Question

我有2个活动类，“Inset_plate_number”类和“CarSelection”类。我试图在“Insert_plate_number”类的文本框中写一些内容然后将我写入的内容放入ArrayList然后存储在sharedpreferences中。接下来，我想检索存储在sharedpreferences中的东西，并在“CarSelection”类的微调器中显示它们。

但是每次运行它都会给我NullPointerException错误。 arraylist是否有问题，存储共享偏好，加载共享首选项或在微调器上显示？

Insert_plate_number类，

public class Insert_plate_number extends ActionBarActivity {

    static ArrayList<String> list;
    static SharedPreferences sharedpreferences;
    static String combo;
    public static final String MyPREFERENCES = "MyPREFERENCES";
    static int size;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.fragment_insert_plate_number);
        list = new ArrayList<String>();
        sharedpreferences = getSharedPreferences(MyPREFERENCES, Context.MODE_PRIVATE);
    }

    public void onClick_Save(View view) {
        Editor editor = sharedpreferences.edit();
        EditText plate_number = (EditText) findViewById(R.id.car_plate_number);
        EditText phone_number = (EditText) findViewById(R.id.phone_number);
        String phoneNo = phone_number.getText().toString();
        String plateNo = plate_number.getText().toString(); 
        combo = plateNo + "," + phoneNo;
        list.add(combo);
        size = list.size();
        editor.putInt("list_size", size);   
        for (int i = 0; i <size; i++) {
            editor.putString("list_"+i, list.get(i));
        }
        editor.commit();
    }
}

CarSelection类，

public class CarSelection extends ActionBarActivity {

    Spinner spinner;
    SharedPreferences sharedpreferences;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.fragment_car_selection);
        sharedpreferences = getSharedPreferences(Insert_plate_number.MyPREFERENCES, Context.MODE_PRIVATE);
        int arraysize = sharedpreferences.getInt("list_size", 0);
        for (int i = 0; i <arraysize; i++) {
            sharedpreferences.getString("list_"+i, null);
        }       
        spinner = (Spinner) findViewById(R.id.spinner1);
        ArrayAdapter<String> adp = new ArrayAdapter<String>(CarSelection.this,android.R.layout.simple_spinner_item, Insert_plate_number.list);
        adp.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        spinner.setAdapter(adp);
    }
}

logcat的

06-07 12:52:01.640: E/AndroidRuntime(979): FATAL EXCEPTION: main
06-07 12:52:01.640: E/AndroidRuntime(979): Process: com.example.myfirstapp, PID: 979
06-07 12:52:01.640: E/AndroidRuntime(979): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.myfirstapp/com.example.myfirstapp.CarSelection}: java.lang.NullPointerException
06-07 12:52:01.640: E/AndroidRuntime(979):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
06-07 12:52:01.640: E/AndroidRuntime(979):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
06-07 12:52:01.640: E/AndroidRuntime(979):  at android.app.ActivityThread.access$800(ActivityThread.java:135)
06-07 12:52:01.640: E/AndroidRuntime(979):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
06-07 12:52:01.640: E/AndroidRuntime(979):  at android.os.Handler.dispatchMessage(Handler.java:102)
06-07 12:52:01.640: E/AndroidRuntime(979):  at android.os.Looper.loop(Looper.java:136)
06-07 12:52:01.640: E/AndroidRuntime(979):  at android.app.ActivityThread.main(ActivityThread.java:5017)
06-07 12:52:01.640: E/AndroidRuntime(979):  at java.lang.reflect.Method.invokeNative(Native Method)
06-07 12:52:01.640: E/AndroidRuntime(979):  at java.lang.reflect.Method.invoke(Method.java:515)
06-07 12:52:01.640: E/AndroidRuntime(979):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
06-07 12:52:01.640: E/AndroidRuntime(979):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
06-07 12:52:01.640: E/AndroidRuntime(979):  at dalvik.system.NativeStart.main(Native Method)
06-07 12:52:01.640: E/AndroidRuntime(979): Caused by: java.lang.NullPointerException
06-07 12:52:01.640: E/AndroidRuntime(979):  at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:330)
06-07 12:52:01.640: E/AndroidRuntime(979):  at android.widget.AbsSpinner.setAdapter(AbsSpinner.java:114)
06-07 12:52:01.640: E/AndroidRuntime(979):  at android.widget.Spinner.setAdapter(Spinner.java:413)
06-07 12:52:01.640: E/AndroidRuntime(979):  at com.example.myfirstapp.CarSelection.onCreate(CarSelection.java:33)
06-07 12:52:01.640: E/AndroidRuntime(979):  at android.app.Activity.performCreate(Activity.java:5231)
06-07 12:52:01.640: E/AndroidRuntime(979):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
06-07 12:52:01.640: E/AndroidRuntime(979):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
06-07 12:52:01.640: E/AndroidRuntime(979):  ... 11 more

Answer 1

sharedpreferences.getString（＆＃34; list _＆＃34; + i，null）;更改null会导致错误..

Answer 2

这里的问题是您正在尝试访问另一个可能为null的类的ArrayList。

实现目标的最佳方法是在onCreate的CarSelection.class中执行此操作：

sharedpreferences = getSharedPreferences(Insert_plate_number.MyPREFERENCES, Context.MODE_PRIVATE);
// You need to create an Array list and fill it with string you have in SharedPreferences
ArrayList<String> list;       
list = new ArrayList<String>();
int arraysize = sharedpreferences.getInt("list_size", 0);
for (int i = 0; i <arraysize; i++) {        
    list.add(sharedpreferences.getString("list_"+i, ""));
}

Spinner spinner = (Spinner) findViewById(R.id.edit_text);
ArrayAdapter<String> adp = new ArrayAdapter<String>(this,android.R.layout.simple_spinner_item, list);
adp.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
spinner.setAdapter(adp);

从arraylist中显示旋转器中的数据

2 个答案: