使用数据库对象时出现Nullpointerexception [英] Nullpointerexception when using database object

问题描述

我是Android新手，我遇到了一些问题。它看起来像是说有一个空对象传递给它，但我不确定。哪里错过了一步？任何帮助表示赞赏。



提前致谢。



Logcat：

I'm new to Android, and i've got some issues. It looks like it's saying that's having an null object passed to it, but I'm not sure. Where I missed a step? Any help is appreciated.

Thanks in advance.

Logcat:

06-11 20:17:16.544 32506-32506/com.androidtutorialpoint.employeemanagementsystem E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.androidtutorialpoint.employeemanagementsystem, PID: 32506
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.androidtutorialpoint.employeemanagementsystem/com.androidtutorialpoint.employeemanagementsystem.AddUpdateEmployee}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.Cursor android.database.sqlite.SQLiteDatabase.query(java.lang.String, java.lang.String[], java.lang.String, java.lang.String[], java.lang.String, java.lang.String, java.lang.String, java.lang.String)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2358)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2420)
at android.app.ActivityThread.access$900(ActivityThread.java:154)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1321)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5294)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:699)
at
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.Cursor android.database.sqlite.SQLiteDatabase.query(java.lang.String, java.lang.String[], java.lang.String, java.lang.String[], java.lang.String, java.lang.String, java.lang.String, java.lang.String)' on a null object reference
at com.androidtutorialpoint.employeemanagementsystem.DB.EmployeeOperations.getEmployee(EmployeeOperations.java:63)
at com.androidtutorialpoint.employeemanagementsystem.AddUpdateEmployee.initializeEmployee(AddUpdateEmployee.java:142)
at com.androidtutorialpoint.employeemanagementsystem.AddUpdateEmployee.onCreate(AddUpdateEmployee.java:68)
at android.app.Activity.performCreate(Activity.java:5990)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2311)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2420)
at android.app.ActivityThread.access$900(ActivityThread.java:154)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1321)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5294)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904)  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:699)

EmployeeOperations：

EmployeeOperations:

package com.androidtutorialpoint.employeemanagementsystem.DB;
 
public class EmployeeOperations {
    public static final String LOGTAG = "EMP_MNGMNT_SYS";
 
    SQLiteOpenHelper dbhandler;
    SQLiteDatabase database;
 
    private static final String[] allColumns = {
            EmployeeDBHandler.COLUMN_ID,
            EmployeeDBHandler.COLUMN_FIRST_NAME,
            EmployeeDBHandler.COLUMN_LAST_NAME,
            EmployeeDBHandler.COLUMN_GENDER,
            EmployeeDBHandler.COLUMN_HIRE_DATE,
            EmployeeDBHandler.COLUMN_DEPT
 
    };
 
    public EmployeeOperations(Context context){
        dbhandler = new EmployeeDBHandler(context);
    }
 
    public void open(){
        Log.i(LOGTAG,"Database Opened");
        database = dbhandler.getWritableDatabase();
 
 
    }
    public void close(){
        Log.i(LOGTAG, "Database Closed");
        dbhandler.close();
 
    }
    public Employee addEmployee(Employee Employee){
        ContentValues values  = new ContentValues();
        values.put(EmployeeDBHandler.COLUMN_FIRST_NAME,Employee.getFirstname());
        values.put(EmployeeDBHandler.COLUMN_LAST_NAME,Employee.getLastname());
        values.put(EmployeeDBHandler.COLUMN_GENDER, Employee.getGender());
        values.put(EmployeeDBHandler.COLUMN_HIRE_DATE, Employee.getHiredate());
        values.put(EmployeeDBHandler.COLUMN_DEPT, Employee.getDept());
        long insertid = database.insert(EmployeeDBHandler.TABLE_EMPLOYEES,null,values);
        Employee.setEmpId(insertid);
        return Employee;
 
    }
 
    // Getting single Employee
    public Employee getEmployee(long id) {
 
        Cursor cursor = database.query(EmployeeDBHandler.TABLE_EMPLOYEES,allColumns,EmployeeDBHandler.COLUMN_ID + "=?",new String[]{String.valueOf(id)},null,null, null, null);
        if (cursor != null)
            cursor.moveToFirst();
 
        Employee e = new Employee(Long.parseLong(cursor.getString(0)),cursor.getString(1),cursor.getString(2),cursor.getString(3),cursor.getString(4),cursor.getString(5));
        // return Employee
        return e;
    }
 
    public List<Employee> getAllEmployees() {
 
        Cursor cursor = database.query(EmployeeDBHandler.TABLE_EMPLOYEES,allColumns,null,null,null, null, null);
 
