Chapter 11: Exceptions

Question 1

What is an exception?

Answer 1

An exception is an event that alters program flow.

Question 2

What superclass (aside from Object) do all exceptions inherit from in Java?

Answer 2

java.lang.Throwable

Question 3

What is an Error and what should it be used for?

Answer 3

Error means something went so horribly wrong that your program should not attempt to recover from it. For example, the disk drive “dissapeared” or the program ran out of memory.

Question 4

What is a checked exception?

Answer 4

A checked exception is an exception that must be declared or handled by the application code where it is thrown.

Question 5

What do all checked exceptions inherit from?

Answer 5

Checked exceptions all inherit Exception (and thus also Throwable & Object)

Question 6

What is an unchecked exception?

Answer 6

An unchecked exception is any exception that does not need to be declared or handled by the application code where it is thrown.

They are also known as runtime exceptions.

Question 7

What do all unchecked exceptions inherit from?

Answer 7

Unchecked exceptions all inherit from either RuntimeException or Error.

Question 8

What is the difference between throw and throws?

Answer 8

Throw is used as a statement inside a code block to throw a new exception.

Throws is used at the end of a method declaration to indicate what exception it might throw.

Question 9

Is the following code valid?

Exception e = new RuntimeException();
throw e;

Answer 9

Yes.

An Exception inherits from the Object class and can therefore be stored in a variable.

Question 10

Is the following code valid?

throw RuntimeException();

Answer 10

No. It is missing the new keyword and is thus never instantiated.

Question 11

Is it okay for a program to catch an Error?

Answer 11

No. An error is not supposed to be recovered from.

Question 12

When is the ArithmeticException thrown?

Is it checked or unchecked (runtime)?

Answer 12

Thrown when code attempts to divide by zero.

Unchecked.

Question 13

When is the ArrayIndexOutOfBoundsException thrown?

Is it checked or unchecked (runtime)?

Answer 13

Thrown when code uses an illegal index to access an array.

Unchecked.

Question 14

When is the ClassCastException thrown?

Is it checked or unchecked (runtime)?

Answer 14

Thrown when an attempt is made to cast an object to a class of which it is not an instance.

Unchecked.

Question 15

When is the NullPointerException thrown?

Is it checked or unchecked (runtime)?

Answer 15

Thrown when there is a null reference where an object is required.

Unchecked.

Question 16

When is the IllegalArgumentException thrown?

Is it checked or unchecked (runtime)?

Answer 16

Thrown by the programmer to indicate that a method has been passed an illegal or inappropriate argument.

Unchecked.

Question 17

When is the NumberFormatException thrown?

Is it checked or unchecked (runtime)?

Answer 17

Subclass of IllegalArgumentException thrown when an attempt is made to convert a string to a numeric type but the string doesn’t have an appropriate format.

Unchecked.

Question 18

When is the IOException thrown?

Is it checked or unchecked (runtime)?

Answer 18

Thrown programmatically when there is a problem reading or writing a file.

Checked.

Question 19

When is the FileNotFoundException thrown?

Is it checked or unchecked (runtime)?

Answer 19

Subclass of IOException thrown programatically when code tries to reference a file that doesn’t exist.

Checked.

Question 20

When is the ExceptionInInitializerError thrown?

Is it checked or unchecked (runtime)?

Answer 20

Thrown when a static initializer throws an exception and doesn’t handle it.

Unchecked.

Question 21

When is the StackOverflowError thrown?

Is it checked or unchecked (runtime)?

Answer 21

Thrown when a method calls itself too many times.

Unchecked.

Question 22

When is the NoClassDefFoundError thrown?

Is it checked or unchecked (runtime)?

Answer 22

Thrown when the class that the code uses is available at compile time but not runtime.

Unchecked.

Question 23

Does the following code compile? (assuming the methods exist)

try
fall();
catch (Exception e)
getUp();

Answer 23

No. Curly braces cannot be ommited with try statements.

Correct would be:

try {
    fall();
} catch (Exception e) {
    getUp();
}

Question 24

Does the following code compile?

try {
} catch (IllegalArgumentException e) {
} catch (NumberFormatException e) {
}

Answer 24

No because NumberFormatException is a subclass of IllegalArgumentException meaning it will never reach that catch clause even if NumberFormatException was thrown.

Question 25

Does the following code compile?

try {
} catch (IllegalArgumentException e1) {
    System.out.println(e1);
} catch (NumberFormatException e2) {
    System.out.println(e1);
}

Answer 25

No because e1 is out of scope in the second catch block.

