Java Exceptions - Try...Catch
Java Exceptions
As mentioned in the Errors chapter, different types of errors can occur while running a program - such as coding mistakes, invalid input, or unexpected situations.
When an error occurs, Java will normally stop and generate an error message. The technical term for this is: Java will throw an exception (throw an error).
Exception Handling (try and catch)
Exception handling lets you catch and handle errors during runtime - so your program doesn't crash.
It uses different keywords:
The try statement allows you to define a block of code to be
tested for errors while it is being executed.
The catch statement allows you to define a block of code to
be executed, if an error occurs in the try block.
The try and catch keywords
come in pairs:
Syntax
try {
// Block of code to try
}
catch(Exception e) {
// Block of code to handle errors
}
Consider the following example:
This will generate an error, because myNumbers[10] does not exist.
public class Main {
public static void main(String[ ] args) {
int[] myNumbers = {1, 2, 3};
System.out.println(myNumbers[10]); // error!
}
}
The output will be something like this:
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 10
at Main.main(Main.java:4)
Note: ArrayIndexOutOfBoundsException occurs when you try to access an index number that does not exist.
If an error occurs, we can use try...catch to catch the error and execute some code to handle it:
Example
public class Main {
public static void main(String[ ] args) {
try {
int[] myNumbers = {1, 2, 3};
System.out.println(myNumbers[10]);
} catch (Exception e) {
System.out.println("Something went wrong.");
}
}
}
The output will be:
Something went wrong.
Finally
The finally statement lets you execute code, after try...catch, regardless of the result:
Example
public class Main {
public static void main(String[] args) {
try {
int[] myNumbers = {1, 2, 3};
System.out.println(myNumbers[10]);
} catch (Exception e) {
System.out.println("Something went wrong.");
} finally {
System.out.println("The 'try catch' is finished.");
}
}
}
The output will be:
Something went wrong.
The 'try catch' is finished.
The throw keyword
The throw statement allows you to create a custom error.
The throw statement is used together with an exception type. There are many exception types available in Java: ArithmeticException,
FileNotFoundException, ArrayIndexOutOfBoundsException, SecurityException, etc:
Example
Throw an exception if age is below 18 (print "Access denied"). If age is 18 or older, print "Access granted":
public class Main {
static void checkAge(int age) {
if (age < 18) {
throw new ArithmeticException("Access denied - You must be at least 18 years old.");
}
else {
System.out.println("Access granted - You are old enough!");
}
}
public static void main(String[] args) {
checkAge(15); // Set age to 15 (which is below 18...)
}
}
The output will be:
Exception in thread "main" java.lang.ArithmeticException: Access denied - You must be at least 18 years old.
at Main.checkAge(Main.java:4)
at Main.main(Main.java:12)
If age was 20, you would not get an exception:
Errors and Exception Types
The table below shows some of the most common errors and exceptions in Java, with a short description of each:
| Error/Exception | Description |
|---|---|
| ArithmeticError | Occurs when a numeric calculation goes wrong |
| ArrayIndexOutOfBoundsException | Occurs when trying to access an index number that does not exist in an array |
| ClassNotFoundException | Occurs when trying to access a class that does not exist |
| FileNotFoundException | Occurs when a file cannot be accessed |
| InputMismatchException | Occurs when entering wrong input (e.g. text in a numerical input) |
| IOException | Occurs when an input or output operation fails |
| NullPointerException | Occurs when trying to access an object referece that is null |
| NumberFormatException | Occurs when it is not possible to convert a specified string to a numeric type |
| StringIndexOutOfBoundsException | Occurs when trying to access a character in a String that does not exist |
Tip: For a list of all errors and exception types, go to our Java Errors and Exception Types Reference.
