Junit Test Cases to add to solve missing code

Question

I want to add few test cases in this code to run the code successfully.Please find the main class and Testclass below and help to resolve this issue.

Main Class: 
```
/**
 * Allow booking of rooms (by name)
 */
public class BookingSystem {
    private AtomicInteger nextUserIdCounter = new AtomicInteger();
    private Map<String, String> userIds = new HashMap<>();
    /**
     * @return new unique userId for user
     */
    public String registerUser(String name) {
        return userIds.computeIfAbsent(name, ignored -> ("user_"+nextUserIdCounter.incrementAndGet()));
    }
    /**
     * @return meetingId
     */
    public String bookRoom(String roomName, int hour, List<String> userIds) {       
        
        
        
        throw new RuntimeException("Not implemented");
    }
    /**
     * @return true if cancel actually cancelled something
     */
    public boolean cancelBooking(String meetingId) {
        throw new RuntimeException("Not implemented");
    }
    /**
     * @return null or the room name the user should be in at the given hour
     */
    public String whatRoomAtTimeForUser(int hour, String userId) {
        throw new RuntimeException("Not implemented");
    }
}


Below is my Test Class and that we need to make it running.

Test Class :

public class BookingSystemTest {
    static class Fixture {
        BookingSystem system = new BookingSystem();
        String bob = system.registerUser("Bob");
        String fred = system.registerUser("Fred");
        String jane = system.registerUser("Jane");
        String chris = system.registerUser("Chris");
        String zach = system.registerUser("Zach");
    }
    private final String room1 = "room1";
    private final String room2 = "room2";
    private List<String> listOf(String ... v) {
        return Arrays.stream(v).collect(Collectors.toList());
    }
    @Test
    public void testRegisterUsers() {
        Fixture f = new Fixture();
        Assert.assertNotEquals(f.bob, f.fred);
        Assert.assertNotEquals(f.bob, f.jane);
        Assert.assertNotEquals(f.zach, f.jane);
    }
    @Test
    public void makeBookings() {
        Fixture f = new Fixture();
        // Book a room at a time with some users
        f.system.bookRoom(room1, 16, listOf(f.bob, f.fred));
      
        // Book another room at a different time with some users
        f.system.bookRoom(room2, 17, listOf(f.chris, f.zach, f.jane));
    }
    @Test
    public void makeConflictingBookings() {
        Fixture f = new Fixture();
        // Book a room at a time with some users
        f.system.bookRoom(room1, 16, listOf(f.bob, f.fred));
        // Book another room at a different time with some users
        f.system.bookRoom(room2, 17, listOf(f.chris, f.zach, f.jane));
        // Book another meeting in same room with different users
        try {
            f.system.bookRoom(room1, 16, listOf(f.bob, f.fred));
            fail("Should not allow booking");
        } catch (RoomTakenException ex) {
            // good
        }
    }
    @Test
    public void cancelBookingsAllowsNewBooking() {
        Fixture f = new Fixture();
        // Book a room at a time with some users
        String b1 = f.system.bookRoom(room1, 16, listOf(f.bob, f.fred));
        // Book another room at a different time with some users
        f.system.bookRoom(room2, 17, listOf(f.chris, f.zach, f.jane));
        boolean cxld = f.system.cancelBooking(b1);
        Assert.assertTrue(cxld);
        // Re-use room, should now be ok
        f.system.bookRoom(room1, 16, listOf(f.bob, f.fred));
    }
    @Test
    public void discoverMeetingsForUser() {
        Fixture f = new Fixture();
        // Book a room at a time with some users
        f.system.bookRoom(room1, 16, listOf(f.bob, f.fred));
        // Book another room at a different time with some users
        f.system.bookRoom(room2, 17, listOf(f.bob, f.zach, f.jane));
        // Discover
        Assert.assertNull(f.system.whatRoomAtTimeForUser(14, f.bob));
        Assert.assertEquals(room1, f.system.whatRoomAtTimeForUser(16, f.bob));
        Assert.assertEquals(room2, f.system.whatRoomAtTimeForUser(17, f.bob));
        Assert.assertNull(f.system.whatRoomAtTimeForUser(16, f.zach));
        Assert.assertEquals(room2, f.system.whatRoomAtTimeForUser(17, f.zach));
    }
}

I want to add missing TestCases in test class.I am using Java 8 and Junit, this is a hackerrank question where we need to add some missing test cases.This is a normal question where all the Testcase need to be passed.The issue is that testregister() is working successfully.