Problem with assertEquals

I have problems with some tests that I am going through, specifically with "assertEquals"

I'm really comparing two "user" objects that have 3 parameters "username, password, fullname"

In the tests I do this:

    public void loginUser() {        
    Platform platform = new Platform();

    // Check that no user is in the platform
    assertEquals((int)platform.getNumUsers(), 0);

    // Register new user
    User u1=platform.registerUser(username1, password1, fullName1);
    assertNotNull(u1);
    assertEquals((int)platform.getNumUsers(), 1);

    // Try to login with the new user credentials
    User u2 = platform.login(username1, password1);
    assertNotNull(u2);
    assertEquals((int)platform.getNumUsers(), 1);

    // User information is correct
    assertEquals(u1, u2);

First I check that no user exists, then I create a user and lastly with User u2 I pass the username and password of U1 and if the information of u1 and u2 is the same I continue forward, the case is that when I come to assertEquals (u1, u2) it tells me that they are not the same but I think that if they are ...

The class where I do the calculations is the Platform class

         package edu.uoc.dpoo;
         import java.util.ArrayList;
         import java.util.List;

      /**
        *
        * @author Guillermo Rodriguez Barcelo
        */
public class Platform {

private List<Competition> competitions;
private List<User> users;

public Platform(){

    competitions = new ArrayList<Competition>();
    users = new ArrayList<User>();

}
public User registerUser(String username ,String password,String fullname){

    User p = findUser(username);
    if (p == null) {
        p =  new User(username, password,fullname);
        users.add(p);
    }
    else {
        p=null;
    }

    return (User) p;
}



public User findUser (String username) {


           User user = null;

    for (User p : users) {
        if (username.equals(p.getUserName())) {
            user = p;
                            p= null;
            break;
        }
    }



    return user;
}


public User login(String username,String password){
    User user = null;

    for (User p : users){
        if(username.equals(p.getUserName())&& (password.equals(p.getPassword())) ){          
            user = p;
            break;
    }else
    {
            return null;
            }        
    }           
    return user;
}


public Integer getNumUsers(){
    Integer  size=users.size();

    return size  ;
}
public Integer getNumCompetitions(){
    return null;
}
public Message sendMessage(User from,String to,String subject,String message){
    return null;
}
private float evaluateAll(){
    return (float) 0.0;

}

public List<Competition> getOpenCompetitions(){
    return null;
}

private User User(User u) {
    throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}





}

The code for assertEquals is as follows

    static public void assertEquals(Object expected, Object actual) {
    assertEquals(null, expected, actual);
}

The code of the assertEquals method (null, expected, current) is as follows

    static public void assertEquals(String message, Object expected,
        Object actual) {
    if (equalsRegardingNull(expected, actual)) {
        return;
    } else if (expected instanceof String && actual instanceof String) {
        String cleanMessage = message == null ? "" : message;
        throw new ComparisonFailure(cleanMessage, (String) expected,
                (String) actual);
    } else {
        failNotEquals(message, expected, actual);
    }
}



No entiendo por que me dice que los objetos u1 y u2 no son iguales.