Try and Catch

Data Types and Exceptions in Java

Jim White

Java Developer

Problems in Java

Problems or issues can occur in applications
- Due to wrong input
- Due to coding errors
- Due to unforeseen circumstances
Problems or issues are called exceptions
- Interrupt the normal control flow
Java throws exceptions

Exception Example

public static void main(String[] args) {
    ArrayList<String> allStars = new ArrayList<String>();
    allStars.add("Mays");
    allStars.add("Aaron");
    allStars.add("Ruth");
    String last = allStars.get(3); // This will cause an exception
    System.out.println(last);
}

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index 3 out of 
bounds for length 3

Try

Wrap exception-generating code with a try-block
- "Try this code and if there is a problem pass it to the catch block"

try {
    // Statements to be executed and watched for exceptions
}

Catch

Place a catch-block after the try
- Acts as a handler to address exception problems
- The exception caught is passed into this block as a parameter
try/catch blocks are required for some types exceptions
- Optional for others

try {
    // Statements to be executed and watched for exceptions
} catch (Exception variable) {
    // Statements to execute if an exception has occurred
}

Catching IndexOutOfBoundsException

try {
  ArrayList<String> allStars = new ArrayList<String>();
  allStars.add("Mays");
  allStars.add("Aaron");
  allStars.add("Ruth");
  String last = allStars.get(3); // Accessing an element that isn't there
  System.out.println(last);
} catch (IndexOutOfBoundsException e) {
  System.out.println("Oops - wrong index");
}

Oops - wrong index

Multiple Catch

Use multiple catch blocks to handle different types of exceptions
Each catch gets its own exception type and exception variable
When exception occurs, Java checks the catch blocks in order until it finds a match

try {
    ArrayList<String> nums 
      = new ArrayList<String>();
    nums.add("one");
    String last = nums.get(0);
    Integer.valueOf(last);
    System.out.println(last);
} catch (IndexOutOfBoundsException eIndex) {
    System.out.println("Oops - wrong index");
} catch (NumberFormatException eValueOf) {
    System.out.println(
      "Oops - String not a number");
}

Oops - String not a number

Catch "All"

Use catch (Exception variable){ } as a catch all
- Handles any exception not handled by the other catch blocks

try {
    // Code to try
} catch (IndexOutOfBoundsException eIndex) {
    System.out.println("Oops - wrong index");
} catch (NumberFormatException eValueOf) {
    System.out.println("Oops - String not a number");
} catch (Exception e) { // Handles all exceptions except the two above
    System.out.println("Something unplanned happened");
}

Finally

Code in finally block always executes

// When exception happens
try {
    Integer.valueOf("one");
} catch (NumberFormatException eValueOf) {
    System.out.println("Oops");
} finally {
    System.out.println("Doing cleanup");
}

Oops
Doing cleanup

// When no exception gets thrown
try {
    Integer.valueOf("1");
} catch (NumberFormatException eValueOf) {
    System.out.println("Oops");
} finally {
    System.out.println("Doing cleanup");
}

Doing cleanup

Exception object

The Exception object passed into the catch block is rich with information
Use methods on the object to learn more about what went wrong
- .getMessage() to get the human readable cause
- .getClass() to get the Exception class or type name
- .printStackTrace() to display the stack trace (next slide)

public class ExceptionExample {

    public static void main(String[] args) {
        try {
            Integer.valueOf("one");
        } catch (NumberFormatException e) {
            System.out.println(e.getMessage());
            e.printStackTrace();
        }
    }
}

Exception stack trace

The stack trace provides a record of the execution flow to the problem
- Below: output from the previous code example - including the printed stack trace

For input string: "one"
java.lang.NumberFormatException: For input string: "one"
    at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:67)
    at java.base/java.lang.Integer.parseInt(Integer.java:661)
    at java.base/java.lang.Integer.valueOf(Integer.java:988)
    at foo/exceptions.ExceptionExample.main(ExceptionExample.java:7)

Let's practice!

Data Types and Exceptions in Java