Ch11 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

When is the IOException thrown?

Is it checked or unchecked (runtime)?

Answer 9

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

Checked.

Question 10

When is the FileNotFoundException thrown?

Is it checked or unchecked (runtime)?

Answer 10

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

Checked.

Question 11

When is the StackOverflowError thrown?

Is it checked or unchecked (runtime)?

Answer 11

Thrown when a method calls itself too many times.

Unchecked.

Question 12

When is the NoClassDefFoundError thrown?

Is it checked or unchecked (runtime)?

Answer 12

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

Unchecked.

Question 13

Does the following code compile?

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

Answer 13

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

Question 14

Does the following code compile?

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

Answer 14

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

Question 15

Does the following code compile?

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

Answer 15

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

correct:

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

Question 16

Does the following code compile?

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

Answer 16

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 17

What is returned?

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

Answer 17

-3

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

Question 18

True or false

The finally block is ALWAYS executed.

Answer 18

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 19

is the following code valid?

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

Answer 19

Yes.

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

Question 20

is the following code valid?

try {
// Code
}

Answer 20

No.

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

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

Question 21

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 21

The implicit finally block by Java will always run first.

Question 22

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

Answer 22

Classes that implement the AutoCloseable interface.

This interface contains a void close() method.

Question 23

is the following code valid?

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

Answer 23

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

Question 24

Is the following code valid?

class CanNotHopException extends Exception { }

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

Answer 24

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 25

Is the following code valid?

class CanNotHopException extends Exception { }

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

Answer 25

This is valid.

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

Question 26

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 26

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 27

Is the following code valid?

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

Answer 27

This is valid because IllegalStateException is an unchecked exception.