Implement all the answers and test them.

src/AscendingHackerThread.java

+public class AscendingHackerThread extends Hacker {
+
+    public AscendingHackerThread(Vault vault) {
+        super(vault);
+        this.setName("AscendingHackerThread");
+        this.setPriority(Thread.MAX_PRIORITY);
+    }
+    @Override
+    public void hack() {
+        for (int guess = 0; guess <= 9999; guess++) {
+            if (vault.isCorrectPassword(guess)) {
+                System.out.println(this.getName() + " guessed the password " + guess);
+                System.exit(0);
+            }
+        }
+    }
+    
+}

src/DescendingHackerThread.java

+public class DescendingHackerThread extends Hacker {
+
+    public DescendingHackerThread(Vault vault) {
+        super(vault);
+        this.setName("DescendingHackerThread");
+        this.setPriority(Thread.MAX_PRIORITY);
+    }
+    
+    @Override
+    public void hack() {
+        for (int guess = 9999; guess >= 0; guess--) {
+            if (vault.isCorrectPassword(guess)) {
+                System.out.println(this.getName() + " guessed the password " + guess);
+                System.exit(0);
+            }
+        }
+    }
+    
+}

src/Hacker.java

+public abstract class Hacker extends Thread {
+    protected final Vault vault;
+
+    public Hacker(Vault vault) {
+        this.vault = vault;
+    }
+
+    public abstract void hack();
+    
+    @Override
+    public void run() { hack(); }
+}

src/Main.java

 public class Main {
-    public static void main(String[] args) throws InterruptedException {
-        System.out.println("Starting threads from Main...");
-
-        MyThread t1 = new MyThread();
-        MyThread t2 = new MyThread();
-
-        t1.start();
-        t2.start();
-
-        t1.join();
-        t2.join();
-
-        System.out.println("All threads finished.");
+    public static void main(String[] args) {
+        Vault vault = new Vault();
+        Thread ascendingHacker = new AscendingHackerThread(vault);
+        Thread descendingHacker = new DescendingHackerThread(vault);
+        Thread police = new PoliceThread(10);
+
+        ascendingHacker.start();
+        descendingHacker.start();
+        police.start();
     }
 }

src/PoliceThread.java

+public class PoliceThread extends Thread {
+    private final int countdownSeconds;
+
+    public PoliceThread(int countdownSeconds) {
+        this.countdownSeconds = countdownSeconds;
+        this.setName("PoliceThread");
+    }
+
+    public void run() {
+        try {
+            for (int i = countdownSeconds; i > 0; i--) {
+                System.out.println("Police will arrive in " + i + " seconds");
+                Thread.sleep(1000);
+            }
+            System.out.println("Police have arrived! Game over for hackers.");
+            System.exit(0);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+        }
+    }
+}    
\ No newline at end of file

src/ThreadExample.java

-class MyThread extends Thread {
-    @Override
-    public void run() {
-        for (int i = 0; i < 5; i++) {
-            System.out.println("Thread running: " + i);
-            try {
-                Thread.sleep(1000); 
-            } catch (InterruptedException e) {
-                System.out.println("Thread interrupted");
-            }
-        }
-    }
-}
\ No newline at end of file

src/Vault.java

+import java.util.concurrent.ThreadLocalRandom;
+
+public class Vault {
+    private final int secretCode;
+
+    public Vault() {
+        this(0, 9999);
+    }
+
+    public Vault(int min, int max) {
+        if (min > max) throw new IllegalArgumentException("min > max");
+        this.secretCode = randBetween(min, max);
+        System.out.println("Vault password is set to " + this.secretCode);
+    }
+
+    public boolean isCorrectPassword(int guess) {
+        try {
+            Thread.sleep(5);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+        }
+        return this.secretCode == guess;
+    }
+
+    private static int randBetween(int min, int max) {
+        if (min > max) throw new IllegalArgumentException("min > max");
+        if (max == Integer.MAX_VALUE) {
+            return (int) ThreadLocalRandom.current().nextLong(min, (long) max + 1L);
+        }
+        return ThreadLocalRandom.current().nextInt(min, max + 1);
+    }
+}
\ No newline at end of file