Exception Handling

Syntax

try {\n  // risky code\n} catch (ExceptionType e) {\n  // handle it\n} finally {\n  // always runs\n}

Examples

try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Error: " + e.getMessage());
}

System.out.println("Program continues running");

public static void process(String input) {
    try {
        int number = Integer.parseInt(input);
        int result = 100 / number;
        System.out.println(result);
    } catch (NumberFormatException e) {
        System.out.println("Not a valid number: " + input);
    } catch (ArithmeticException e) {
        System.out.println("Cannot divide by zero");
    }
}

process("abc");  // Not a valid number: abc
process("0");     // Cannot divide by zero
process("5");     // 20

public static void readFile() {
    System.out.println("Opening file...");
    try {
        throw new RuntimeException("File not found");
    } catch (RuntimeException e) {
        System.out.println("Error: " + e.getMessage());
    } finally {
        System.out.println("Closing file...");  // always runs
    }
}

readFile();

class InsufficientFundsException extends Exception {
    public InsufficientFundsException(String message) {
        super(message);
    }
}

class BankAccount {
    private double balance;

    public BankAccount(double balance) { this.balance = balance; }

    public void withdraw(double amount) throws InsufficientFundsException {
        if (amount > balance) {
            throw new InsufficientFundsException("Not enough funds for this withdrawal");
        }
        balance -= amount;
    }
}

BankAccount account = new BankAccount(100);
try {
    account.withdraw(200);
} catch (InsufficientFundsException e) {
    System.out.println("Transaction failed: " + e.getMessage());
}

Best practices

• Catch specific exception types rather than a broad catch (Exception e), so unrelated bugs are not accidentally silenced
• Use finally (or try-with-resources) for cleanup code that absolutely must run, like closing files or database connections
• Create custom exception classes for domain-specific errors, extending Exception (checked) or RuntimeException (unchecked) depending on whether callers should be forced to handle them
• Don't use exceptions for routine control flow when a simple if check would do - they carry real performance overhead and should signal genuinely exceptional situations

At a glance

Purpose

General-purpose application development

File extension

.java

Runs in

Java Virtual Machine

Usually used with

JDK and Java libraries

Specifications & further reading

• Java Language Specification

Related Java documentation