ʕ·ᴥ·ʔ

When you open the .java file, you see this code:

import java.io.*;
import java.util.*;  
public class SafeOpener {
    public static void main(String args[]) throws IOException {
        BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in));
        Base64.Encoder encoder = Base64.getEncoder();
        String encodedkey = "";
        String key = "";
        int i = 0;
        boolean isOpen;


        while (i < 3) {
            System.out.print("Enter password for the safe: ");
            key = keyboard.readLine();

            encodedkey = encoder.encodeToString(key.getBytes());
            System.out.println(encodedkey);

            isOpen = openSafe(encodedkey);
            if (!isOpen) {
                System.out.println("You have  " + (2 - i) + " attempt(s) left");
                i++;
                continue;
            }
            break;
        }
    }
    
    public static boolean openSafe(String password) {
        String encodedkey = "cGwzYXMzX2wzdF9tM18xbnQwX3RoM19zYWYz";

        if (password.equals(encodedkey)) {
            System.out.println("Sesame open");
            return true;
        }
        else {
            System.out.println("Password is incorrect\n");
            return false;
        }
    }
}

The password you entered is first encoded by something called encoder and then compared to the encodedkey. The encoder is a base64 encoder as stated in the first few lines of the code, so then you just use a base64 decoder and decode the encoded key.

Flag: picoCTF{pl3as3_l3t_m3_1nt0_th3_saf3}