如何解决 Singelton is NULL 问题

Question

我正在尝试建立一个学生注册系统，并将这些学生保存在 firestore 中。我不希望它在添加学生时添加相同编号的学生，为此，我创建了一个单身人士 class。这个单例 class 有一个标志值。如果 firestore 中有相同编号的学生，我使用此标志提供控制。

但它始终 returns 为空。没看懂。

我只是分享必要的代码。

我加学生class

private ActivityOgrenciEkleBinding binding;
private FirebaseFirestore mFirestore =  FirebaseFirestore.getInstance();
private LinkedHashMap<String,String> linkedHashMap;
private OgrenciyiKontrolEt ogrenciyiKontrolEt;
Singleton singleton;

public void init(){

    linkedHashMap = new LinkedHashMap<>();
    singleton = Singleton.getInstance();
    btn_Ogrenci_EKLE();


}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    binding = ActivityOgrenciEkleBinding.inflate(getLayoutInflater());
    View view = binding.getRoot();
    setContentView(view);
    init();
}

public void btn_Ogrenci_EKLE(){


    binding.btnEkleOgrenci.setOnClickListener(new View.OnClickListener() 
   {

        @Override
        public void onClick(View view) {

            String email_ogr = binding.emailOgrenci.getText().toString();
            String ad_ogr = binding.isimOgrenci.getText().toString();
            String soyisim_ogr = binding.soyisimOgrenci.getText().toString();
            String no_ogr = binding.numaraOgrenci.getText().toString();
            String parola_ogr = binding.sifreOgrenci.getText().toString();

           ogrenciyiKontrolEt = new 
           OgrenciyiKontrolEt(no_ogr,OgrenciEkleActivity.this);// I 
           //created object from 
           //control class to check students


            if(email_ogr.equals("") || ad_ogr.equals("") || soyisim_ogr.equals("") || no_ogr.equals("") || parola_ogr.equals("")){
                AlertDialog.Builder builder = new AlertDialog.Builder(OgrenciEkleActivity.this);
                builder.setTitle("UYARI !");
                builder.setMessage("Boş alan bırakmayınız !");
                builder.setIcon(R.drawable.warningicon);
                builder.setPositiveButton("Tamam", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialogInterface, int i) {

                    }
                });
                builder.show();
            }
            else{

                ogrenciyiKontrolEt.OGR_KONTROL();
                System.out.println("singleton get flag"+singleton.getflag()); //singleton returns null here and and it never 
                       //goes into the if loop
                
                if("100".equals(singleton.getflag())){

                    AlertDialog.Builder builder = new AlertDialog.Builder(OgrenciEkleActivity.this);
                    builder.setIcon(R.drawable.warningicon);
                    builder.setMessage(no_ogr+" numaralı öğrenci zaten kayıtlı lütfen farklı bir numara giriniz.");
                    builder.setTitle("UYARI");
                    builder.setPositiveButton("TAMAM", new DialogInterface.OnClickListener() {
                        @Override
                        public void onClick(DialogInterface dialogInterface, int i) {

                        }
                    });
                    builder.show();

                }
                else{


                    linkedHashMap.put("name",ad_ogr);
                    linkedHashMap.put("lastname",soyisim_ogr);
                    linkedHashMap.put("number",no_ogr);
                    linkedHashMap.put("email",email_ogr);
                    linkedHashMap.put("password",parola_ogr);

                    mFirestore.collection("Students").add(linkedHashMap).addOnSuccessListener(new OnSuccessListener<DocumentReference>() {
                        @Override
                        public void onSuccess(@NonNull DocumentReference documentReference) {
                            Toast toast =   Toast.makeText(OgrenciEkleActivity.this,"Öğrenci başaralı bir şekilde eklendi.",Toast.LENGTH_LONG);
                            toast.show();
                        }
                    }).addOnFailureListener(new OnFailureListener() {
                        @Override
                        public void onFailure(@NonNull Exception e) {
                            Toast toast =   Toast.makeText(OgrenciEkleActivity.this,"Öğrenci eklerken bir hata ile karşılaşıldı.",Toast.LENGTH_LONG);
                            toast.show();
                        }
                    });
                }


            }
        }
    });

}

控制class我创建的控制学生

public class OgrenciyiKontrolEt {


protected String ogrenci_no;
protected String firestore_ogr_no;
protected FirebaseFirestore firebaseFirestoreDb = FirebaseFirestore.getInstance();
public Context context;
Singleton singleton  = Singleton.getInstance();

public OgrenciyiKontrolEt(String ogr_no,Context context){
    this.ogrenci_no = ogr_no;
    this.context    = context;

}



public void OGR_KONTROL(){


    firebaseFirestoreDb.collection("Students")
            .get()
            .addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
                @Override
                public void onSuccess(@NonNull QuerySnapshot queryDocumentSnapshots) {
                    List<DocumentSnapshot> snapshotList = queryDocumentSnapshots.getDocuments();

                    for(DocumentSnapshot snapshot: snapshotList ) {

                        firestore_ogr_no = snapshot.getString("number");

                        if(firestore_ogr_no.equals(ogrenci_no)) {
                            singleton.setflag("100"); //If there is a student with the same 
                                              number, I set the flag in the singleton to 100.
                        }
                    }
                }

            })

            .addOnFailureListener(new OnFailureListener() {
            @Override
            public void onFailure(@NonNull Exception e) {
                Toast toast = Toast.makeText(context,"Bir hata ile karşılaşıldı.",Toast.LENGTH_LONG);
                toast.show();
            }
        });





 }

}

我的单身狗class

public class Singleton {

**EDIT**
private String flag="1";
**EDIT**

private static Singleton singleton;

private Singleton() {

}

public String getflag() {
    return flag;
}

public void setflag(String flag) {
    this.flag = flag;
}

public static Singleton getInstance() {
    if (singleton == null) {
        singleton = new Singleton();
    }

    return singleton;

 }
 }

Answer 1

你的标志在你调用它时没有被初始化，要么将一些东西设置为默认值，要么在构造函数中初始化，比如

private Singleton() {
    this.flag = "";
}

或在第一次出现 getFalg

之前致电您的 setFlag

如何解决 Singelton is NULL 问题

How can I solve Singelton is NULL problem

java

singleton

android

firebase

google-cloud-firestore