Android中onCreate的NullPointerException

Question

所以我正在爲學生製作一個非常簡單的類組織者，並且我在AddCreate中爲Add類活動獲取了nullPointerException。我真的不知道我爲什麼要這樣做。任何幫助，將不勝感激！

下面是在MainActivity：

public class MainActivity extends Activity { 

    ListView classList; 
    Button addClass; 
    ArrayAdapter<Class> adapterClass; 
    ArrayList<Class> currClasses = new ArrayList<Class>(); 
    ClassesSingleton myClasses; 
    int REQUEST_CODE_ADD = 318; 
    int REQUEST_CODE_EDIT = 319; 

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

     classList = (ListView) findViewById(R.id.classList); 
     final Context context = this; 
     adapterClass = new ArrayAdapter<Class>(this, R.layout.single_list_item, R.id.label, currClasses); 

     classList.setAdapter(adapterClass); 

     addClass = (Button) this.findViewById(R.id.addClass); 

     classList.setOnItemClickListener(new AdapterView.OnItemClickListener() { 
      public void onItemClick(AdapterView <?> arg0, View view, int position, long id){ 
       Intent intent = new Intent(MainActivity.this, ShowClass.class); 
       startActivityForResult(intent, REQUEST_CODE_EDIT); 
      } 
     }); 

     addClass.setOnClickListener(new OnClickListener() { 

      public void onClick(View arg0){ 
       Intent intent = new Intent(MainActivity.this, AddClass.class); 
       startActivityForResult(intent, REQUEST_CODE_ADD); 
      } 
     }); 
    } 

    protected void onActivityResult(int requestCode, int resultCode, Intent data){ 
     super.onActivityResult(requestCode, resultCode, data); 

     if(resultCode == RESULT_OK && requestCode == REQUEST_CODE_ADD){ 
      Class newClass = new Class(); 
      if(data.hasExtra("className")){ 
       newClass.setName(data.getExtras().getString("name")); 
      } 
      if(data.hasExtra("number")){ 
       newClass.setNumber(data.getExtras().getInt("number")); 
      } 
      if(data.hasExtra("students")){ 
       ArrayList<Student> s = data.getExtras().getParcelableArrayList("students"); 
       newClass.setStudents(s); 
      } 

      myClasses = ClassesSingleton.getInstance(); 
      updateClassList(newClass); 
      myClasses.setClassArray(currClasses); 
     } 

     else if(resultCode == RESULT_OK && requestCode == REQUEST_CODE_EDIT){ 

     } 
    } 

    public void updateClassList(Class n){ 

     adapterClass = null; 
     currClasses.add(n); 
     adapterClass = new ArrayAdapter<Class>(this, R.layout.single_list_item, R.id.label, currClasses); 
     classList.setAdapter(adapterClass); 
     adapterClass.notifyDataSetChanged(); 
    } 
} 

這裏的第二活動：

public class AddClass extends Activity{ 

    Button addStudent, saveClass, cancelClass; 
    EditText className, classNumber; 
    ListView studentList; 
    ArrayAdapter<Student> adapterStudent; 
    ArrayList<Student> currStudents = new ArrayList<Student>(); 
    int REQUEST_CODE_ADDSTU = 317; 
    int REQUEST_CODE_EDSTU = 316; 

    public void onCreate(Bundle savedInstanceState){ 
     super.onCreate(savedInstanceState); 
     Intent intent = getIntent(); 
     setContentView(R.layout.new_class); 

     addStudent = (Button) findViewById(R.id.addStudent); 
       saveClass = (Button) findViewById(R.id.confirmClassAdd); 
     className = (EditText) findViewById(R.id.newClassName); 
     classNumber = (EditText) findViewById(R.id.newClassNumber); 
     studentList = (ListView) findViewById(R.id.newStudentList); 
     adapterStudent = new ArrayAdapter<Student>(this, R.layout.single_list_item, R.id.label, currStudents); 

     studentList.setAdapter(adapterStudent); 

     studentList.setOnItemClickListener(new AdapterView.OnItemClickListener() { 
      public void onItemClick(AdapterView <?> arg0, View view, int position, long id){ 
       Intent intent2 = new Intent(AddClass.this, EditStudent.class); 
       startActivityForResult(intent2, REQUEST_CODE_EDSTU); 
      } 
     }); 

