# 17 - Exceptions

Michele

# Goals

• Understand what exceptions are
• Understand why exceptions are needed
• Know how to handle exception
• Recognize most common Java exceptions

# Slides

# Exercises

# Exercise 1 - Getting to know exceptions

• In your main method, use a Scanner to get a user input, and try to convert it to Integer. What happens if you run your program and input something that's not convertible to an integer?

Scanner scanner = new Scanner(System.in);
String userInput = scanner.nextLine();

// if the user has inserted a string
// this will throw an exception
Integer.parseInt(userInput);

• In your main method, create a List<String> and print the string at position 3 in the list. What happens when you run your program?

List<String> emptyList = new ArrayList<>();

// this will throw an exception in any case!
emptyList.get(5) 

• Try reading a file using a FileInputStream. Does it work? If not, why not? Advice: You can try reading a file from /some/path like this:

// this does not work just like this, since FileNotFoundException is a checked exception
FileInputStream inputStream = new FileInputStream(new File("/some/path"));
// do stuff with inputStream
inputStream.close();

# Exercise 2 - Handling exceptions

• How can you check that the user has actually entered an integer? How can you use exception-catching to solve this issue?

Scanner scanner = new Scanner(System.in);
try {
	String userInput = scanner.nextLine();
} catch (NumberFormatException nfe) {
	// oh oh! something went wront
	// let's just print the stack trace
	nfe.printStackTrace();
}

• Can you make your Pizza/Moma/Bank code from last week more reliable by catching exceptions on user input?

# Exercise 3 - finally clause

• Can you close the inputStream from exercise 1 after using it? You can print "Closing input stream" when calling inputStream.close() to make sure you're actually doing it!

FileInputStream inputStream = null;
try {
    System.out.println("Opening stream from file");
    inputStream = new FileInputStream(
            new File("/some/path"));

    // what if an exception happens here?

} catch (FileNotFoundException fnfE) {
    System.out.println("Something bad happened: " + fnfE.getMessage());
    fnfE.printStackTrace();
} finally {
    System.out.println("Closing input stream");
    inputStream.close();
}

try (FileInputStream inputStream = new FileInputStream(new File("/some/path"))) {

    inputStream.read();
    // anything bad happening here doesn't matter
    // the input stream will be closed at the end of the block
    // without us having to do anything!
    
} catch (IOException e) {
    // we still need to catch exceptions thrown by the inputstream
}

# Recap

Let's make a silly calculator. A calculator that can only do very few, very simple operations.

When the application starts, the user should be asked which operation they want to perform. The two following operations are possible:

1. Divide 3 by a number entered by the user (3/x).
2. Calculate the square root of a number provided by the user (√x)
3. Sum 5 to a number entered by the user(5+x).

After the user has chosen one of these operations, they should be asked to enter the number they want to use for their operation (x). After entering the number, the result should be shown!

• Remember: It's not possible to divide by 0, and the square root is only defined for positive numbers.
• Try to keep your code clean! Use different methods for the various operations if possible.

# Additional Resources

• StackOverflow - Understanding checked vs unchecked exceptions
• StackOverflow - Differences between Exception and Error
• Java Exceptions on Baeldung