🔐 Why use `char[]` over `String` for passwords in Java?

When handling sensitive data like passwords, security is a big concern. Java offers two common ways to store password input:

• Using a String
• Using a char[] (character array)

At first, both seem okay — but there are important security reasons why char[] is the preferred choice.

📋 Quick Comparison

Aspect	char[]	String
Mutable	✅ Yes	❌ No (Immutable)
Can erase?	✅ Yes (overwrite with nulls)	❌ No (stays in memory until GC runs)
Stored in Pool?	❌ No	✅ Yes (String pool)
Memory lifespan	Shorter (can control manually)	Longer (waits for GC)
Security	✅ High (can be cleared)	❌ Low (persists in memory)
Performance	✅ Fast	⚠️ Slightly slower

💥 Why is String risky for passwords?

🔹 1. Immutability

• Once created, a String cannot be changed.
• If you store a password as a String, it stays in memory until garbage collected, which you can’t control directly.
• Even if you set the reference to null, the actual string data remains in memory.

🔹 2. String Pool

• Java stores String literals in a common memory pool.
• Passwords stored as String may persist longer than needed.
• Tools like heap dump analyzers or memory scanners could potentially recover passwords from memory.
• Multiple references to the same string share the same memory location.

🔹 3. Memory Dumps and Logs

• Heap dumps may contain password strings
• Application logs might accidentally log password strings
• Debug tools can inspect string contents in memory
• JVM crash dumps preserve string data

✅ Why char[] is safer

🔸 1. Mutable

• You can manually overwrite the contents of the array after use:

Arrays.fill(password, '0'); // Overwrites each character

🔸 2. Control Over Lifetime

• You decide when to clear the password from memory.
• Can clear immediately after authentication.
• No dependency on garbage collection.

🔸 3. No String Pool

• Character arrays are not interned.
• Each array is a separate memory location.
• No risk of sharing memory with other parts of the application.

🛡️ Complete Secure Password Example

import java.util.Arrays;
import java.util.Scanner;

public class SecurePasswordExample {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter password: ");
        char[] password = scanner.nextLine().toCharArray();

        try {
            // Use password for authentication
            if (authenticate(password)) {
                System.out.println("Authentication successful!");
            } else {
                System.out.println("Authentication failed!");
            }
        } finally {
            // CRITICAL: Clear password from memory immediately
            clearPassword(password);
            scanner.close();
        }
    }

    private static boolean authenticate(char[] password) {
        // Simulate authentication
        String storedHash = "5e884898da28047151d0e56f8dc6292773603d0d6aabbdd62a11ef721d1542d8"; // "password"
        String inputHash = hashPassword(password);
        return storedHash.equals(inputHash);
    }

    private static String hashPassword(char[] password) {
        // In real applications, use proper hashing like BCrypt, PBKDF2, or Argon2
        // This is just a simple example
        return "hashed_" + new String(password);
    }

    private static void clearPassword(char[] password) {
        if (password != null) {
            // Overwrite with zeros
            Arrays.fill(password, '\0');
            System.out.println("Password cleared from memory.");
        }
    }
}