我创建了3个类（1. MainActivity，DBhelper和Student类）。我是新手，他们使用简单的代码，而且始终无法将DBhelper类与MainActivity连接起来。我认为一切正常，但是在单击“ buttonAdd”后，我得到一条声明，“我的应用已停止”。 在MainActivity中，我仅使用了DBhelper的“ loadStudents”和“ addStudent”方法，因为即使在那些基本级别上我也遇到了问题。
应用程序的构想->将新的学生添加到editText，将其删除等。我知道，更好的方法是使用scrollList，但我只想知道如何首先使用SQLite。
学生类->我在“ DBhelper类”和“ MainActivity”中使用它
public class Student {
private int studentID;
private String studentName;
public Student(int id, String studentName) { //konstruktor
this.studentID = id;
this.studentName = studentName;
}
public int getStudentID() {
return studentID;
}
public void setStudentID(int studentID) {
this.studentID = studentID;
}
public String getStudentName() {
return studentName;
}
public void setStudentName(String studentName) {
this.studentName = studentName;
}
}
DBhelper类
public class DBhelper extends SQLiteOpenHelper {
private static final int DATABASE_VERSION = 1;
private static final String DATABASE_NAME = "studentDB.db";
public static final String TABLE_NAME = "student";
public static final String COLUMN_ID = "studentID";
public static final String COLUMN_NAME = "studentName";
//initialize the database
public DBhelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL("CREATE TABLE " + TABLE_NAME + "("
+ COLUMN_ID + "INTEGER PRIMARYKEY,"
+ COLUMN_NAME + " TEXT " + ")");
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP TABLE IF EXISTS contacts");
onCreate(db);
}
public void addStudent(Student student) {
SQLiteDatabase db = this.getWritableDatabase();
ContentValues values = new ContentValues();
values.put(COLUMN_ID, student.getStudentID());
values.put(COLUMN_NAME, student.getStudentName());
db.insert(TABLE_NAME, null, values);
db.close();
}
public String loadStudents() {
String result = "";
String query = "Select*FROM " + TABLE_NAME;
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(query, null);
while (cursor.moveToNext()) {
int result_0 = cursor.getInt(0);
String result_1 = cursor.getString(1);
result += String.valueOf(result_0) + " " + result_1 +
System.getProperty("line.separator");
}
cursor.close();
db.close();
return result;
}
public Student findStudent(int ID, String studentName) {
String query = "Select * FROM " + TABLE_NAME + "WHERE" + COLUMN_NAME + "
= " + "'" + studentName + "'";
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(query, null);
Student student = new Student(ID, studentName);
if (cursor.moveToFirst()) {
cursor.moveToFirst();
student.setStudentID(Integer.parseInt(cursor.getString(0)));
student.setStudentName(cursor.getString(1));
cursor.close();
} else {
student = null;
}
db.close();
return student;
}
public boolean deleteStudent(int ID, String studentName) {
boolean result = false;
String query = "Select*FROM" + TABLE_NAME + "WHERE" + COLUMN_ID + "= '"
+ String.valueOf(ID) + "'";
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(query, null);
Student student = new Student(ID, studentName);
if (cursor.moveToFirst()) {
student.setStudentID(Integer.parseInt(cursor.getString(0)));
db.delete(TABLE_NAME, COLUMN_ID + "=?",
new String[] {
String.valueOf(student.getStudentID())
});
cursor.close();
result = true;
}
db.close();
return result;
}
public boolean updateStudent(int ID, String name) {
SQLiteDatabase db = this.getWritableDatabase();
ContentValues args = new ContentValues();
args.put(COLUMN_ID, ID);
args.put(COLUMN_NAME, name);
return db.update(TABLE_NAME, args, COLUMN_ID + "=" + ID, null) > 0;
}
}
MainActivity
public class MainActivity extends AppCompatActivity {
Button buttonLoad;
Button buttonAdd;
Button buttonFind;
Button buttonDelete;
Button buttonUpdate;
TextView textViewStudents;
EditText editTextID;
EditText editTextName;
DBhelper myDB;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
buttonLoad = (Button) findViewById(R.id.buttonLoad);
buttonAdd = (Button) findViewById(R.id.buttonAdd);
buttonFind = (Button) findViewById(R.id.buttonFind);
buttonDelete = (Button) findViewById(R.id.buttonDelete);
buttonUpdate = (Button) findViewById(R.id.buttonUpdate);
textViewStudents = (TextView) findViewById(R.id.textViewStudents);
editTextID = (EditText) findViewById(R.id.editTextID);
