Exceptions and Assertions

Question 1

Throw and Throws definition

Answer 1

Throw means an exception is actually being thrown

Throws indicates that the method merely has the potential to thrw that exception to the calling method

Question 2

What should we decide on creating our own exception class?

Answer 2

We should decide on whether it should be checked or unchecked exceptions

Question 3

Three most common constructors defined by the Exception classes

Answer 3

1. public someclass(){
{super();}
2. public someclass(Exception e){
super(e);
}
3. public someclass(String message){
super(message);
}

Question 4

How to print the stack trace of our own?

Answer 4

e.printStackTrace();

Question 5

what are the two approaches before java 1.7 for catching multiple exceptions?

Answer 5

1. To have multiple catch clause to catch each of the expected exceptions and handle them
2. To have a same method defined in each catch class that handles the exception

Question 6

What is the approach after 1.7 for catching multiple exceptions?

Answer 6

Use multi-catch approach

Question 7

Syntax of multi-catch

Answer 7

1. try{
}
catch(Exception 1 | Exception 2 e){
// exception handler
}

Question 8

1. Catch(Exception1 e1 | Exception2 e2| Exception3 e3)
2. catch(Exception1 | Exception2 | Exception3 e)
3. catch(Exception1 e| Exception2 e| Exception3 e)

Which one compiles?

Answer 8

2 one compiles.

Question 9

Does java allows you to specify redundant types in multi-catch?

Answer 9

No, compiler throws the error

Question 10

catch(FileNotFoundException | IOException e).

What happens?

Answer 10

Exception is thrown.

The Exception FileNotFoundException is already caught by the alternative IOException

Question 11

Can we reassign a variable in a catch block in normal single catch Block. Is the below one legal?

Eg.., try{
}
catch(RuntimeException e)
{
e = new RuntimeException();
}

Answer 11

Yes it is legal, this is allowed but we should avoid using like this.

Question 12

Is this allowed?
try{
throw new IOException();
}catch (IOException | RunTimeException e){
e=new RuntimeException();
}

Answer 12

No, It is not allowed in multicatch statements.

Multi-catch are effectively final.

Question 13

What is the issue with finally clause?

Answer 13

The finally clause may have some redundant codes because it should close all the resources

Question 14

what does java 7 introduces for managing the resources automatically?

Answer 14

try-with-Resources approach

Question 15

What is the Try-With-Resources class called as?

Answer 15

Automatic Resource Management

Question 16

Can we have the catch/Finally clauses still used in try-with-resources approach?

Answer 16

Yes, we can still have the catc/finally with Try-with-resources approach

Question 17

Can we have 2 or more finally blocks in traditional tryblocks?

Answer 17

No, it is not legal,

Question 18

What is the scope of resources declared in try-with-resources block?

Answer 18

Its scope is only to try blocks. It cannot be accessed in the catch or finally blocks

Question 19

try(Scanner s = new Scanner(System.in)){
			s.nextLine();
		}
		catch(Exception e)
		{
			s.nextInt();
		}
		finally{
			s.nextInt();
		}

what is the output?

Answer 19

It doesnt compile. Bacuse the scope of the scanner is with in try only

Question 20

What is the criteria of declaring a resource in try with resources block?

Answer 20

The resource should implement AutoCloseable interface.

Question 21

What will happen here?

try(SomeClass class = new SomeClass())
{
System.out.println(class);
}

Answer 21

Exception: The resource type Turkey doesnot implement java.lang.AutoCloseable interface

Question 22

What is the method to be overridden for AutoCloseable interface?

Answer 22

public void close() throws Exception;

Question 23

what is idempotent?

Answer 23

Idempotent means that the method can be called multiple times without any side effects or change of state

Question 24

public class AutoCloseableIssue implements AutoCloseable {

	@Override
	public void close() throws Exception{
		throw new Exception();
	}

	public static void main(String[] args) {
		try(AutoCloseableIssue issue = new AutoCloseableIssue()){
			System.out.println("Hello");
		}
	}
}

What happens to this code?

Answer 24

The code doesnt compile because the close method throws an Exception and the place where this class is declared in try with resources has not caught tis Exception. So, it throws error

Question 25

What does Java recommends for close method()

Answer 25

1. close() method not throw Exception
2. It is better to throw any specific Exception
3. The close method should be idempotent

Question 26

What are the difference between Closeable and AutoCloseable interface?

