Exceptions

Question 1

What exception does zero division lead to?

Answer 1

ArithmeticException

Question 2

What exception does an invalid array index lead to?

Answer 2

ArrayIndexOutOfBoundsException

Question 3

What exception occurs from using an object reference that hasn’t been assigned?

Answer 3

NullPointerException

Question 4

When does NullPointerException occur?

Answer 4

When an attempt is made to used an object reference that hasn’t been assigned to an object

Question 5

What exception occurs when a specific file doesn’t exist?

Answer 5

FileNotFoundException

Question 6

How do exceptions occur and what do they contain?

Answer 6

The method creates a new exception object which is handed off to the runtime system.

This object contains information about the state of the program when it occur and the method-call stack.

Question 7

What is the hierarchy of Java exceptions?

Answer 7

All derive from Throwable.

Errors are those that happen in JVM. They should not be caught be applications as they tend to be unrecoverable.

Exceptions represent situations that occur in a Java program.

Question 8

How are exceptions caught?

Answer 8

A method can catch them directly using a try … catch … block.

Otherwise they are passed up the call stack to their calling method, eventually reaching the Java default handler.

Question 9

What is the structure of a try-catch block?

Answer 9

try { … } catch (Exception e) { };

n.b. can have multiple catch blocks to handle different exceptions

Question 10

What happens if multiple catch clauses match the exception raised?

Answer 10

The first one is matched and handled

Question 11

What exception is raised if a number cannot be parsed?

Answer 11

NumberFormatException

Question 12

What is important to note when using multiple catch clauses?

Answer 12

Specific ones must come before more general ones - otherwise a compile time error is raised

Question 13

What is the effect of nested try..catch blocks?

Answer 13

It allows the handling of errors that occur within the catch clauses, such as if they re throw the exception

Question 14

What is the effect of a finally block?

Answer 14

it is guaranteed to be executed. This allows the cleanup of actions that occurred in the try block.

Notably it is still called even if the code in the try block includes return, continue or break

Question 15

How are exceptions triggered?

Answer 15

Using the “throw” keyword.

Exceptions are objects so it is common to go: throw new Exception(‘…’);

Question 16

How are methods that raise exceptions declared?

Answer 16

With “throws” i.e. public static void method() throws Exception { …

Question 17

How are multiple exceptions declared using throws?

Answer 17

As a comma separated list

Question 18

How are exceptions classed?

Answer 18

Runtime exceptions occur within Java runtime system: arithmetic exceptions, pointer exceptions and indexing exceptions etc.

Non-runtime exceptions occur in code outside of the java runtime system i.e. during IO

Question 19

In what way is exception handling enforced?

Answer 19

The compiler enforces catch-or-declare: methods that use throw must either use a catch block or declare that exception using throws