CS1021 - Exception Handling Review

name: inverse
layout: true
class: taylor msoe
---
class: center, middle

.title[
  # Exception Handling
]

???

* **P** speaker view
* **C** clone slideshow
* **?** help

---
# Exception Handling

* Exception handling is a technique in Java for recovering from exceptional situations

--
* Exception handling can make our software more robust

--
* A common source of exceptional situation is incorrect/unexpected user input

--
* We would like to recover gracefully from incorrect input

--
* Typically discover issues with user input in library code (e.g., `Scanner.nextInt()`)

--
* Would like to deal with incorrect input at the user interface layer of the code

--
* Exceptions allow us to avoid lots of input validation code

---
# Exception Handling Mechanics

* `throw` when a problem is discovered

--
* `catch` where responding to the problem is appropriate

--
* When a problem is encountered:
    + Create an exception
    + Throw the exception
--
* When exception is thrown
    + Execution in the current method terminates
    + Exception is thrown to the calling method
    + Exception continues to be propagated up the calling stack until either
        - It reaches the JVM (and the program crashes)
        - It reaches code that is prepared to respond to the exception

---
# Catching an Exception

* In order to catch an exception, you must `try`
``` Java
    int weight = -1;
    do {
        try {
            System.out.println("Please enter your weight in lbs");
            weight = in.nextInt();
            System.out.println("Thank you for entering an integer value");
        } catch(InputMismatchException exception) {
            in.nextLine();
            System.out.print("To the nearest integer value... ");
        }
    } while(weight<0);
```

---
# Program Flow
* If exception is encountered in a `try` block:
    + Skip any remaining code in the `try` block
    + Look for matching `catch` block
        - If found, execute `catch` block and then resume execution **after** the `try`-`catch` block
        - If not found, exception is immediately thrown up to the calling method
--
* Can have multiple `catch` blocks
    + Must place more specific exceptions first:

```
} catch(ArrayIndexOutOfBoundsException exception) { // ...
} catch(IndexOutOfBoundsException exception) { // ...
```

--
* Can have `catch` block with multiple matches, e.g., 
    + `catch(InvalidPathException | NoSuchFileException e)`

---
# Checked and Unchecked Exceptions
* Checked exceptions must be handled

--
* Two ways to handle a checked exception:
    + Catch the exception
    + Specify that your method potentially throws the checked exception

--
```
public String readFile(String filename) throws IOException {
```

---
# Common Exceptions

.right-column[
* Colored classes indicate Checked exceptions
* Should not catch `Error`s
]
.left-column[
<img alt="Exceptions Class Hierarchy" src="exceptions.png" style="width: 70%" />
]

---
# Best Practices
* Throw exceptions in library code

--
* Catch exceptions in user interface code

--
* For the MVC pattern:
    + Throw in Model
    + Catch in Controller
--
* If you can partially handle an exception, can catch and re-throw
```
    // ...
    catch(IOException exception) {
        LOGGER.log(Level.SEVERE, exception.toString(), exception);
        throw exception;
    }
```
--
* Don't catch exceptions if you don't know how to recover

--
* Catching an exception can make debugging harder

--
* Calling `exception.printStackTrace()` is not handling an exception

---
# `finally` Block
* The `finally` block is used to force code to run "no matter what"

--
* Appears either:

.left-column[
After the last `catch` block, or

``` Java
    try {
        // ...
    } catch(NumberFormatException exception) {
        // ...
*   } catch(ClassCastException exception) {
        // ...
*   } finally {
        // ...
    }
```
]
.right-column[
After a `try` block (if there are no `catch` blocks)

``` Java
*   try {
        // ...
*   } finally {
        // ...
    }
```
]

---
# Try-with-Resources
* Prefer using try-with-resources when possible

--
* Applies to any object that implements `Closable` and is needed only within a `try` block

``` Java
*   try(FileOutputStream out = new FileOutputStream(new File("stuff.txt"));
*       DataOutputStream dOut = new DataOutputStream(out)) {
        // ...
    } catch(FileNotFoundException exception) {
        // ...
    } catch(IOException exception) {
        // ...
    }
```

---
# Example
.left-column[
From [Scanner JavaDoc](https://javadoc.taylorial.com/java.base/util/Scanner.html)
``` Java
String input = "1 fish 2 fish red fish blue fish";
Scanner s = null;
try {
    s = new Scanner(input).useDelimiter("\\s*fish\\s*");
    System.out.println(s.nextInt());
    System.out.println(s.nextInt());
    System.out.println(s.next());
    System.out.println(s.next());
} finally {
    if(s!=null) {
        s.close();
    }
}
```
]
.right-column[
Output:
```
1
2
red
blue
```
]

???

* `\\s*fish\\s*` - match on "fish" with zero or more whitespace characters on either side
* Java requires the backslash to be escaped (why there are two)

---
# Alternative try-with-resources
```
    //try {
    //    Scanner s = new Scanner(input).useDelimiter("\\s*fish\\s*");
*   try(Scanner s = new Scanner(input).useDelimiter("\\s*fish\\s*")) {
        System.out.println(s.nextInt());
        System.out.println(s.nextInt());
        System.out.println(s.next());
        System.out.println(s.next());
    //} finally {
    //    if(s!=null) {
    //        s.close();
    //    }
    }
```

---
# Recapping

* Advantages:
    * Separate error handling code from other code
    * Handle exception where it makes sense
    * Grouping and Differentiating Exception types

--
* `Error`s -- serious problem, don't catch

--
* `RunTimeException`s -- can occur anywhere, may be numerous

--
* All other exceptions are _Checked_ exceptions

--
* If you expect calling code to be able to recover from your exception, it should be a checked exception