            List<Employee> employees = new ArrayList<>();
        if(cursor.getCount() > 0){
            while(cursor.moveToNext()){
                Employee employee = new Employee();
                employee.setEmpId(cursor.getLong(cursor.getColumnIndex(EmployeeDBHandler.COLUMN_ID)));
                employee.setFirstname(cursor.getString(cursor.getColumnIndex(EmployeeDBHandler.COLUMN_FIRST_NAME)));
                employee.setLastname(cursor.getString(cursor.getColumnIndex(EmployeeDBHandler.COLUMN_LAST_NAME)));
                employee.setGender(cursor.getString(cursor.getColumnIndex(EmployeeDBHandler.COLUMN_GENDER)));
                employee.setHiredate(cursor.getString(cursor.getColumnIndex(EmployeeDBHandler.COLUMN_HIRE_DATE)));
                employee.setDept(cursor.getString(cursor.getColumnIndex(EmployeeDBHandler.COLUMN_DEPT)));
                employees.add(employee);
            }
        }
        // return All Employees
        return employees;
    }
 
 
 
 
    // Updating Employee
    public int updateEmployee(Employee employee) {
 
        ContentValues values = new ContentValues();
        values.put(EmployeeDBHandler.COLUMN_FIRST_NAME, employee.getFirstname());
        values.put(EmployeeDBHandler.COLUMN_LAST_NAME, employee.getLastname());
        values.put(EmployeeDBHandler.COLUMN_GENDER, employee.getGender());
        values.put(EmployeeDBHandler.COLUMN_HIRE_DATE, employee.getHiredate());
        values.put(EmployeeDBHandler.COLUMN_DEPT, employee.getDept());
 
        // updating row
        return database.update(EmployeeDBHandler.TABLE_EMPLOYEES, values,
                EmployeeDBHandler.COLUMN_ID + "=?",new String[] { String.valueOf(employee.getEmpId())});
    }
 
    // Deleting Employee
    public void removeEmployee(Employee employee) {
 
        database.delete(EmployeeDBHandler.TABLE_EMPLOYEES, EmployeeDBHandler.COLUMN_ID + "=" + employee.getEmpId(), null);
    }
 
 
 
}

MainActivity：

MainActivity:

public class MainActivity extends AppCompatActivity{
 
    private Button addEmployeeButton;
    private Button editEmployeeButton;
    private Button deleteEmployeeButton;
    private Button viewAllEmployeeButton;
    private EmployeeOperations employeeOps;
    private static final String EXTRA_EMP_ID = "com.androidtutorialpoint.empId";
    private static final String EXTRA_ADD_UPDATE = "com.androidtutorialpoint.add_update";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        employeeOps = new EmployeeOperations(MainActivity.this);
        super.onCreate(savedInstanceState);
 
        setContentView(R.layout.activity_main);
        addEmployeeButton = (Button) findViewById(R.id.button_add_employee);
        editEmployeeButton = (Button) findViewById(R.id.button_edit_employee);
        deleteEmployeeButton = (Button) findViewById(R.id.button_delete_employee);
        viewAllEmployeeButton = (Button)findViewById(R.id.button_view_employees);
 
 
 
        addEmployeeButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent i = new Intent(MainActivity.this,AddUpdateEmployee.class);
                i.putExtra(EXTRA_ADD_UPDATE, "Add");
                startActivity(i);
            }
        });
        editEmployeeButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                getEmpIdAndUpdateEmp();
            }
        });
        deleteEmployeeButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                getEmpIdAndRemoveEmp();
            }
        });
        viewAllEmployeeButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent i = new Intent(MainActivity.this, ViewAllEmployees.class);
                startActivity(i);
            }
        });
 
    }
 
 
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.employee_menu, menu);
        return true;
    }
 
    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();
        if (id == R.id.menu_item_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
 
 
    public void getEmpIdAndUpdateEmp(){
 
        LayoutInflater li = LayoutInflater.from(this);
        View getEmpIdView = li.inflate(R.layout.dialog_get_emp_id, null);
 
        AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(this);
        // set dialog_get_emp_id.xml to alertdialog builder
        alertDialogBuilder.setView(getEmpIdView);
 
        final EditText userInput = (EditText) getEmpIdView.findViewById(R.id.editTextDialogUserInput);
 
