example

Answer 3

Question 4

What does NumberFormatException tell a user?

Answer 4

It’s thrown when a number’s format is inappropriate.

Question 5

What causes the NumberFormatException to be thrown?

Answer 5

It’s thrown my one of the parse methods (Integer.parseInt, Long.parseLong, Double.parseDouble etc.)

When there’s an attempt to convert a string to a number and the string’s character don’t form a valid number.

Question 6

How large can try blocks be?

Answer 6

Big or small, depends on the need.

It is legal to surround an entire method body with a try block, but that’s usually counterproductive because it makes it harder to identify the “dangerous” code.

Question 7

What should you do if you have a chunk of code that has several “dangerous” method/constructor calls?

Answer 7

• Add a seperate try-catch structure for each scuh call might result in cluttered code
• To prevent this and improve program readability, consider using a single try block that surrounds the calls.

Question 8

What does the JVM do if an exception is thrown?

Answer 8

It immediately jumps out of the current try block and looks for a matching catch block. The immediacy of the jump means that if there are statements in the try block after the exception-throwing statement, those statements are skipped.

Question 9

What does the compiler assume in regards to a try block?

Answer 9

It knows that statements inside a try block might be skipped, so in turn it assumes that all statements inside a try block get skipped.

Question 10

What can happen if an assignments is within a try block, but the same assignment doesn’t exist outside of the try block?

Answer 10

It can give you the following compilation error:

variable x might not have been initialized

Question 11

How do you fix the variable x might not have been initialized error?

Answer 11

Initialize the variable prior to the try block.

Question 12

What are the two types of exceptions?

Answer 12

Checked and Unchecked.

Question 13

What is a check exception?

Answer 13

Checked exceptions are required to be checked with a try-catch mechanism.

Question 14

What is an unchecked exception?

Answer 14

Unchecked exceptions are not required to be checked with a try-catch mechanism (but, as an option, unchecked exceptions may be checked with a try-catch mechanism)

Question 15

How can you tell if a particular exception is classified as checked or unchecked?

Answer 15

• Look up its associated class in the API documentation
• On the class’s API page, look at its class hierarchy tree. If you find that the class is derived from the RuntimeException class or from the Error exceptions class, then it’s an unchecked exception. Otherwise, it’s a check exception.

Question 16

Class hierarchy for Exception Classes visual

Answer 16

Question 17

What happens if an unchecked exception is thrown and not caught with a try-catch mechanism?

Answer 17

The program will crash

Question 18

How do you handle code that might thrown an unchecked exception?

Answer 18

• Use a try-catch mechanism

or

• Don’t attempt the catch the exception, but write the code carefully so as to avoid the possibility of the exception being thrown.

Question 19

Where should you look if you want to use a method or constructor from one of the API classes, but you’re unsure about it?

Answer 19

The API codumentation.

Question 20

What can the API documentation be used for?

Answer 20

- To determine if the method/constructor call could possible throw an exception (On the API documentation page for the method/constructor, look for a throws sections. If there’s a throws section, that means the method/constructor can possible throw an exception.)

- To determine whether the method/constructor call throw is a checked or unchecked exception.

Question 21

What must you add if a method/constructor can possible throw a checked exception?

Answer 21

A try-catch mechanism

Question 22

What do you do if a method/constructor can possible throw an unchecked exception?

Answer 22

Read teh API documentation to figure out the nature of the exception and then either:

• Use a try-catch mechanism
• Use careful code so that the exception won’t be thrown.

Question 24

What should you do if several statements within a try block can possible throw an exception and the exceptions are of different types?

Answer 24

• Provide a generic catch block that handles every type of exception that might be thrown.

or

• Provide a sequence of specific catch blocks, with each catch block handling a different exception type or group of exception types.

Question 25

How do you define a catch block?

Answer 25

With an Exception parameter, e.

Example:

Question 26

In a catch block heading, what happens if you use an Exception parameter?

Answer 26

All thrown exceptions will match up with the exception parameter.

Question 27

What is required for a thrown exception to be caught by the catch block?

Answer 27

The throw exception’s class equals the catch heading’s parameter’s class or the thrown exception’s class is a subclass of the catch heading’s parameter’s class.

Question 28

What happens since every thrown exception’s class is a subclass of the Exception class?

Answer 28

All thrown exceptions will match up with a generic Exception parameter.

Question 29

What should you do when a try block might throw more than one type of exception?

Answer 29

Instead of providing one generic catch block, you can provide a sequence of catch blocks, with each catch block handling a different exception type or group of exception types.

Question 30

What happens if multiple catch blocks are used and an exception is thrown?

Answer 30

The first catch block that matches the type of exception thrown is the one excecuted and the other catch blocks are then skipped.

Question 31

What should you do if one of the catch block’s exception class is derived from another catch block’s exception class?

Answer 31

To avoid a compilation error, you must arrange the catch blocks with the more general exception classes at the bottom (the superclasses go at the bottom)

Question 32

What does the JVM do as part of a runtime error?

Answer 32

The JVM prints the exception that was thrown and then prints the call-stack trace.

The call-stack trace shows the methods that were call prior to the crash.

Question 33

When should you move the try and catch blocks from the called method back to the calling method?

Answer 33

If you have a utility method that might throw an exception, but the original causes of that exception are in the calling methods.

or

You have a non-void return method that sometimes throws an exception and that method’s exception handler cannot know what value to have the method return.

Question 34

How do you move the try and catch blocks from the called method back to the calling method?

Answer 34

Append throws “exception-type” to the called method’s header.

Example:

Question 35

What do you have to provide sometimes that executes regardless of whether an exception is thrown?

Answer 35

Cleanup code.

For example: After writing a file, you must close the files to complete the writing process. Closing also releases system resources and improves system performance.

Question 36

What could be skipped if some operation between file opening and file closing throws an exception?

Answer 36

The close operation

Question 37

How do you deal with the close operation being potentially skipped?

Answer 37

Put an explicit close method call in a seperate finally block located immediately after the last catch block.

Example:

Question 38

What do all classes that implement closeable also implement?

Answer 38

Autocloseable

When an object is autocloseable, instead of closing it with an explicit close statement, we can ask the JVM to close it for us by opening the Autocloseable object with a try-with-resources header