Android的工作室：Android的運行時致命異常

Question

這裏是LOG：

E/AndroidRuntime: FATAL EXCEPTION: main 
         Process: com.example.chong.medicinereminder, PID: 3221 
         java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.chong.medicinereminder/com.example.chong.medicinereminder.AddMedicine}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setOnClickListener(android.view.View$OnClickListener)' on a null object reference 
          at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646) 
          at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) 
          at android.app.ActivityThread.-wrap12(ActivityThread.java) 
          at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) 
          at android.os.Handler.dispatchMessage(Handler.java:102) 
          at android.os.Looper.loop(Looper.java:154) 
          at android.app.ActivityThread.main(ActivityThread.java:6077) 
          at java.lang.reflect.Method.invoke(Native Method) 
          at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) 
          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755) 
         Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setOnClickListener(android.view.View$OnClickListener)' on a null object reference 
          at com.example.chong.medicinereminder.AddMedicine.onCreate(AddMedicine.java:36) 
          at android.app.Activity.performCreate(Activity.java:6664) 
          at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118) 
          at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2599) 
          at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)  
          at android.app.ActivityThread.-wrap12(ActivityThread.java)  
          at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)  
          at android.os.Handler.dispatchMessage(Handler.java:102)  
          at android.os.Looper.loop(Looper.java:154)  
          at android.app.ActivityThread.main(ActivityThread.java:6077)  
          at java.lang.reflect.Method.invoke(Native Method) 

我真的需要幫助解決這個問題，因爲我所有的語法似乎沒有錯誤可言，請幫助我做一個醫學提醒應用程序和應用程序崩潰每次我按下'AddMedicine'按鈕。下面是問題的接口的編碼：

public class AddMedicine extends AppCompatActivity { 
    EditText med_name, med_reason, med_dosage; 
    List<MedicineList> MedicineList = new ArrayList<MedicineList>(); 


    @Override 
    protected void onCreate(final Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_add_medicine); 

     med_name = (EditText) findViewById(R.id.text_med_name); 
     med_reason = (EditText) findViewById(R.id.text_med_reason); 
     med_dosage = (EditText) findViewById(R.id.text_med_dosage); 

     final Button button_save = (Button) findViewById(R.id.button_save); 
     button_save.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View view) { 
       saveMedicine(med_name.getText().toString(), med_reason.getText().toString(), med_dosage.getText().toString()); 
       Toast.makeText(getApplicationContext(), med_name.getText().toString() + " has been added to the list", Toast.LENGTH_SHORT).show(); 
      } 

     }); 

     med_name.addTextChangedListener(new TextWatcher() { 
      @Override 
      public void beforeTextChanged(CharSequence s, int start, int count, int after) { 

      } 

      @Override 
      public void onTextChanged(CharSequence s, int start, int before, int count) { 
       button_save.setEnabled(!med_name.getText().toString().trim().isEmpty()); 
      } 

      @Override 
      public void afterTextChanged(Editable s) { 

      } 
     }); 
    } 

    private void saveMedicine(String name, String reason, String dosage) { 
     MedicineList.add(new MedicineList(name, reason, dosage)); 
    } 

    private class MedicineListAdapter extends ArrayAdapter<MedicineList> { 


     public MedicineListAdapter() { 
      super(AddMedicine.this, R.layout.medicine_list, MedicineList); 
     } 

     @Override 
     public View getView(int position, View view, ViewGroup parent) { 
      if (view == null) 
       view = getLayoutInflater().inflate(R.layout.medicine_list, parent, false); 
      MedicineList currentMedicine = MedicineList.get(position); 

      TextView name = (TextView) view.findViewById(R.id.medicine_name); 
      name.setText(currentMedicine.getName()); 

      TextView reason = (TextView) view.findViewById(R.id.medicine_reason); 
      reason.setText(currentMedicine.getReason()); 

      TextView dosage = (TextView) view.findViewById(R.id.medicine_dosage); 
      dosage.setText(currentMedicine.getDosage()); 

      return view; 
     } 


    } 


    public boolean onCreateOptionsMenu(Menu menu) { 
     getMenuInflater().inflate(R.menu.menu_medicine_add, menu); 
     return true; 
    } 

} 

Answer 1

它看起來像的button_save ID的Button元素在R.layout.activity_add_medicine XML文件丟失。

因爲findViewById()返回給定id或null的元素，如果沒有找到具有此id的元素。