android - 在 Android Studio 中从 Api 创建 Spinner

Question

我正在尝试从 API 数据在 Android 3.2 版中创建 Spinner。编写所有代码后，我得到了致命异常。我还创建了一个 TOAST 来显示弹出我的 Api 数据，但没有显示弹出窗口，Spinner 获得空值。

这是我的 API 数据

[
    {
        "oid": "PCATxxx008",
        "pcat_text": "008",
        "pcat_name": "MDSP Project (Cyclone Center)",
        "pcat_actv": "1"
    },
    {
        "oid": "PCATxxx001",
        "pcat_text": "001",
        "pcat_name": "Plastic",
        "pcat_actv": "1"
    },
    {
        "oid": "PCATxxx002",
        "pcat_text": "002",
        "pcat_name": "PVC",
        "pcat_actv": "1"
    },
    {
        "oid": "PCATxxx003",
        "pcat_text": "003",
        "pcat_name": "Household",
        "pcat_actv": "0"
    },
    {
        "oid": "PCATxxx004",
        "pcat_text": "004",
        "pcat_name": "Furniture",
        "pcat_actv": "0"
    },
    {
        "oid": "PCATxxx005",
        "pcat_text": "005",
        "pcat_name": "LGED",
        "pcat_actv": "1"
    },
    {
        "oid": "PCATxxx006",
        "pcat_text": "006",
        "pcat_name": "Export",
        "pcat_actv": "1"
    },
    {
        "oid": "PCATxxx007",
        "pcat_text": "007",
        "pcat_name": "Tender",
        "pcat_actv": "1"
    }
]  

这是 Java 接口(interface)

import java.util.List;
import retrofit2.Call;
import retrofit2.http.GET;

public interface api {
    String BASE_URL ="http://192.168.1.95:55069/api/";

    @GET("Category")
    Call<List<Category>> getCategory();
}

这是类别类

public class Category {
    private String oid;
    private String pcat_text;
    private String pcat_name;
    private String pcat_actv;

    public Category(String oid, String pcat_text, String pcat_name, String pcat_actv) {
        this.oid = oid;
        this.pcat_text = pcat_text;
        this.pcat_name = pcat_name;
        this.pcat_actv = pcat_actv;
    }

    public String getOid() {
        return oid;
    }

    public String getPcat_text() {
        return pcat_text;
    }

    public String getPcat_name() {
        return pcat_name;
    }

    public String getPcat_actv() {
        return pcat_actv;
    }
}

主 Activity

public class MainActivity extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener {

    DrawerLayout drawerLaout;
    ActionBarDrawerToggle toggle;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        drawerLaout = findViewById(R.id.drawerId);

        NavigationView navigationView = findViewById(R.id.navigationId);
        navigationView.setNavigationItemSelectedListener(this);

        toggle = new ActionBarDrawerToggle(this,drawerLaout,R.string.nav_open,R.string.nav_close);
        drawerLaout.addDrawerListener(toggle);
        toggle.syncState();
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);

        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(api.BASE_URL)
                .addConverterFactory(GsonConverterFactory.create()) //Here we are using the GsonConverterFactory to directly convert json data to object
                .build();

        //creating the api interface
        api api = retrofit.create(api.class);

        //now making the call object
        //Here we are using the api method that we created inside the api interface
        Call<List<Category>> call = api.getCategory();

        //then finallly we are making the call using enqueue()
        //it takes callback interface as an argument
        //and callback is having two methods onRespnose() and onFailure
        //if the request is successfull we will get the correct response and onResponse will be executed
        //if there is some error we will get inside the onFailure() method
        call.enqueue(new Callback<List<Category>>() {
            @Override
            public void onResponse(Call<List<Category>> call, Response<List<Category>> response) {
                List<Category> Category = response.body();
            }

            @Override
            public void onFailure(Call<List<Category>> call, Throwable t) {
               Toast.makeText(getApplicationContext(), t.getMessage(), Toast.LENGTH_LONG+5000).show();
            }
        });
    }

