Learn Java Programming

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

// Multiple catch blocks (most specific first)
try {
    String[] arr = {"hello"};
    int num = Integer.parseInt(arr[5]);
} catch (ArrayIndexOutOfBoundsException e) {
    System.out.println("Index error");
} catch (NumberFormatException e) {
    System.out.println("Parse error");
} catch (Exception e) {
    System.out.println("General: " + e.getMessage());
}

// Multi-catch (Java 7+)
try {
    riskyMethod();
} catch (IOException | SQLException e) {
    System.out.println("IO or DB error: " + e.getMessage());
}

import java.io.*;

// Resources in try() are auto-closed (must implement AutoCloseable)
try (BufferedReader br = new BufferedReader(new FileReader("data.txt"));
     PrintWriter pw = new PrintWriter(new FileWriter("out.txt"))) {

    String line;
    while ((line = br.readLine()) != null) {
        pw.println(line.toUpperCase());
    }
} catch (IOException e) {
    System.out.println("IO Error: " + e.getMessage());
}
// br and pw are automatically closed here — even if exception occurs!

// Custom checked exception with additional context
class InsufficientFundsException extends Exception {
    private final double shortfall;

    public InsufficientFundsException(double shortfall) {
        super("Insufficient funds. Short by: $" + String.format("%.2f", shortfall));
        this.shortfall = shortfall;
    }

    public double getShortfall() { return shortfall; }
}

// Using in a class
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(amount - balance);
        }
        balance -= amount;
    }
}

// Handling
BankAccount acc = new BankAccount(100);
try {
    acc.withdraw(150);
} catch (InsufficientFundsException e) {
    System.out.println(e.getMessage());       // Insufficient funds. Short by: $50.00
    System.out.println(e.getShortfall());     // 50.0
}

// throw — creates and throws an exception object (inside method body)
public static void validateAge(int age) {
    if (age < 0 || age > 150) {
        throw new IllegalArgumentException("Invalid age: " + age);
    }
}

// throws — declares that a method MAY throw (in method signature)
public static String loadConfig() throws IOException, FileNotFoundException {
    return Files.readString(Path.of("config.properties"));
}

// Wrapping exceptions (preserve original cause)
public static void processData() {
    try {
        loadConfig();
    } catch (IOException e) {
        throw new RuntimeException("Config load failed", e);  // wrap with cause
    }
}