“这个类应该有一个默认构造函数”[Android]

Posted 2023-04-14

【中文标题】“这个类应该有一个默认构造函数”[Android]

【英文标题】："This class should have a default constructor" [Android]

【发布时间】：2014-11-21 15:27:30

【问题描述】：

每当我尝试在 android Studio 中生成签名的 APK 时，我都会收到此错误。

Error:(69) Error: This class should provide a default constructor (a public constructor with no arguments) (com.example.msdproject.DBManager) [Instantiatable]

谁能告诉我为什么会这样？

这是导致问题的类：

包 com.example.msdproject;

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

import java.sql.SQLException;


public class DBManager 

    public static final String COL_ROWID = "_id";
    public static final String COL_NAME = "name";
    public static final String COL_VENUE = "venue";
    public static final String COL_DATE = "date";
    public static final String COL_COMMENTS = "comments";

    private static final String DB_NAME = "Concerts";
    private static final String DB_TABLE = "Concert_Info";
    private static final int DB_VERSION = 1;

    private static final String DB_CREATE =
            "create table " + DB_TABLE +
    " (_id integer primary key autoincrement, " +
            "name text not null, " +
            "venue text not null, " +
            "comments text not null, " +
            "date text not null);";

    private final Context context;

    private DatabaseHelper DBHelper;
    private SQLiteDatabase db;

    public DBManager(Context ctx)
    
        super();
        this.context = ctx;
        DBHelper = new DatabaseHelper(context);
    

    public static class DatabaseHelper extends SQLiteOpenHelper
    
        DatabaseHelper(Context context)
        
            super(context, DB_NAME, null, DB_VERSION);
        

        @Override
        public void onCreate(SQLiteDatabase db)
        
            db.execSQL(DB_CREATE);
        

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion)
        
            //to do
        
    

    public DBManager open() throws SQLException
    
        db = DBHelper.getWritableDatabase();
        return this;
    

    public void close()
    
        DBHelper.close();
    

    public long insertConcert(String name, String venue, String date, String comments)
    
        ContentValues initialValues = new ContentValues();
        initialValues.put(COL_NAME, name);
        initialValues.put(COL_VENUE, venue);
        initialValues.put(COL_DATE, date);
        initialValues.put(COL_COMMENTS, comments);
        return db.insert(DB_TABLE, null, initialValues);
    

    public boolean deleteConcert(String name)
    
        return db.delete(DB_TABLE, "name='"+ name+"'", null) > 0;

    

    public Cursor getAllConcerts() 
        return db.query(DB_TABLE, new String[]
                        
                                COL_ROWID,
                                COL_NAME,
                                COL_VENUE,
                                COL_DATE,
                                COL_COMMENTS
                        ,
                        null,
                        null,
                        null,
                        null,
                        COL_NAME+ " ASC"
                );
    

    public Cursor getConcert(long ROW_ID) throws SQLException
    
        Cursor mCursor =
                db.query(DB_TABLE, new String[]
                        
                                COL_ROWID,
                                COL_NAME,
                                COL_VENUE,
                                COL_DATE,
                                COL_COMMENTS
                        ,
                        COL_ROWID + "=" + ROW_ID,
                        null,
                        null,
                        null,
                        null
                );
        if (mCursor != null)
        
            mCursor.moveToFirst();
        

        return mCursor;
    

    public boolean updateConcert(String name, String venue, String date, String comments)
    
        ContentValues updateValues = new ContentValues();
        updateValues.put(COL_NAME, name);
        updateValues.put(COL_VENUE, venue);
        updateValues.put(COL_DATE, date);
        updateValues.put(COL_COMMENTS, comments);
        return db.update(DB_TABLE, updateValues, "name='"+ name+"'", null) > 0;
    

    public void reset () throws SQLException 
        db.delete(DB_TABLE, null, null);
        db.close();
        this.DBHelper.onCreate(this.db);
    

【问题讨论】：

最适合我的解决方案this

【参考方案1】：

public DBManager(Context ctx)

    super(); // Remove this
    this.context = ctx;
    DBHelper = new DatabaseHelper(context);

删除 super(); 你没有扩展任何类。

同时删除final，所以你应该有private Context context;

【讨论】：

我假设这是作为一般性更正而不是修复，因为我仍然遇到同样的错误。不管怎样，谢谢！

是的，从上下文中删除最终属性。

【参考方案2】：

运行时需要一个空的构造函数来实例化类而不需要参数。只需添加一个空的构造函数public DBManager ()

【讨论】：

我添加了这个，Android Studio 在“private final Context context;”这一行给了我一个错误说变量“上下文”可能没有被初始化，建议之一是将它放在新创建的 DBManager 方法中并将其初始化为空。这是正确的吗？

删除最后。无论在什么条件下，最终变量都必须是初始化之王。你关心在哪里使用它而不是散布它是很好的。请阅读并保持良好的工作

【参考方案3】：

public final class UserProfile 

private UserProfile()



public static UserProfile getProfile()

    UserProfile userProfile = new UserProfile();
    return userProfile;

class Users implements BaseColumn

    public static final String TABLE_NAME = "UserInfo";
    public static final String COL_ID = "_ID";
    public static final String COL_USERNAME  = "userName ";
    public static final String COL_DOB = "dateOfBirth";
    public static final String COL_GENDER = "Gender";
    public static final String COL_PASSWORD = "Password";

    private int id;
    private String username;
    private String dob;
    private String gender;
    private String password;


    public int getId() 
        return id;
    

    public void setId(int id) 
        this.id = id;
    

    public String getUsername() 
        return username;
    

    public void setUsername(String username) 
        this.username = username;
    

    public String getDob() 
        return dob;
    

    public void setDob(String dob) 
        this.dob = dob;
    

    public String getGender() 
        return gender;
    

    public void setGender(String gender) 
        this.gender = gender;
    

    public String getPassword() 
        return password;
    

    public void setPassword(String password) 
        this.password = password;
    


public Users getUser()
    Users users = new Users();

    return users;

【讨论】：

虽然此代码可能会回答问题，但提供有关它如何和/或为什么解决问题的额外上下文将提高​​答案的长期价值。