CollectionActivity 我的 Spinner 应该在其中显示数据

public class CollectionActivity extends AppCompatActivity {

    private Spinner spinner1, spinner2;
    private Button btnSubmit;
    DatePicker picker;
    TextView dateText;
    EditText dateView;
    DatePickerDialog.OnDateSetListener setListener;
    private static final String CHANNEL_ID = "Notification";
    private static final String CHANNEL_NAME = "Test Notification";
    private static final String CHANNEL_DESC = "Test Notification Desc";


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_collection);
        addItemsOnSpinner2();
        addListenerOnButton();

        if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.O){
            NotificationChannel channel = new NotificationChannel(CHANNEL_ID, CHANNEL_NAME, NotificationManager.IMPORTANCE_DEFAULT);
            channel.setDescription(CHANNEL_DESC);
            NotificationManager manager = getSystemService(NotificationManager.class);
            manager.createNotificationChannel(channel);
        }

        dateText = findViewById(R.id.dateText);
        dateView = findViewById(R.id.dateView);

        Calendar calendar = Calendar.getInstance();
        final int year = calendar.get(Calendar.YEAR);
        final int month = calendar.get(Calendar.MONTH);
        final int day = calendar.get(Calendar.DAY_OF_MONTH);

        dateView.setOnClickListener(new View.OnClickListener() {
             @Override
             public void onClick(View v){
                DatePickerDialog datePickerDialog = new DatePickerDialog(
                        CollectionActivity.this,android.R.style.Theme_Holo_Light_Dialog_MinWidth
                        ,setListener,year,month,day);
                datePickerDialog.getWindow().setBackgroundDrawable(new ColorDrawable(Color.TRANSPARENT));
                datePickerDialog.show();
             }
        });

        setListener = new DatePickerDialog.OnDateSetListener() {
            @Override
            public void onDateSet(DatePicker view, int year, int month, int dayOfMonth) {
                month = month + 1;
                String date = day+"/"+month+"/"+year;
                dateView.setText(date);
            }
        };

        addListenerOnSpinnerItemSelection();
    }

    // add items into spinner dynamically
    public void addItemsOnSpinner2() {

        spinner2 = (Spinner) findViewById(R.id.spinner2);

        //List<String> list = new ArrayList<String>();
        //list.add("LGED");
        //list.add("PVC");
        //list.add("Plastic");

        Retrofit retrofit =new Retrofit.Builder()
                .baseUrl(api.BASE_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .build();
        api api =retrofit.create(api.class);
        Call<List<Category>> call = api.getCategory();

        call.enqueue(new Callback<List<Category>>() {
            @Override
            public void onResponse(Call<List<Category>> call, Response<List<Category>> response) {
                List<Category> CategoryList = response.body();

                String[] Categorys = new String[CategoryList.size()];

                for(int i=0; i< CategoryList.size(); i++){
                    Categorys[i]= CategoryList.get(i).getPcat_name();

                    ArrayAdapter<String> spinnerArrayAdapter = new ArrayAdapter<String>(CollectionActivity.this, android.R.layout.simple_spinner_item, Categorys);
                    spinnerArrayAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item); // The drop down view
                    spinner2.setAdapter(spinnerArrayAdapter);
                }



                /*ArrayAdapter<String> dataAdapter = new ArrayAdapter<String>(this,
                        android.R.layout.simple_spinner_item, Categorys);
                dataAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
                spinner2.setAdapter(dataAdapter);*/
            }

            @Override
            public void onFailure(Call<List<Category>> call, Throwable t) {
                Toast.makeText(getApplicationContext(), t.getMessage(), Toast.LENGTH_SHORT).show();
            }
        });
    }

    public void addListenerOnSpinnerItemSelection() {
        spinner1 = (Spinner) findViewById(R.id.spinner1);
        spinner1.setOnItemSelectedListener(new CustomOnItemSelectedListener());
    }

