在数组中它存储的是NULL而不是PDF文件名。

Question

public class cp extends AppCompatActivity {

    ListView ListPdf;
    DatabaseReference databaseReference;
    List<upload> uploadPdf;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_cp);

        ListPdf = (ListView) findViewById(R.id.List);
        uploadPdf = new ArrayList<>();
        ViewAllPdf();
    }

    private void ViewAllPdf() {
        databaseReference = FirebaseDatabase.getInstance().getReference("year_1");
        databaseReference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                for(DataSnapshot PostSnapShot: dataSnapshot.getChildren()){

                    upload Pdf = PostSnapShot.getValue(upload.class);
                    uploadPdf.add(Pdf);

                }

                String[] uploads = new String[uploadPdf.size()];
                for(int i=0;i<uploads.length;i++){
                    uploads[i] = uploadPdf.get(i).getName();
                }
                ArrayAdapter<String> adapter = new ArrayAdapter<String>(getApplicationContext(),android.R.layout.simple_list_item_1,uploads);
                ListPdf.setAdapter(adapter);
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });
    }


}

上课

public class upload {

    public String name;

    public upload() {

        this.name = name;
    }



    public String getName() {
        return name;
    }
}

在uploads[]中，它存储的是null而不是文件名，我不知道问题出在哪里。我试着用logcat调试，但显示Pdf对象有三个文件，但在uploads[]中却存储了null。请您研究一下

Firebase数据库截图

Answer 1

你的代码的问题在于你的POJO类。Upload. 请看Firebase文档中的一个例子

public class User {

    public String username;
    public String email;

    public User() {
        // Default constructor required for calls to DataSnapshot.getValue(User.class)
    }

    public User(String username, String email) {
        this.username = username;
        this.email = email;
    }

}

另外，你的实时数据库中的字段名与你的POJO类不匹配。