        // set dialog message
        alertDialogBuilder
                .setCancelable(false)
                .setPositiveButton("OK",new DialogInterface.OnClickListener() {
                            public void onClick(DialogInterface dialog,int id) {
                                // get user input and set it to result
                                // edit text
                                Intent i = new Intent(MainActivity.this,AddUpdateEmployee.class);
                                i.putExtra(EXTRA_ADD_UPDATE, "Update");
                                i.putExtra(EXTRA_EMP_ID, Long.parseLong(userInput.getText().toString()));
                                startActivity(i);
                            }
                        }).create()
                .show();
 
    }
 
 
    public void getEmpIdAndRemoveEmp(){
 
        LayoutInflater li = LayoutInflater.from(this);
        View getEmpIdView = li.inflate(R.layout.dialog_get_emp_id, null);
 
        AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(this);
        // set dialog_get_emp_id.xml to alertdialog builder
        alertDialogBuilder.setView(getEmpIdView);
 
        final EditText userInput = (EditText) getEmpIdView.findViewById(R.id.editTextDialogUserInput);
 
        // set dialog message
        alertDialogBuilder
                .setCancelable(false)
                .setPositiveButton("OK",new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog,int id) {
                        // get user input and set it to result
                        // edit text
                        employeeOps = new EmployeeOperations(MainActivity.this);
                        employeeOps.removeEmployee(employeeOps.getEmployee(Long.parseLong(userInput.getText().toString())));
                        Toast t = Toast.makeText(MainActivity.this,"Employee removed successfully!",Toast.LENGTH_SHORT);
                        t.show();
                    }
                }).create()
                .show();
 
    }
 
 
 
    @Override
    protected void onResume() {
        super.onResume();
        employeeOps.open();
    }
 
    @Override
    protected void onPause() {
        super.onPause();
        employeeOps.close();
 
    }
}

我的尝试：



使用数据库对象时的NullPointerException

What I have tried:

NullPointerException when using database object

推荐答案

900（ActivityThread.java： 154 ）
在android.app.ActivityThread

900(ActivityThread.java:154) at android.app.ActivityThread

H.handleMessage（ActivityThread.java： 1321 ）
在android.os.Handler.dispatchMessage（Handler.java： 102 ）
在android.os.Looper.loop（ Looper.java: 135 ）
在android.app.ActivityThread.main（ActivityThread.java： 5294 ）
at java.lang.reflect.Method.invoke（Native Method）
at java.lang.reflect.Method.invoke（Method.java： 372 ）
com.android.internal.os.ZygoteInit

H.handleMessage(ActivityThread.java:1321) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5294) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit

MethodAndArgsCaller.run（ZygoteInit.java： 904 ）com.android.internal.os.Zy上的
goteInit.main（ZygoteInit.java： 699 ）
at
引起：java.lang.NullPointerException：尝试调用虚方法' android.database.Cursor android.database.sqlite.SQLiteDatabase.query（java.lang.String，java.lang.String [] ，null。对象引用$，java.lang.String，java.lang.String []，java.lang.String，java.lang.String，java.lang.String，java.lang.String）' b $ b at com.androidtutorialpoint.employeemanagementsystem.DB.EmployeeOperations.getEmployee（EmployeeOperations.java: 63 ）
at com.androidtutorialpoint.employeemanagementsystem.AddUpdateEmployee.initializeEmployee （AddUpdateEmployee.java: 142 ）
at com.androidtutorialpoint.employeemanagementsystem.AddUpdateEmployee.onCreate（AddUpdateEmployee.java： 68 ）
在android.app.Activity.performCreate（Activity.java： 5990 ）
在android.app.Instrumentation.callActivityOnCreate（Instrumentation.java： 1106 ）
在android.app.ActivityThread .performLaunchActivity（ActivityThread.java： 2311 ）
在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java： 2420 ）
在android.app.ActivityThread.access

MethodAndArgsCaller.run(ZygoteInit.java:904) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:699) at Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.Cursor android.database.sqlite.SQLiteDatabase.query(java.lang.String, java.lang.String[], java.lang.String, java.lang.String[], java.lang.String, java.lang.String, java.lang.String, java.lang.String)' on a null object reference at com.androidtutorialpoint.employeemanagementsystem.DB.EmployeeOperations.getEmployee(EmployeeOperations.java:63) at com.androidtutorialpoint.employeemanagementsystem.AddUpdateEmployee.initializeEmployee(AddUpdateEmployee.java:142) at com.androidtutorialpoint.employeemanagementsystem.AddUpdateEmployee.onCreate(AddUpdateEmployee.java:68) at android.app.Activity.performCreate(Activity.java:5990) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2311) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2420) at android.app.ActivityThread.access

这篇关于使用数据库对象时出现Nullpointerexception的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！