editTextName = (EditText) findViewById(R.id.editTextName);
myDB = new DBhelper(this);
//load
buttonLoad.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
textViewStudents.setText(myDB.loadStudents());
editTextID.setText("");
editTextName.setText("");
}
});
buttonAdd.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
int id = Integer.parseInt(editTextID.getText().toString());
String name = editTextName.getText().toString();
Student student = new Student(id, name);
myDB.addStudent(student);
editTextID.setText("");
editTextName.setText("");
}
});
}
// Maybe it will be better to use the methos under the onCreate method but I
//don't know what should I write as a paramether instead of "view" when I
//will use this method in the onCreate
// public void addStudent() {
// DBhelper myDB = new DBhelper(this);
// int id = Integer.parseInt(editTextID.getText().toString());
// String name = editTextName.getText().toString();
// Student student = new Student(id, name);
// myDB.addStudent(student);
// editTextID.setText("");
// editTextName.setText("");
// }
}
答案 0 :(得分：0)
在 loadStudents 方法中，您有String query = "Select*FROM " + TABLE_NAME;
这将导致语法错误，因为SELECT与*和*与FROM之间没有空格。
这应该是 String query = "Select * FROM " + TABLE_NAME;
由于您省略了数字，因此需要检查代码中是否缺少空格。
列名和类型之间的空格，以及PRIMARY和KEY之间的空格，因此，studentID将不是 rowid 列的别名，因此将为null而不是自动生成的唯一ID。 **如果未解决此问题（不会导致错误），则会导致find / delete / update / Students方法无法正常工作。
在 WHERE 关键字之前，TABLE_NAME + "WHERE"还有一个空格。
在deleteStudent方法中复制了相同的初始错误Select*FROM。
注意 可能还有其他语法错误，以上内容仅通过直观的代码即可发现。 < / p>
我建议进行所有修改，然后删除该应用程序的数据或卸载该应用程序或增加分配给DATABASE_VERSION的值，然后重新运行该应用程序。
您还应该在“日志停止工作”之后检查日志，该日志将详细说明遇到的错误。
以下是经过测试的代码，还包含一些建议，例如仅使用学生的ID来查找学生。
DBhelper.java类（为方便起见，已使用DBhelper2类）：-
public class DBhelper2 extends SQLiteOpenHelper { //<<<<<<<<<< Change DBhelper2 to DBhelper
private static final int DATABASE_VERSION = 1;
private static final String DATABASE_NAME = "studentDB.db";
public static final String TABLE_NAME = "student";
public static final String COLUMN_ID = "studentID";
public static final String COLUMN_NAME = "studentName"; //initialize the database
public DBhelper2(Context context) { //<<<<<<<<<< Change DBhelpe2 to DBhelper
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL("CREATE TABLE " + TABLE_NAME + "("
+ COLUMN_ID + " INTEGER PRIMARY KEY,"
+ COLUMN_NAME + " TEXT " + ")");
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP TABLE IF EXISTS contacts");
onCreate(db);
}
public void addStudent(Student student) {
SQLiteDatabase db = this.getWritableDatabase();
ContentValues values = new ContentValues();
values.put(COLUMN_ID, student.getStudentID());
values.put(COLUMN_NAME, student.getStudentName());
db.insert(TABLE_NAME, null, values);
db.close();
}
//<<<<<<<<<< ADDED add student allowing ID to be auto generated
//!!!NOTE!!! ideally long should be used for ID and thus should return long not int
// won't be an issue with a small number of Students
public int addNewStudent(String studentName) {
SQLiteDatabase db = this.getWritableDatabase();
ContentValues values = new ContentValues();
values.put(COLUMN_NAME,studentName);
return (int) db.insert(TABLE_NAME,null,values);
}
public String loadStudents() {
String result = "";
String query = "Select * FROM " + TABLE_NAME; //<<<<<<<<<< ADDED SPACES
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(query, null);
while (cursor.moveToNext()) {
int result_0 = cursor.getInt(0);
String result_1 = cursor.getString(1);
result += String.valueOf(result_0) + " " + result_1 +
System.getProperty("line.separator");
}
cursor.close();
db.close();
return result;
}
//<<<<<<<<<< Get all Students as an ArrayList of Student objects
//<<<<<<<<<< Alternative to loadStudents
public ArrayList<Student> getAllStudents() {
ArrayList<Student> result = new ArrayList<>();
SQLiteDatabase db = this.getWritableDatabase();
Cursor csr = db.query(TABLE_NAME,null,null,null,null,null,null);
while (csr.moveToNext()) {
result.add(
new Student(
csr.getInt(csr.getColumnIndex(COLUMN_ID)),
csr.getString(csr.getColumnIndex(COLUMN_NAME)
)
)
);
}
csr.close();
db.close();
return result;
}
public Student findStudent(int ID, String studentName) {
