不同的DateTimePicker問題

Question

在這個程序中，我希望在按鈕上顯示時間。但它會生成NullpointerException。我已經嘗試過Log。它顯示在日誌中。但是，開始在按鈕上顯示時立即顯示錯誤。請，任何人都可以告訴我這個程序有什麼錯誤。

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:tools="http://schemas.android.com/tools" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    android:orientation="vertical" 
    android:paddingLeft="@dimen/activity_horizontal_margin" 
    android:paddingRight="@dimen/activity_horizontal_margin" 
    android:paddingTop="@dimen/activity_vertical_margin" 
    android:paddingBottom="@dimen/activity_vertical_margin" 
    tools:context=".MainActivity"> 

    <Button android:id="@+id/button_date" 
     android:layout_width="fill_parent" 
     android:layout_height="wrap_content" /> 

    <Button android:id="@+id/button_time" 
     android:layout_width="fill_parent" 
     android:layout_height="wrap_content" /> 


</LinearLayout> 

public class MainActivity extends AppCompatActivity 
{ 
    //Declaration for class 
    Views views; 
    // Declartion for member vairables 
    int hours; 
    int minutes; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     Views views = new Views(); 

     // pick up the default date using Calender class 
     Calendar calendar = Calendar.getInstance(Locale.getDefault()); 

     hours = calendar.get(Calendar.HOUR); 
     minutes = calendar.get(Calendar.MINUTE); 
     setupTime(hours, minutes); 
    } 

    private void setupTime(int hours, int minutes){ 
     // Log.i("TAG", "------------ " + hours + " "); 
     // Log.i("TAG", "------------ " + minutes + " "); 
     views.button_time.setText(String.valueOf(hours)); 

    } 
    class Views{ 
     Button button_date; 
     Button button_time; 

     public Views(){ 
      Button button_date = (Button) findViewById(R.id.button_date); 
      Button button_time = (Button) findViewById(R.id.button_time); 
     } 
    } 
} 

Answer 1

在Views類構造函數中，您將視圖分配給其他變量，而不是在類中聲明的視圖。

試試這個：

public Views(){ 

     button_date = (Button) findViewById(R.id.button_date); 
     button_time = (Button) findViewById(R.id.button_time); 
} 

說明

您聲明2個變量在這裏：

class Views{ 
    Button button_date; 
    Button button_time; 

button_date和button_time，這些變量將長達大約爲視圖對象是活的

然後，在你的構造函數，你再次聲明他們：

public Views(){ 
     Button button_date = (Button) findViewById(R.id.button_date); 
     Button button_time = (Button) findViewById(R.id.button_time); 
    } 

即使這樣，它們具有相同的名稱，它們是不同的變量，該問題之前宣佈。這兩個變量只會在構造函數中生存，並且在構造函數完成其工作後，它們的值不會持續。

現在，試着猜測你在首先聲明的變量中有什麼值？如果你猜對了，那是正確的。當你試圖打電話時會發生什麼？

views.button_time.setText(...) 

您對null變量調用setText，因此會發生NullPointerException。

更新2個

同樣的問題也出現在這裏也：

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    Views views = new Views(); 

改成這樣：

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    views = new Views(); 

Answer 2

試試這個：

public Views() { 
    this.button_date = (Button) findViewById(R.id.button_date); 
    this.button_time = (Button) findViewById(R.id.button_time); 
} 

Answer 3

讓我們改變這個類 - 這部分代碼

class Views{ 
    Button button_date; 
    Button button_time; 

    public Views(){ 
     Button button_date = (Button) findViewById(R.id.button_date); 
     Button button_time = (Button) findViewById(R.id.button_time); 
    } 
} 

到

class Views{ 
    Button button_date; 
    Button button_time; 

    public Views(){ 
     button_date = (Button) findViewById(R.id.button_date); 
     button_time = (Button) findViewById(R.id.button_time); 
    } 
} 

Answer 4

試試這個

public class MainActivity extends AppCompatActivity 
{ 
//Declaration for class 
Views views; 
// Declartion for member vairables 
int hours; 
int minutes; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    views = new Views(this); 

    // pick up the default date using Calender class 
    Calendar calendar = Calendar.getInstance(Locale.getDefault()); 

    hours = calendar.get(Calendar.HOUR); 
    minutes = calendar.get(Calendar.MINUTE); 
    setupTime(hours, minutes); 
} 

    private void setupTime(int hours, int minutes){ 
    // Log.i("TAG", "------------ " + hours + " "); 
    // Log.i("TAG", "------------ " + minutes + " "); 
    views.button_time.setText(String.valueOf(hours)); 

} 

class Views{ 
    Button button_date; 
    Button button_time; 

    public Views(Activity activity){ 
     button_date = (Button)  activity.findViewById(R.id.button_date); 
     button_time = (Button)  activity.findViewById(R.id.button_time); 
    } 
} 

}