Java SQLite数据库图形错误

这是我的MainActivity.java文件。我正在尝试创建一个简单的应用程序来输入学生姓名，然后使用另外两个类DatabaseManager.java和Students.java将其存储在数据库中。问题是，每当我尝试运行应用程序并单击“添加”按钮时，应用程序就会冻结，下面的错误会显示在AppLogCat中

MainActivity.java

package com.example.android.dbms;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {
EditText inputText;
TextView outputText;
DatabaseManager manager;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    inputText = (EditText)findViewById(R.id.dataEntryText);
    outputText = (TextView)findViewById(R.id.textView);
    manager = new DatabaseManager(this,null,null,1);
    printDatabase();
}

public void onAddButtonClick(View view){
    String studentName = inputText.getText().toString();
    Students student = new Students(studentName);
    manager.addStudent(student);
    printDatabase();
}

public void onDeleteButtonClick(View view){
    String studentName = inputText.getText().toString();
    manager.deleteStudent(studentName);
    printDatabase();
}

public void printDatabase(){
    String dbString;
    dbString = manager.databaseToString();
    outputText.setText(dbString);
    inputText.setText("");
}


}

Students.java

package com.example.android.dbms;



public class Students {
private int _id;
private String _studentname;

public Students(){}

public Students(String _studentname) {
    this._studentname = _studentname;
}

public int get_id() {
    return _id;
}

public void set_id(int _id) {
    this._id = _id;
}

public String get_studentname() {
    return _studentname;
}

public void set_studentname(String _studentname) {
    this._studentname = _studentname;
}
}

DatabaseManager.java

package com.example.android.dbms;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;




public class DatabaseManager extends SQLiteOpenHelper {

private static final int DATABASE_VERSION = 1;
private static final String DATABASE_NAME = "students.db";
private static final String TABLE_STUDENTS = "students";
private static final String COLUMN_ID ="_id";
private static final String COLUMN_STUDENTNAME = "_studentname";

public DatabaseManager(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {
    super(context, DATABASE_NAME, factory, DATABASE_VERSION);
}

@Override
public void onCreate(SQLiteDatabase db) {
    String query = "CREATE TABLE "+ TABLE_STUDENTS + " ("+
            COLUMN_ID + " INTEGER PRIMARY KEY AUTOINCREMENT,"+
            COLUMN_STUDENTNAME + " TEXT );";
    db.execSQL(query);
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    db.execSQL("DROP TABLE IF EXISTS" + TABLE_STUDENTS +";");
    onCreate(db);
}

//add a new row to the database

public void addStudent(Students students){
    ContentValues values = new ContentValues();
    values.put(COLUMN_STUDENTNAME,students.get_studentname());
    SQLiteDatabase db = getWritableDatabase();
    db.insert(TABLE_STUDENTS,null,values);
    db.close();
}

//delete a row from the table

public void deleteStudent(String studentName){
    SQLiteDatabase db = getWritableDatabase();
    db.execSQL("DELETE FROM " + TABLE_STUDENTS + " WHERE " + COLUMN_STUDENTNAME + "=\"" + studentName + "\" ;");
}

//print out the database as string

public String databaseToString(){
    String dbString = "";
    SQLiteDatabase db = getWritableDatabase();
    String query = "SELECT * FROM "+ TABLE_STUDENTS + " WHERE 1;";

    //Cursor point to locate data in table
    Cursor c = db.rawQuery(query,null);
    c.moveToFirst();

    while (!c.isAfterLast()){
        if(c.getString(c.getColumnIndex("_studentname"))!=null){
            dbString += c.getString(c.getColumnIndex("_studentname"));
            dbString += "\n";
        }
    }
    db.close();
    return dbString;
}
}