    // get the selected dropdown list value
    public void addListenerOnButton() {

        spinner1 = (Spinner) findViewById(R.id.spinner1);
        spinner2 = (Spinner) findViewById(R.id.spinner2);
        btnSubmit = (Button) findViewById(R.id.btnSubmit);
        dateView =(EditText)findViewById(R.id.dateView);
        //picker= (DatePicker)findViewById(R.id.date);

        btnSubmit.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {

                Toast.makeText(CollectionActivity.this,
                        "You have selected : " +
                                "\nLocation : "+ String.valueOf(spinner1.getSelectedItem()) +
                                "\nCategory : "+ String.valueOf(spinner2.getSelectedItem()) +
                                //"\nSelected Date: "+ picker.getDayOfMonth()+"/"+ (picker.getMonth() + 1)+"/"+picker.getYear(),
                                "\nDate : " + dateView.getText().toString(),
                        Toast.LENGTH_LONG).show();
                addNotifications();
            }

        });
    }

    private void addNotifications() {
        spinner1 = (Spinner) findViewById(R.id.spinner1);
        spinner2 = (Spinner) findViewById(R.id.spinner2);
        dateView =(EditText)findViewById(R.id.dateView);

        NotificationCompat.Builder builder =
                new NotificationCompat.Builder(this,CHANNEL_ID)
                .setSmallIcon(R.drawable.ic_sms_black_24dp)
                .setContentTitle("New Collection Added")
                .setContentText(
                        "You have selected : " +
                        "\nLocation : "+ String.valueOf(spinner1.getSelectedItem()) +
                        "\nCategory : "+ String.valueOf(spinner2.getSelectedItem()) +
                        "\nDate : " + dateView.getText().toString()
                )
                .setPriority(NotificationCompat.PRIORITY_DEFAULT);

        NotificationManagerCompat mNotificationMgr = NotificationManagerCompat.from(this);
        mNotificationMgr.notify(1,builder.build());
    }
}

错误

AndroidRuntime: FATAL EXCEPTION: main
     Process: com.example.TEST, PID: 23249
    java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
        at com.example.talukdergroup.CollectionActivity$3.onResponse(CollectionActivity.java:114)
        at retrofit2.ExecutorCallAdapterFactory$ExecutorCallbackCall$1$1.run(ExecutorCallAdapterFactory.java:70)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

我已经给出了我编码的所有内容，当我输入时，我的 CollectionActivity 应用程序崩溃了。我想知道我在哪里犯了这个愚蠢的错误。

Answer 1

尝试使用您的 CollectionActivity 的以下代码，希望它会起作用。使用前应检查列表的空值。

 import android.app.DatePickerDialog;
 import android.app.NotificationChannel;
 import android.app.NotificationManager;
 import android.graphics.Color;
 import android.graphics.drawable.ColorDrawable;
 public class CollectionActivity extends AppCompatActivity {

private Spinner spinner1, spinner2;
private Button btnSubmit;
DatePicker picker;
TextView dateText;
EditText dateView;
DatePickerDialog.OnDateSetListener setListener;
private static final String CHANNEL_ID = "Notification";
private static final String CHANNEL_NAME = "Test Notification";
private static final String CHANNEL_DESC = "Test Notification Desc";


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_collection);
    addItemsOnSpinner2();
    addListenerOnButton();

    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        NotificationChannel channel = new NotificationChannel(CHANNEL_ID, CHANNEL_NAME, NotificationManager.IMPORTANCE_DEFAULT);
        channel.setDescription(CHANNEL_DESC);
        NotificationManager manager = getSystemService(NotificationManager.class);
        manager.createNotificationChannel(channel);
    }

    dateText = findViewById(R.id.dateText);
    dateView = findViewById(R.id.dateView);

    Calendar calendar = Calendar.getInstance();
    final int year = calendar.get(Calendar.YEAR);
    final int month = calendar.get(Calendar.MONTH);
    final int day = calendar.get(Calendar.DAY_OF_MONTH);

    dateView.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            DatePickerDialog datePickerDialog = new DatePickerDialog(
                    CollectionActivity.this, android.R.style.Theme_Holo_Light_Dialog_MinWidth
                    , setListener, year, month, day);
            datePickerDialog.getWindow().setBackgroundDrawable(new ColorDrawable(Color.TRANSPARENT));
            datePickerDialog.show();
        }
    });

    setListener = new DatePickerDialog.OnDateSetListener() {
        @Override
        public void onDateSet(DatePicker view, int year, int month, int dayOfMonth) {
            month = month + 1;
            String date = day + "/" + month + "/" + year;
            dateView.setText(date);
        }
    };

    addListenerOnSpinnerItemSelection();
}

// add items into spinner dynamically
public void addItemsOnSpinner2() {

    spinner2 = (Spinner) findViewById(R.id.spinner2);