Question 26

Does the following code compile?

try {
} catch (ArrayIndexOutOfBoundsException e | NumberFormatException e) {
}

Answer 26

No because only a single identifier is allowed for all exception types.

correct:

try {
} catch (ArrayIndexOutOfBoundsException | NumberFormatException e) {
}

Question 27

Does the following code compile?

try {
} catch (IllegalArgumentException | NumberFormatException e) {
}

Answer 27

No because Java intends multi-catch to be used for exceptions that are not related. If it is, it will not compile.

Since NumberFormatException is a subclass of IllegalArgumentException, it won’t compile. The corrent way is just to remove IllegalArgumentException from the clause.

Question 28

Does the following code compile?

try {
} catch (ArrayIndexOutOfBoundsException | NumberFormatException e) {
}

Answer 28

Yes it does :)

Question 29

What is the finally block?

Answer 29

The finally block is an optional block that always executes at the end, whether or not an exception was thrown.

Question 30

What is the finally block typically used for?

Answer 30

Closing resources such as files or databases.

Question 31

What is returned?

try {
    return -1;
} catch (Exception e) {
    return -2;
} finally {
    return -3
}

Answer 31

-3

When the return statement is reached in the try block, it executes the finally block first and returns -3 instead.

Question 32

True or false

The finally block is ALWAYS executed.

Answer 32

True.

It is always executed, however it may not always finish.

There is one exception: if System.exit() is called in the try block, finally will not be executed.

Question 33

What is the following try-statement called?

try (someResourceHere) {
} catch (IOException e) {
}

Answer 33

try-with-resource OR automatic resource management

Question 34

is the following code valid?

try (FileInputStream in = new FileInputStream("file.txt")) {
    // Read file data
}

Answer 34

Yes.

With try-with-resource statements, the catch/finally block is optional.

Question 35

is the following code valid?

try {
    // Code
}

Answer 35

No.

With try-statements, it is required to either use the catch block or the finally block (optionally you can use both).

They catch/finally block are optional for try-with-resources

Question 36

When using try-with-resources, Java automatically inserts a finally block to close the resource. If another finally block is explicitely written by the programmer, which finally block runs first?

Answer 36

The implicit finally block by Java will always run first.

Question 37

Using a try statement, can I add a finally block when no catch block was added?

Answer 37

Yes, either catch or finally is required

Note: they are optional for try-with-resources

Question 38

Using a try statement, can I add a second finally block?

Answer 38

No, only one finally block is legal.

Question 39

Using a try-with-resource statement, can I add a second finally block?

Answer 39

No, only one finally block is legal.

Question 40

What classes are supportable with try-with-resource statements, such that Java automatically closes the resource?

Answer 40

Classes that implement the AutoCloseable interface.

This interface contains a void close() method.

Question 41

is the following code valid?

try (FileInputStream in = new FileInputStream("file.txt"), FileInputStream in2 = new FileInputStream("file.txt")) {
    // Read file data
}

Answer 41

No. Multiple resources must be seperated with a semicolon (;).

Question 42

is the following code valid?

try (FileInputStream in = new FileInputStream("file.txt"), in2 = new FileInputStream("file.txt")) {
    // Read file data
}

Answer 42

No, each variable must be declared in a seperate statement.

Question 43

With try-with-resource, when are the resources closed?

Answer 43

After the try clause ends and before any catch/finally clauses.

Question 44

Given the following code

try (FileInputStream in = new FileInputStream("file.txt"); FileInputStream in2 = new FileInputStream("file.txt")) {
    // Read file data
}

In what order are the resources closed?

Answer 44

in2 first,
in second.

Resources are closed in reverse order.

Question 45

Is the following code valid?

class CanNotHopException extends Exception { }

class Hopper {
    public void hop() {}
}

class Bunny extends Hopper {
   public void hop() throws CanNotHopException { }
}

Answer 45

This code is not valid.

When a class overrides a method from a super-class or implements a method from an interface, it’s not allowed to add new checked exceptions to the method signature.

Question 46

Is the following code valid?

class CanNotHopException extends Exception { }

class Hopper {
    public void hop() throws CanNotHopException {}
}

class Bunny extends Hopper {
   public void hop() { }
}

Answer 46

This is valid.

Subclasses are allowed to declare fewer exceptions than the superclass or interface, because callers are already handling them.

Question 47

Is the following code valid?

class CanNotHopException extends Exception { }

class Hopper {
    public void hop() throws Exception {}
}

class Bunny extends Hopper {
   public void hop() throws CanNotHopException { }
}

Answer 47

This is valid.

Subclasses are allowed to declare fewer (in this case its a subclass) exceptions than the superclass or interface, because callers are already handling them.

Question 48

Is the following code valid?

class Hopper {
    public void hop() {}
}

class Bunny extends Hopper {
   public void hop() throws IllegalStateException { }
}

Answer 48

This is valid because IllegalStateException is an unchecked exception.

(page 437)