显示的错误：

05-31 22:32:15.734 14795-14795/? I/art: Late-enabling -Xcheck:jni
05-31 22:32:15.765 14795-14801/? I/art: Debugger is no longer active
05-31 22:32:15.786 14795-14795/? W/System: ClassLoader referenced unknown 
path: /data/app/com.example.android.dbms-1/lib/arm
05-31 22:32:15.922 14795-14795/? W/art: Before Android 4.1, method android.graphics.PorterDuffColorFilter android.support.graphics.drawable.VectorDrawableCompat.updateTintFilter(android.graphics.PorterDuffColorFilter, android.content.res.ColorStateList, android.graphics.PorterDuff$Mode) would have incorrectly overridden the package-private method in android.graphics.drawable.Drawable
05-31 22:32:16.051 14795-14810/? D/OpenGLRenderer: Use EGL_SWAP_BEHAVIOR_PRESERVED: true
05-31 22:32:16.114 14795-14810/? I/Adreno-EGL: <qeglDrvAPI_eglInitialize:379>: EGL 1.4 QUALCOMM build:  (Ifd751822f5)
                                           OpenGL ES Shader Compiler Version: XE031.06.00.05
                                           Build Date: 01/26/16 Tue
                                           Local Branch: AU12_SBA
                                           Remote Branch: 
                                           Local Patches: 
                                           Reconstruct Branch: 
05-31 22:32:16.116 14795-14810/? I/OpenGLRenderer: Initialized EGL, version 1.4
05-31 22:32:28.290 14795-14805/com.example.android.dbms W/art: Suspending all threads took: 5.405ms
05-31 22:32:28.312 14795-14805/com.example.android.dbms I/art: Background partial concurrent mark sweep GC freed 96(3KB) AllocSpace objects, 90(8MB) LOS objects, 40% free, 8MB/14MB, paused 5.890ms total 32.806ms
05-31 22:32:32.319 14795-14805/com.example.android.dbms W/art: Suspending all threads took: 5.073ms
05-31 22:32:34.343 14795-14805/com.example.android.dbms I/art: Background sticky concurrent mark sweep GC freed 84(3KB) AllocSpace objects, 81(10MB) LOS objects, 38% free, 8MB/14MB, paused 5.083ms total 18.483ms
05-31 22:32:36.545 14795-14801/com.example.android.dbms W/art: Suspending all threads took: 11.236ms
05-31 22:32:37.036 14795-14801/com.example.android.dbms W/art: Suspending all threads took: 5.119ms
05-31 22:32:41.672 14795-14805/com.example.android.dbms W/art: Suspending all threads took: 7.867ms

确定问题在databaseToString方法中，将其替换为以下内容，因为它不会退出循环

 public String databaseToString(){
        String dbString = "";
        SQLiteDatabase db = this.getWritableDatabase();
        String query = "SELECT * FROM "+ TABLE_STUDENTS;//+ " WHERE "+COLUMN_ID+"=1  if you want data for particular id

        //Cursor point to locate data in table
        Cursor c = db.rawQuery(query,null);

       if( c.moveToFirst()){
        do{
            if(c.getString(c.getColumnIndex("_studentname"))!=null){
                dbString += c.getString(c.getColumnIndex("_studentname"));
                dbString += "\n";
            }
        }while (c.moveToNext());
       }
        db.close();
        return dbString;
    }

我还建议您将DatabaseManager构造函数替换为

所以在活动oncreate replace中

 manager = new DatabaseManager(this,null,null,1);

与

你也有问题，在删除学生替换为下面的问题是你正在增加价值，而不是

---

此链接为您提供了插入、删除、更新和选择的基本概念

这将帮助您遵循示例。此处未找到任何有用的内容。非常感谢您的帮助。还有一件事需要添加，那就是检查表是否为空集。如果表返回空集，应用程序将崩溃。我通过使用cursor.getCount==0并返回dbString并通知空数据库来设置约束，或者您可以使用cursor.moveToFirst添加约束；如答案中所编辑

 manager = new DatabaseManager(this,null,null,1);

 manager = new DatabaseManager(this);

public String databaseToString(){
        String dbString = "";
        SQLiteDatabase db = this.getWritableDatabase();
        String query = "SELECT * FROM "+ TABLE_STUDENTS;//+ " WHERE "+COLUMN_ID+"=1

        //Cursor point to locate data in table
        Cursor c = db.rawQuery(query,null);
        c.moveToFirst();

        do{
            if(c.getString(c.getColumnIndex("_studentname"))!=null){
                dbString += c.getString(c.getColumnIndex("_studentname"));
                dbString += "\n";
            }
        }while (c.moveToNext());

        db.close();
        return dbString;
    }