     addStudent.setOnClickListener(new OnClickListener() { 

      public void onClick(View arg0){ 
       Intent intent2 = new Intent(AddClass.this, AddStudent.class); 
       startActivityForResult(intent2, REQUEST_CODE_ADDSTU); 
      } 
     }); 

     saveClass.setOnClickListener(new OnClickListener() { 
      public void onClick(View arg0){ 
       Intent data = new Intent(); 
       int numData = 0; 
       if((className.getText() != null) && (classNumber.getText() != null) && (currStudents.isEmpty() != true)){ 
        String nameData = className.getText().toString(); 
        String numStr = classNumber.getText().toString(); 
        ArrayList students = currStudents; 

        boolean intTrue = true; 
        try{ 
         numData = Integer.parseInt(numStr); 
        }catch(NumberFormatException e){ 
         Toast.makeText(AddClass.this, "Please be sure to enter a numberic value in the number field.", Toast.LENGTH_LONG).show(); 
         intTrue = false; 
        } 

        if(intTrue){ 
         data.putExtra("className", nameData); 
         data.putExtra("number", numData); 
         data.putParcelableArrayListExtra("students", students); 
         setResult(RESULT_OK, data); 
         finish(); 
        } 
       } 
       else{ 
        Toast.makeText(AddClass.this, "Don't leave your class name or number field blank!", Toast.LENGTH_LONG).show(); 
       } 
      } 
     }); 
    } 

    //onActivityResult callback 
    protected void onActivityResult(int requestCode, int resultCode, Intent data){ 
     super.onActivityResult(requestCode, resultCode, data); 

     if(resultCode == RESULT_OK && requestCode == REQUEST_CODE_ADDSTU){ 
      Student newStudent = new Student(); 
      if(data.hasExtra("name")){ 
       newStudent.setName(data.getExtras().getString("name")); 
      } 
      if(data.hasExtra("id")){ 
       newStudent.setId(data.getExtras().getInt("id")); 
      } 

      updateStudentList(newStudent); 
     } 
    } 

    public void updateStudentList(Student s){ 
     adapterStudent = null; 
     currStudents.add(s); 
     adapterStudent = new ArrayAdapter<Student>(this, R.layout.single_list_item, R.id.label, currStudents); 
     studentList.setAdapter(adapterStudent); 
     adapterStudent.notifyDataSetChanged(); 
    } 

} 

這裏的堆棧跟蹤：

FATAL EXCEPTION: main 
Process: com.example.congelassign2, PID: 1654 
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.congelassign2/com.example.congelassign2.AddClass}: java.lang.NullPointerException 
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2176) 
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2226) 
at android.app.ActivityThread.access$700(ActivityThread.java:135) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1397) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:137) 
at android.app.ActivityThread.main(ActivityThread.java:4998) 
at java.lang.reflect.Method.invokeNative(Native Method) 
at java.lang.reflect.Method.invoke(Method.java:515) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:593) 
at dalvik.system.NativeStart.main(Native Method) 
Caused by: java.lang.NullPointerException 
at com.example.congelassign2.AddClass.onCreate(AddClass.java:55) 
at android.app.Activity.performCreate(Activity.java:5243) 
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087) 
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2140) 
... 11 more 

編輯：我添加了參考saveClass按鈕（這是一個愚蠢的錯誤，感謝球員），但我仍然在同一條線上得到同樣的錯誤...

Answer 1

所以它是null

saveClass.setOnClickListener(new OnClickListener() { 

這種說法是導致NullPointerException你還沒有指定什麼saveClass按鈕。

你應該賦予它這樣的事情：

saveClass = (Button) findViewById(R.id.saveClass); 

希望這有助於。

Answer 2

你需要獲取到saveClass按鈕的引用以及（可能cancelClass在某些時候也是如此），這應該是這樣的：

addStudent = (Button) findViewById(R.id.addStudent); 
saveClass = (Button) findViewById(R.id.saveClass); 
cancelClass = (Button) findViewById(R.id.cancelClass);