Question

这是用于学生安置活动的Java代码，每当我单击按钮而不填充字段时，在单击按钮之前它都没有显示任何语法错误或其他错误，或者在崩溃时都会填充字段，这是一个尝试学习android的初学者请提供帮助。

  public class company_profile extends AppCompatActivity {
        EditText title, address;
        SQLiteDatabase sqlitedb;
        Button Add;
        String companytitle, companyaddress, SQLiteDataBaseQueryHolder;
        Boolean EditTextEmpty;

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_company_profile);
            title = (EditText) findViewById(R.id.com_name);
            address = (EditText) findViewById(R.id.com_address);
           Add = (Button) findViewById(R.id.addComapny);

            Add.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {

                    SQLiteDataBaseBuild();

                    SQLiteTableBuild();

                    CheckEditTextStatus();

                    InsertDataIntoSQLiteDatabase();


                }
            });
        }

        public void SQLiteDataBaseBuild() {

            sqlitedb = openOrCreateDatabase("PlacementDb", Context.MODE_PRIVATE, null);
        }

        public void SQLiteTableBuild() {

            sqlitedb.execSQL("CREATE TABLE IF NOT EXISTS Company(id INTEGER PRIMARY KEY AUTOINCREMENT, ctitle VARCHAR(255), caddress VARCHAR(255));");

        }

        public void CheckEditTextStatus() {

            companytitle = title.getText().toString();
            companyaddress = address.getText().toString();

            if (TextUtils.isEmpty(companytitle) || TextUtils.isEmpty(companyaddress)) {

                EditTextEmpty = false;

            } else {

                EditTextEmpty = true;
            }
        }
        public void InsertDataIntoSQLiteDatabase(){

            if(EditTextEmpty == true)
            {

                sqlitedb.execSQL("INSERT INTO Company (ctitle,caddress) VALUES('"+companytitle+"','"+companyaddress+"');");

                sqlitedb.execSQL(SQLiteDataBaseQueryHolder);

                Toast.makeText(this,"Data Inserted Successfully", Toast.LENGTH_LONG).show();

            }
            else {

                Toast.makeText(this,"Please Fill All The Required Fields.", Toast.LENGTH_LONG).show();

            }

        }


    }

这是一个错误日志：

FATAL EXCEPTION: main
    Process: com.example.studentplacement
    java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.String.trim()' on a null object reference

Answer 1

可能是如何实现SQLite数据库的。尝试像这样使用它

DatabaseHelper.java

package YOUR_PACKAGE_NAME;

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

import java.util.ArrayList;
import java.util.HashMap;

public class DatabaseHelper extends SQLiteOpenHelper {
    ArrayList<HashMap<String, String>> YourList = new ArrayList<>();
    public static final String DATABASE_NAME = "MyDBName.db";
    public static final String CONTACTS_TABLE_NAME = "Your_TableName";
    public static final String CONTACTS_COLUMN1 = "column1";
    public static final String CONTACTS_COLUMN2 = "column2";
    public static final String CONTACTS_COLUMN3 = "column3";
    private HashMap hp;

    public DatabaseHelper(Context context){
        super(context ,DATABASE_NAME,null,1);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        // TODO Auto-generated method stub
        db.execSQL(
                "create table Your_TableName(column1 VARCHAR,column2 VARCHAR,column3 VARCHAR)"
        );
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // TODO Auto-generated method stub
        db.execSQL("DROP TABLE IF EXISTS Your_TableName");
        onCreate(db);
    }
//Insert Data to SQLite database
    public boolean insertData(String column1, String column2  ,String column3) {
        SQLiteDatabase db = this.getWritableDatabase();

            ContentValues contentValues = new ContentValues();
            contentValues.put("column1", column1);
            contentValues.put("column2", column2);
            contentValues.put("column2", column3);
            db.insert("Your_TableName", null, contentValues);


        return true;
    }
//delete all data in SQLite database
    public Cursor deleteData() {
        SQLiteDatabase db = this.getReadableDatabase();
        db.execSQL("delete FROM Your_TableName");
        return null;
    }
//counting the number of items in your database
    public int numberOfRows(){
        SQLiteDatabase db = this.getReadableDatabase();
        int numRows = (int) DatabaseUtils.queryNumEntries(db, CONTACTS_TABLE_NAME);
        return numRows;
    }
//Get all item in your database
    public ArrayList<HashMap<String, String>> getAllDatabaseData() {
        //hp = new HashMap();
        SQLiteDatabase db = this.getReadableDatabase();
        Cursor res =  db.rawQuery( "select * from Your_TableName", null );
        res.moveToFirst();

        while(res.isAfterLast() == false){
            HashMap<String, String> array_list = new HashMap<>();
            array_list.put("column1",res.getString(res.getColumnIndex(CONTACTS_COLUMN1)));
            array_list.put("column2",res.getString(res.getColumnIndex(CONTACTS_COLUMN2)));
            array_list.put("column3",res.getString(res.getColumnIndex(CONTACTS_COLUMN3)));
            YourList.add(array_list);
            res.moveToNext();
        }

        return YourList;
    }
}

，然后在您的“活动”中实施此操作。

public class company_profile extends AppCompatActivity {
        EditText title, address;
        SQLiteDatabase sqlitedb;
        Button Add;
        String companytitle, companyaddress;
        Boolean EditTextEmpty;

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_company_profile);
            title = (EditText) findViewById(R.id.com_name);
            address = (EditText) findViewById(R.id.com_address);
           Add = (Button) findViewById(R.id.addComapny);

            Add.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {

                    column1= title.getText().toString(); 
                    column2= address.getText().toString();

                    new DatabaseHelper(this).insertData(column1, column2, column3); //calling the method [inserData] in DatabaseHelper Class

                }
            });
        }
}

每当我尝试单击Android按钮时，应用程序崩溃

1 个答案: