问hashSet上的android空指针异常

EN

package com.example.alon.a2018_17_12_userloginexhomework;

public static final String USERNAME = "username";
public static final String SP_LOGGED_USER = "spLoggedUser";
public static final String PREFS = "prefs";
public static final int REQUEST_CODE = 123;
public static final String DESTINATION = "destination";
public static final String REGISTERED_USERS_SET = "registeredUsersSet";
EditText etUsername, etPassword;
Button btnLogin, btnSignup;
public static HashMap<String, User> hashMap;
public static SharedPreferences sharedPreferences;
Set<String> registeredUsersSet;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    init();
    checkDestination();
    checkForLoggedOrRegisteredUsers();
}

private void checkDestination() {
    int destination = getIntent().getIntExtra(DESTINATION, -1);
    if (destination == 0)
        finish();
}

private void checkForLoggedOrRegisteredUsers() {
    String loggedInUser = getSharedPreferences(PREFS, MODE_PRIVATE)
            .getString(SP_LOGGED_USER, null);
    if (loggedInUser != null){
        User user = new User(loggedInUser);
        Intent intent = new Intent(this, ActivityUserLoggedIn.class);
        intent.putExtra(USERNAME, user.getUsername());
        startActivityForResult(intent,REQUEST_CODE);
    }
    try {
        registeredUsersSet = getSharedPreferences(PREFS, MODE_PRIVATE)
                .getStringSet(REGISTERED_USERS_SET, null);
    }catch (NullPointerException e){
        e.printStackTrace();
    }
    if (registeredUsersSet != null){
        for (String s : registeredUsersSet){
            User u = new User(s);
            hashMap.put(u.getUsername(),u);
        }
    }
}

private void init() {
    registeredUsersSet = new HashSet<>();
    etUsername = findViewById(R.id.etUsername);
    etPassword = findViewById(R.id.etPassword);
    btnLogin = findViewById(R.id.btnLogin);
    btnSignup = findViewById(R.id.btnSignup);
    hashMap = new HashMap<>();
    sharedPreferences = getSharedPreferences(PREFS,MODE_PRIVATE);
    btnLogin.setOnClickListener(this);
    btnSignup.setOnClickListener(this);
}

@Override
public void onClick(View v) {
    switch (v.getId()){
        case R.id.btnSignup:
            signUp();
            break;
        case R.id.btnLogin:
            login();
            break;
    }
}

private void login() {
    String user = etUsername.getText().toString();
    String password = etPassword.getText().toString();
    //logic check:
    if (user.length() < 1 || password.length() < 1){
        Toast.makeText(this,
                "user or pass must be above 1 chars",
                Toast.LENGTH_SHORT).show();
    }
    //username check
    if (hashMap.containsKey(user)){
        //password check:
        if (Objects.requireNonNull(hashMap.get(user)).getPassword().equals(password)){
            //all checks are okay, logging in:
            Intent intent = new Intent(this, ActivityUserLoggedIn.class);
            intent.putExtra(USERNAME, user);
            sharedPreferences.edit().putString(SP_LOGGED_USER,user).apply();
            startActivityForResult(intent, REQUEST_CODE);
        } else {
            //password error:
            Toast.makeText(this,
                    "password does not match, please retry"
                    , Toast.LENGTH_SHORT).show();
            etPassword.setText("");
        }
    } else {
        //username error:
        Toast.makeText(this,
                "User does not exists, please try again"
                , Toast.LENGTH_SHORT).show();
        clearEditTexts();
    }
}

private void signUp() {
    String user = etUsername.getText().toString();
    String password = etPassword.getText().toString();
    if (user.length() < 1 || password.length() < 1){
        Toast.makeText(this,
                "user or pass must be above 1 chars",
                Toast.LENGTH_SHORT).show();
    } else if (hashMap.containsKey(user)){
        Toast.makeText(this,
                "user already exists, please choose a different usename",
                Toast.LENGTH_SHORT).show();
        clearEditTexts();
    } else {
        User newUser = new User(user,password);
        hashMap.put(user,newUser);
        registeredUsersSet.add(newUser.toString());
        Toast.makeText(this,
                "successfully registered used name " + user,
                Toast.LENGTH_SHORT).show();
        clearEditTexts();
    }
}

private void clearEditTexts(){
    etUsername.setText("");
    etPassword.setText("");
}

@Override
protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    if (requestCode == REQUEST_CODE && resultCode == RESULT_OK){
        if (data != null) {
            int destination = data.getIntExtra(DESTINATION, -1);
            if (destination == 0)
                finish();
        }
    }
}

@Override
protected void onStop() {
    super.onStop();
    sharedPreferences.edit().putStringSet(REGISTERED_USERS_SET, registeredUsersSet).apply();
}