Cannot use 'this' in static context

Question

My aim is to have a class containing all functions which perform database operations cleanly and neatly - which can also be called with a single line of code, eg; DbFunctions.AddContact("fName", "lName");

I have a DBAdapter class which I have read in a tutorial:

public class DBAdapter {
static final String KEY_ROWID = "_id";
static final String KEY_NAME = "name";
static final String KEY_EMAIL = "email";
static final String TAG = "DBAdapter";

static final String DATABASE_NAME = "MyDB";
static final String DATABASE_TABLE = "contacts";
static final int DATABASE_VERSION = 1;

static final String DATABASE_CREATE =
        "create table contacts (_id integer primary key autoincrement, "
        + "name text not null, email text not null);";

final Context context;
DatabaseHelper DBHelper;
SQLiteDatabase db;

public DBAdapter(Context ctx)
{
    this.context = ctx;
    DBHelper = new DatabaseHelper(context);
}

private static class DatabaseHelper extends SQLiteOpenHelper
{
    DatabaseHelper(Context context)
    {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db)
    {
        try
        {
            db.execSQL(DATABASE_CREATE);
        }
        catch (SQLException ex)
        {
            ex.printStackTrace();
        }
    }
}

//---opens the database---
public DBAdapter open() throws SQLException
{
    db = DBHelper.getWritableDatabase();
    return this;
}

//---closes the database---
public void close()
{
    DBHelper.close();       
}   

// some other database functions here... inserts, updates etc
}

And I have created my own class to handle all calls to the DBAdapter:

    public static class DatabasesActivity extends Activity
{       
    static DBAdapter db;

    // Called when activity is first created
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    public static long addContact(String name, String email)
    {
        if (db == null) {
            db = new DBAdapter(this); // <--- compiler error here
        }

        db.open();
        long id = db.insertContact("Joe Bloggs", "joe@bloggs.com");
        db.close();
        return id;
    }
}

In the addContact method, on the line: db = new DBAdapter(this);, I get the following error: Cannot use 'this' in a static context.

I am familiar with OOP concepts so I understand why I am getting this error - but being new to java itself, I am looking for alternate methods on what I'm trying to achieve. The DBAdapter class constructor takes in a context parameter, but I am unsure why as I have not written that class myself.

To Clarify: I understand why the error is occurring. The DBAdapter class constructor takes in a context parameter, and I don't know what to pass in as the context parameter when I'm using it statically. I want the class to be static as I don't want to have to instantiate it every time I want to use it.

I guess my real question would be "why does SQLiteOpenHelper require a context?"

Answer 1

You have defined a static method here :

  public static long addContact(String name, String email)

Static methods and class variables are tied to the Class and not to any specific instance of the Class. You cannot use the this keyword inside it as it refers to the current instance. One of the choice will be to declare the method as instance method removing the static keyword from the method declaration, if indeed the method logic depends on the state of the current instance.

I believe the problem in using this inside a static method will be that during runtime if your code calls the static method as ClassName.staticMethodName() , the runtime will have no idea how to resolve this in this context.

Answer 2

this refers to the current instance of that class.

Static methods and objects exist outside of any instance, which is why you are not allowed to use instance specific data, like the current instance, within them

Answer 3

You cannot cannot access fields or methods inside a Static method .

 Static methods do not point to any instance of the enclosing class.

So this keyword this refers to the instance of the class.

In a static context:
you won't have instance reference to current instance.

So error occurs.

Answer 4

You are accessing this in a static function which is not correct

make the function non-static by removing static modifier then it will work fine

Answer 5

You can't use "this" in static context because it implies you have an instance of the object, and statically you can't call to instances.

Why you need a static class?

Answer 6

By looking at this method

// Called when activity is first created
@Override
public void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
}

I don't think you should be using static class, static field and static method at all.

Just create an instance of Activity and work with it. If you need one Activity object through all your program - then use Singleton Pattern. Lazy and Eager instantiation examples are provided.

for example eager initialization will be:

public class DatabaseActivity extends Activity {
private static final DatabaseActivity INSTANCE= new DatabaseActivity();

private DatabaseActivity () {}

public static DatabaseActivity getInstance() {
    return INSTANCE;
}

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);     // calls nonstatic method onCreate from Activity class
    setContentView(R.layout.activity_main);
} 
}

Answer 7

You have made your class as static.

Only nested classes can be static. By doing so you can use the nested class without having an instance of the outer class.

class OuterClass{
    public static class StaticNestedClass{
    }

    public class InnerClass{
    }

    public InnerClass getAnInnerClass(){
        return new InnerClass();
    }

    //This method doesn't work
    public static InnerClass getAnInnerClassStatically(){
        return new InnerClass();
    }
}

class OtherClass{
    //Use of a static nested class:
    private OuterClass.StaticNestedClass staticNestedClass = new OuterClass.StaticNestedClass();

    //Doesn't work
    private OuterClass.InnerClass innerClass = new OuterClass.InnerClass();

    //Use of an inner class:
    private OuterClass outerclass= new OuterClass();
    private OuterClass.InnerClass innerClass2 = outerclass.getAnInnerClass();
    private OuterClass.InnerClass innerClass3 = outerclass.new InnerClass();
}

Answer 8

this is a pointer to an object.. in static context you don't have an object.

why is the DatabaseActivy a static class? So make a normal class and create an object of it