    //List<String> list = new ArrayList<String>();
    //list.add("LGED");
    //list.add("PVC");
    //list.add("Plastic");

    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl(api.BASE_URL)
            .addConverterFactory(GsonConverterFactory.create())
            .build();
    api api = retrofit.create(api.class);
    Call<List<Category>> call = api.getCategory();

    call.enqueue(new Callback<List<Category>>() {
        @Override
        public void onResponse(Call<List<Category>> call, Response<List<Category>> response) {
            List<Category> CategoryList = response.body();

            if (CategoryList != null && CategoryList.size() > 0) {
                String[] Categorys = new String[CategoryList.size()];

                for (int i = 0; i < CategoryList.size(); i++) {
                    Categorys[i] = CategoryList.get(i).getPcat_name();

                    ArrayAdapter<String> spinnerArrayAdapter = new ArrayAdapter<String>(CollectionActivity.this, android.R.layout.simple_spinner_item, Categorys);
                    spinnerArrayAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item); // The drop down view
                    spinner2.setAdapter(spinnerArrayAdapter);
                }
            }



            /*ArrayAdapter<String> dataAdapter = new ArrayAdapter<String>(this,
                    android.R.layout.simple_spinner_item, Categorys);
            dataAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
            spinner2.setAdapter(dataAdapter);*/
        }

        @Override
        public void onFailure(Call<List<Category>> call, Throwable t) {
            Toast.makeText(getApplicationContext(), t.getMessage(), Toast.LENGTH_SHORT).show();
        }
    });
}

public void addListenerOnSpinnerItemSelection() {
    spinner1 = (Spinner) findViewById(R.id.spinner1);
    spinner1.setOnItemSelectedListener(new CustomOnItemSelectedListener());
}

// get the selected dropdown list value
public void addListenerOnButton() {

    spinner1 = (Spinner) findViewById(R.id.spinner1);
    spinner2 = (Spinner) findViewById(R.id.spinner2);
    btnSubmit = (Button) findViewById(R.id.btnSubmit);
    dateView = (EditText) findViewById(R.id.dateView);
    //picker= (DatePicker)findViewById(R.id.date);

    btnSubmit.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {

            Toast.makeText(CollectionActivity.this,
                    "You have selected : " +
                            "\nLocation : " + String.valueOf(spinner1.getSelectedItem()) +
                            "\nCategory : " + String.valueOf(spinner2.getSelectedItem()) +
                            //"\nSelected Date: "+ picker.getDayOfMonth()+"/"+ (picker.getMonth() + 1)+"/"+picker.getYear(),
                            "\nDate : " + dateView.getText().toString(),
                    Toast.LENGTH_LONG).show();
            addNotifications();
        }

    });
}

private void addNotifications() {
    spinner1 = (Spinner) findViewById(R.id.spinner1);
    spinner2 = (Spinner) findViewById(R.id.spinner2);
    dateView = (EditText) findViewById(R.id.dateView);

    NotificationCompat.Builder builder =
            new NotificationCompat.Builder(this, CHANNEL_ID)
                    .setSmallIcon(R.drawable.ic_sms_black_24dp)
                    .setContentTitle("New Collection Added")
                    .setContentText(
                            "You have selected : " +
                                    "\nLocation : " + String.valueOf(spinner1.getSelectedItem()) +
                                    "\nCategory : " + String.valueOf(spinner2.getSelectedItem()) +
                                    "\nDate : " + dateView.getText().toString()
                    )
                    .setPriority(NotificationCompat.PRIORITY_DEFAULT);

    NotificationManagerCompat mNotificationMgr = NotificationManagerCompat.from(this);
    mNotificationMgr.notify(1, builder.build());
}
}