String query = "Select * FROM " + TABLE_NAME + " WHERE " + COLUMN_NAME + //<<<<<<<<<< ADDED SPACES
" = " + "'" + studentName + "'"; //<<<<<<<<<< ADDED missing double quote
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(query, null);
Student student = new Student(ID, studentName);
if (cursor.moveToFirst()) {
cursor.moveToFirst(); //<<<<<<<<<< NOT NEEDED as already at first
student.setStudentID(Integer.parseInt(cursor.getString(0)));
student.setStudentName(cursor.getString(1));
cursor.close();
} else {
student = null;
}
db.close();
return student;
}
//<<<<<<<<<< alternative get student just by id
// uses the convenience query method rather than rawQuery
// recommended to use COLUMN NAMES not column offsets
public Student findStudent(int studentID) {
Student result = null;
String whereclause = COLUMN_ID + "=?";
String[] whereargs = new String[]{String.valueOf(studentID)};
SQLiteDatabase db = this.getWritableDatabase();
Cursor csr = db.query(TABLE_NAME,null,whereclause,whereargs,null,null,null);
if (csr.moveToFirst()) {
result = new Student(
csr.getInt(csr.getColumnIndex(COLUMN_ID)),
csr.getString(csr.getColumnIndex(COLUMN_NAME)
)
);
}
csr.close();
db.close();
return result;
}
// Deleting just by id (see findStudent above) could/should be implemented
public boolean deleteStudent(int ID, String studentName) {
boolean result = false;
String query = "Select * FROM " + TABLE_NAME + " WHERE " + COLUMN_ID + "= '" //<<<<<<<<<< ADDED SPACES
+ String.valueOf(ID) + "'";
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(query, null);
Student student = new Student(ID, studentName);
if (cursor.moveToFirst()) {
student.setStudentID(Integer.parseInt(cursor.getString(0)));
db.delete(TABLE_NAME, COLUMN_ID + "=?",
new String[] {
String.valueOf(student.getStudentID())
});
cursor.close();
result = true;
}
db.close();
return result;
}
// Updating just by id (see findStudent above) could/should be implemented
public boolean updateStudent(int ID, String name) {
SQLiteDatabase db = this.getWritableDatabase();
ContentValues args = new ContentValues();
args.put(COLUMN_ID, ID);
args.put(COLUMN_NAME, name);
return db.update(TABLE_NAME, args, COLUMN_ID + "=" + ID, null) > 0;
}
}
在活动的onCreate方法中，以下代码用于测试上述内容：-
mDBStudents = new DBhelper2(this);
//Delete all students for testing purposes only
mDBStudents.getWritableDatabase().delete(DBhelper2.TABLE_NAME,null,null);
// Load some data and test methods
mDBStudents.addStudent(new Student(1,"Fred"));
mDBStudents.addStudent(new Student(2,"Mary"));
Log.d("STUDENTS",mDBStudents.loadStudents());
Student a_student_found = mDBStudents.findStudent(1,"Fred");
Log.d("STUDENTFOUND", a_student_found.getStudentName());
mDBStudents.updateStudent(1,"Bert");
a_student_found = mDBStudents.findStudent(1,"Bert");
Log.d("STUDENtFOUND",a_student_found.getStudentName());
mDBStudents.deleteStudent(2,"Mary");
Log.d("STUDENTS",mDBStudents.loadStudents());
//Test alternative suggested methods
mDBStudents.addNewStudent("Harold");
mDBStudents.addNewStudent("Susan");
ArrayList<Student> all_students = mDBStudents.getAllStudents();
boolean after_first_row = false;
StringBuilder sb = new StringBuilder("Students are :-");
for (Student s: mDBStudents.getAllStudents()) {
sb.append("\n\t")
.append("Student ID is ")
.append(String.valueOf(s.getStudentID()))
.append(" Student name is ")
.append(s.getStudentName());
}
Log.d("STUDENTS",sb.toString());
Student another = mDBStudents.findStudent(3);
Log.d("STUDENT 3","Student with an ID of 3 is " + another.getStudentName());
以下是测试生成的日志的输出：-
2018-10-06 08:49:27.410 2382-2382/so52598847.so52598847 D/STUDENTS: 1 Fred
2 Mary
2018-10-06 08:49:27.411 2382-2382/so52598847.so52598847 D/STUDENTFOUND: Fred
2018-10-06 08:49:27.418 2382-2382/so52598847.so52598847 D/STUDENtFOUND: Bert
2018-10-06 08:49:27.426 2382-2382/so52598847.so52598847 D/STUDENTS: 1 Bert
2018-10-06 08:49:27.444 2382-2382/so52598847.so52598847 D/STUDENTS: Students are :-
Student ID is 1 Student name is Bert
Student ID is 2 Student name is Harold
Student ID is 3 Student name is Susan
2018-10-06 08:49:27.445 2382-2382/so52598847.so52598847 D/STUDENT 3: Student with an ID of 3 is Susan
结果符合预期。那是