如何解决 Singelton 为 NULL 问题[重复]

Posted 2023-04-19

【中文标题】如何解决 Singelton 为 NULL 问题[重复]

【英文标题】：How can I solve Singelton is NULL problem [duplicate]

【发布时间】：2022-01-11 00:53:18

【问题描述】：

我正在尝试制作一个学生注册系统，并将这些学生保存在 Firestore 中。我不希望它在添加学生时添加具有相同编号的学生，为此，我创建了一个单例类。这个单例类有一个标志值。我使用这个标志来控制firestore中是否有相同号码的学生。

但它总是返回 null。我不明白。

我只是分享必要的代码。

我的添加学生班级

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();
                        
                    );
                


            
        
    );

我为控制学生而创建的控制类

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();
            
        );





 

我的单身班

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;

 
 

【参考方案1】：

你的 flag 在你调用它时没有被初始化，要么设置为默认值，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

【讨论】：

谢谢它的工作，但我将标志设置为 1 作为默认值，即使有相同号码的学生，标志也永远不会 100。它不会进入 if 循环根本没有，并添加好像没有相同数量的学生。为什么会发生这种情况？

这是极具误导性的。实际的问题是回调是异步的，这意味着标志没有按照 OP 期望的顺序初始化。

我明白你告诉了