Java Exception Handling Introduction
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What is an Exception?
An exception is an object Java creates and throws when something disrupts a program's normal flow, like dividing by zero or accessing a null reference. Instead of letting the program crash outright, Java lets you catch that object and decide how to respond.
Example: What is an Exception?
public class Main {
public static void main(String[] args) {
try {
int result = 10 / 0; // throws ArithmeticException
} catch (ArithmeticException e) {
System.out.println("Caught: " + e.getMessage());
}
}
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Checked vs Unchecked Exceptions
Checked exceptions (like IOException) must be declared or caught at compile time because the compiler forces you to acknowledge the failure is possible; unchecked exceptions (like NullPointerException) extend RuntimeException and can surface without any such warning.
Example: Checked vs Unchecked Exceptions
public class Main {
public static void main(String[] args) {
try {
throw new RuntimeException("Unchecked"); // no need to declare it
} catch (RuntimeException e) {
System.out.println(e.getMessage());
}
}
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Why Handle Exceptions?
Handling exceptions keeps one failure from silently corrupting program state or crashing the whole application. It also lets you give the user a meaningful message instead of a raw stack trace, or fall back to a safe default and keep running.
Example: Why Handle Exceptions?
public class Main {
public static void main(String[] args) {
try {
int[] arr = new int[2];
System.out.println(arr[5]);
} catch (ArrayIndexOutOfBoundsException e) {
System.out.println("Handled gracefully instead of crashing");
}
}
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Common Exception Types
Some of the exceptions you'll hit constantly include NullPointerException (calling a method on null), ArrayIndexOutOfBoundsException (invalid array index), and ArithmeticException (like dividing an int by zero). Recognizing these by name speeds up debugging considerably.
Example: Common Exception Types
public class Main {
public static void main(String[] args) {
try {
String s = null;
s.length();
} catch (NullPointerException e) {
System.out.println("Caught a NullPointerException");
}
}
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The try-catch-finally Concept
The try block holds code that might fail, catch defines how to respond if it does, and finally runs regardless of whether an exception occurred — typically used to release resources like open files or network connections.
Example: The try-catch-finally Concept
public class Main {
public static void main(String[] args) {
try {
int result = 10 / 0;
} catch (ArithmeticException e) {
System.out.println("Error occurred");
} finally {
System.out.println("Runs regardless");
}
}
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