Exceptions

Question 1

What is parent class of all exception classes?

Answer 1

Throwable

Question 2

What classes extend Throwable?

Answer 2

Exception and Error

Question 3

What are checked exceptions?

Answer 3

Those exceptions that need to be declared or handled. They are expected and checked at compile time.

Question 4

What are unchecked exceptions?

Answer 4

Exceptions that are optionally handled. They can appear at any time. They are not checked at compile time.

Question 5

What main types of exceptions are unchecked exceptions?

Answer 5

Error, RuntimeException

Question 6

What main types of exceptions are checked exceptions?

Answer 6

Throwable, Exception

Question 7

Can you catch exceptions from methods that are empty?

Answer 7

Only if they are unchecked exceptions. Checked exceptions must be declared in method signature.

Question 8

What do try catch need to have?

Answer 8

They must contain {}

Question 9

How does “finally” work?

Answer 9

It executes before return, break, continue keywords in try or catch blocks, then it returns to block it came from.

Question 10

When is catch block not necessary?

Answer 10

When you have finally block or you use try-with-resources

Question 11

What is potential pitfall when using exceptions and its “throw” keyword?

Answer 11

Need to watch out for “throw” and “throws”

Question 12

What is rule of overriding methods that throw a checked exception?

Answer 12

Overridden method can throw same exception, narrower exception or not declare exception at all. But it cannot declare different exception or broader exception.

Question 13

Overridden method can throw narrower exception or not declare exception at all. Why?

Answer 13

Liskov Substitution Principle. Overridden method can potentially fix what throws a exception in super class, or can granulate problem into a subtype of the super method’s exception.

Question 14

Name some RuntimeExceptions

Answer 14

NullPointerException, IllegalArgumentException, NumberFormatException, ArithmeticException, ArrayIndexOutOfBoundsException, ClassCastException

Question 15

What exception does NumberFormatException extends?

Answer 15

IllegalArgumentException

Question 16

What can throw ArithmeticException?

Answer 16

Dividing by 0

Question 17

What are Helpful NullPointerExceptions?

Answer 17

It is a new feature of NullPointerExceptions, where you can get detailed information about parameters that were reason of this exception. It shows number of parameter that threw it.

Question 18

How can we get name of params instead of numbers in Helpful NullPointerExceptions?

Answer 18

-g:vars command at java startup

Question 19

Name some Error exceptions

Answer 19

NoClassDefFoundError, ExceptionInitializerError, StackOverflowError

Question 20

Name some checked Exceptions

Answer 20

SQLException, ParseException, FileNotFoundException, IOException, NotSerializableException

Question 21

What exceptions extend IOException?

Answer 21

FileNotFoundException, NotSerializableException

Question 22

What is the rule about resources closure in try-with-resources section?

Answer 22

They are being closed from last to first

Question 23

What is the rule about resources that can be declared in try-with-resources section?

Answer 23

Classes must implement AutoCloseable interface

Question 24

What is the scope of try-with-resources?

Answer 24

Only in try block

Question 25

Can var be used in try-with-resources section?

Answer 25

Yes

Question 26

Can resources be declared outside try-with-resources section?

Answer 26

Yes, but they need to be final or effectively final

Question 27

Do try-with-resources need catch?

Answer 27

No, it is being implicitly created by compiler

Question 28

What do we need to watch out when using try-with-resources?

Answer 28

Not to close resources that are needed later, or already closed

Question 29

What are stacked or suppressed exceptions?

Answer 29

If multiple exceptions are occurred while throwing first exception, first exception is main exception, and others are considered suppressed but readable

Question 30

When are resources being closed when using try-with-resources?

Answer 30

Right after try block ends

Question 31

What is implicit finally block in try-with-resources?

Answer 31

It is a finally block that compiler adds right after try and catch blocks and before explicit finally block. It contains generated resource close calls. It is ran before finally block.

Question 32

What happens if run a method that throws an exception, but as a field?

Answer 32

Then classes constructor needs to me marked that it throws that exception