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
}Runnable example
public class Main {
public static void main(String[ ] args) {
int[] myNumbers = {1, 2, 3};
System.out.println(myNumbers[10]); // error! }
}Consider the following example
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.");
}
}
}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 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
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...)
}
}If age was 20, you would not get an exception:
checkAge(20);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 .