Answer 26

1. The closeable restricts the specific type of exception thrown to be IOException
2. Closeable requires implementation to be Idempotent

Question 27

What should we do when the close() method throws an Exception?

Answer 27

We must handle that exception. SO, we must have a catch block to catch that Exception.

Since, the try-with-resource automatically calls the Close(), if it throws an exception, we need to handle it with catch clause.

Question 28

When multiple exceptions are thrown in try-with-resources, what are the first exception called?

Answer 28

All but the first exceptions are called as ‘Suppressed Exceptions’

Question 29

What is the output of the below code:

@Override
	public void close() throws Exception{
		throw new Exception("Suppressed");
	}

	public static void main(String[] args) { //throws Exception{
		try(AutoCloseableIssue issue = new AutoCloseableIssue()){
			throw new IllegalStateException("This is primary");
		}
		catch(Exception e){
			System.out.println("caught: " + e);
			for(Throwable e1 : e.getSuppressed()){
				System.out.println(e1.getMessage());
			}
		}
	}

Answer 29

caught: java.lang.IllegalStateException: This is primary

Suppressed

Question 30

What happens if the primary exceptions thrown doesnt match with the Exception type declared in ‘catch’ block? Does we get compiler error?

Answer 30

1. It eventually throws the exception to the caller and prints the stack trace. No we don’t get compiler error

Question 31

what is the output of thie below?

static int i = 0;
	@Override
	public void close() throws RuntimeException{
		throw new RuntimeException("from close " + i++ );
	}

	public static void main(String[] args) { //throws Exception{
		try(AutoCloseableIssue issue1 = new AutoCloseableIssue();
				AutoCloseableIssue issue2 = new AutoCloseableIssue()){
			System.out.println("Hello");;
		}
		catch(RuntimeException e){
			System.out.println("caught: " + e);
			for(Throwable e1 : e.getSuppressed()){
				System.out.println(e1.getMessage());
			}
		}
	}

Answer 31

Hello
caught: java.lang.RuntimeException: from close 0
from close 1

Question 32

What does the suppressed exception applicable only to?

Answer 32

The suppressed exception is applicable only to ‘try clause’

Question 33

What exception does this throw?
static int i = 0;
	@Override
	public void close() throws RuntimeException{
		throw new RuntimeException("from close " + i++ );
	}

	public static void main(String[] args) { //throws Exception{
		try(AutoCloseableIssue issue1 = new AutoCloseableIssue();
				AutoCloseableIssue issue2 = new AutoCloseableIssue()){
			System.out.println("Hello");;
		}
		finally{
			throw new RuntimeException("And we lost others");
		}

Answer 33

java.lang.RuntimeException: And we lost others

at com.org.AutoCloseableIssue.main(AutoCloseableIssue.java:17)

Question 34

What should we keep in mind always on using try-with-resources statement

Answer 34

1. Resources are closed after the try clause ends and before any catch/finally clause runs
2. Resorces are closed in reverse order from which they are created.

Question 35

What to keep on mind on rethrowing an exception?

Answer 35

If we call a method that throws some exception, we can catch them by having
catch(Exception e) in our code.

Java considers only the exceptions that are thrown.

Which means ‘Exception type’ in the catch block, but it really means limited set of exceptions and doesnt need to keep on changing the catch blocks while adding or updating any exceptions in method signature.

More info on 305-306 page

Question 36

What are assertions?

Answer 36

Assertions are boolean expressions that you place at a point that you consider to be true

Question 37

Syntax for Assert expressions?

Answer 37

assert boolean_expression

assert boolean_expression : error_message

Question 38

What type of error does assertion throws if it fails?

Answer 38

AssertionError. Since programmers cannot catch Error, assertion failures are fatal and ends the program

Question 39

command to enable assertions

Answer 39

java -ea Class_name
(or)
java -enableassertions class_name

Question 40

Which are the all the classes the assertions are enabled when using -ea without arguments

Answer 40

all classes except System class

Question 41

syntax for enabling the assertins only for classes in a package and its sub packages

Answer 41

java -ea package_name… class_name.Main

Close look on 3 dots. It means any class in package or subpackages

Question 42

How can we disable assertions?

Answer 42

-da or -disableassertions