尝试在Android Studio中添加到SQLite数据库时出错

Question

所以我在第87和97行周围的这段代码出现错误。这是代码

package username.myandroidproject;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import org.w3c.dom.Text;


public class MainActivity extends AppCompatActivity {

Button addButton;
Button button;
Button usefulLinks;
Button button2;
EditText BookText;
EditText authorView;
EditText categoryView;
MyDBhandler myDB;
String[] bookNames;
String[] bookAuthor;
String[] bookCategory;



@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    BookText = findViewById(R.id.BookText);
    authorView = findViewById(R.id.authorView);
    categoryView = findViewById(R.id.categoryView);
    addButton = findViewById(R.id.addButton);



    //begining of button stuff for navigation
    button = findViewById(R.id.button);
    button.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            openactivity2();

        }
    });

    usefulLinks = findViewById(R.id.usefulLinks);
    usefulLinks.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            openusefullinks();

        }
    });

    button2 = findViewById(R.id.button2);
    button2.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            openlistact();

        }
    });
    //end of buttons
}
public void openusefullinks() {
    Intent intent = new Intent(this, usefulWebsites.class);
    startActivity(intent);
}

public void openactivity2() {
    Intent intent = new Intent(this, Main2Activity.class);
    startActivity(intent);
}

public void openlistact() {
    Intent intent = new Intent(this, listact.class);
    startActivity(intent);
}

public void addButtonClicked(View v){
    String newEntry = BookText.getText().toString();
    String newEntry2 = authorView.getText().toString();
    String newEntry3 = categoryView.getText().toString();
    if(BookText.length() != 0) {
        AddData(newEntry, newEntry2, newEntry3);

        BookText.setText("");
    }
    else {
        Toast.makeText(MainActivity.this, "Please enter text",Toast.LENGTH_LONG).show();
    }
}

public void AddData(String newEntry, String newEntry2, String newEntry3){
    boolean insertData = myDB.addData(newEntry, newEntry2, newEntry3);
    if(insertData==true){
        Toast.makeText(MainActivity.this, "Its done boi",Toast.LENGTH_LONG).show();
    }
    else{
        Toast.makeText(MainActivity.this, "error",Toast.LENGTH_LONG).show();
    }

}

}

这是错误代码

  

由于：java.lang.NullPointerException：尝试调用虚拟   方法'布尔值   username.myandroidproject.MyDBhandler.addData（java.lang.String，   null对象引用上的java.lang.String，java.lang.String）           在username.myandroidproject.MainActivity.AddData（MainActivity.java:97）         username.myandroidproject.MainActivity.addButtonClicked（MainActivity.java:87）

    public boolean addData(String BOOKNAME, String AUTHOR, String CATEGORY) {
    SQLiteDatabase db = this.getWritableDatabase();
    ContentValues contentValues = new ContentValues();
    contentValues.put(COL2, BOOKNAME);
    contentValues.put(COL3, AUTHOR);
    contentValues.put(COL4, CATEGORY);