java - 如果用户尚未登录，如何为 getDisplayName() 写入 if 语句？

Question

我在运行应用程序时遇到此问题:

Attempt to invoke virtual method java.lang.String com.google.firebase.auth.FirebaseUser.getDisplayName() on a null object reference

我想制作一个用户无需先登录即可浏览的应用程序。如何将 name.setText("hi, "+ currentUser.getDisplayName()); 放入 if 语句中 我应该在 if 语句的编码中写什么？

这是我的MainActivity java 类:

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

    name=findViewById(R.id.name);

    //        //display customer's account name
    name.setText("hi, " + currentUser.getDisplayName());

    //ini
    firebaseAuth = FirebaseAuth.getInstance();
    currentUser = firebaseAuth.getCurrentUser();
    databaseReference = FirebaseDatabase.getInstance().getReference("Cust");



@Override
public boolean onCreateOptionsMenu(Menu menu) {
    MenuInflater inflater = getMenuInflater();
    inflater.inflate(R.menu.menu_option,menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(@NonNull MenuItem item) {
    int id= item.getItemId();
    if (id == R.id.logout){
        logout();
        return true;
    }
    else if (id==R.id.login){
        login();
        return true;
    }
    return super.onOptionsItemSelected(item);
}

private void login() {
    Intent intent= new Intent(this,LoginActivity.class);
    startActivity(intent);

}

private void logout() {
    FirebaseAuth.getInstance().signOut();
    Intent intent = new Intent(MainActivity.this, LoginActivity.class);
    intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
    startActivity(intent);
    finish();
}
}

Answer 1

检查 currentUser 是否不为空:

if (currentUser != null) name.setText("hi, " + currentUser.getDisplayName());

而且，在您的代码中，您在 从 firebase 初始化它之前请求 getDisplayName，因此它将始终为 null:

firebaseAuth = FirebaseAuth.getInstance();
currentUser = firebaseAuth.getCurrentUser();
databaseReference = FirebaseDatabase.getInstance().getReference("Cust");

if (currentUser != null) name.setText("hi, " + currentUser.getDisplayName());