Android的onCreate中的NullPointerException

Question

因此，我為學生創建了一個非常簡單的班級組織者，並且在onCreate中為Add Class活動得到了nullPointerException。 我真的不知道為什么要得到這個。 任何幫助，將不勝感激！

這是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();
    }
}

這是第二個活動：

    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();
    }

}

這是堆棧跟蹤：

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：我添加了對saveClass按鈕的引用（這是一個愚蠢的錯誤，謝謝大家），但是我仍然在同一行收到相同的錯誤...

Answer 1

您尚未為saveClass按鈕分配任何內容，因此為null

saveClass.setOnClickListener(new OnClickListener() {

該語句導致NullPointerException 。

您應該為它分配以下內容：

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

希望這可以幫助。

Answer 2

您還需要獲取對saveClass按鈕的引用（ saveClass也可能需要cancelClass ），該引用應類似於：

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