null pointer exception with ROOM database

问题

I am developing an android app using android studio my app has ROOM database when i store in the database a run time error appears and I can not solve it

this is my database code

@Database(entities = {Cart.class},version = 1,exportSchema = false)
public  abstract class CartDatabase extends RoomDatabase {

private static Context context;

public abstract CartDAO cartDAO();

private static CartDatabase instance;


public static CartDatabase getInstance() {
    if (instance == null) {
        instance = Room.databaseBuilder(context.getApplicationContext(), CartDatabase.class, "Cart")//if we want in memory builder  ithink we can add it here
                .allowMainThreadQueries().build();
    }
    return instance;


}


}

and this insert into database code

 addBTn.setOnClickListener( new View.OnClickListener() {
                            @Override
                            public void onClick(View view) {
                                Cart cart=new Cart();
                                cart.setName( (String) productNameTxt.getText() );
                                cart.setPrice(Integer.parseInt( (String ) productPriceTxt.getText()  ));
                                CartDatabase.getInstance( ).cartDAO().insertToCart( cart );

                                Toast.makeText(getActivity(),"Item added successfully",Toast.LENGTH_SHORT).show();



                            }
                        } );

and this is the error

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.queueskip, PID: 3544
    java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
        at com.example.queueskip.Database.Local.CartDatabase.getInstance(CartDatabase.java:25)
        at com.example.queueskip.ui.home.HomeFragment$2$1$1.onClick(HomeFragment.java:169)
        at android.view.View.performClick(View.java:5721)
        at android.widget.TextView.performClick(TextView.java:10936)
        at android.view.View$PerformClick.run(View.java:22620)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:148)
        at android.app.ActivityThread.main(ActivityThread.java:7406)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)

回答1:

Your Context is null, it would be better if you create a Context parameter in your getInstance() method instead of storing it in your CartDatabase class.

Try the following code

public static CartDatabase getInstance(Context context) {
    if (instance == null) {
        instance = Room.databaseBuilder(context, CartDatabase.class, "Cart")//if we want in memory builder  ithink we can add it here
                .allowMainThreadQueries().build();
    }
    return instance;
}

Then on your onClick()

CartDatabase.getInstance(getApplicationContext()).cartDAO().insertToCart(cart);

回答2:

Add a constructor and pass context to it from the activity:

In your activity:

CartDatabase cart = new CartDatabase(this);

and in the CartDatabase class :

   public CartDatabase(Context             context){
    this.context=context;
     }

or pass context to the class using getInstance from your activity:

CartDatabase.getInstanse(this).cartDAO().insertToCart( cart);

and in the CartDatabase class :

public static CartDatabase getInstance(Context context) ...

回答3:

You are getting this because you are not passing any context to the CartDatabase. You can simply add a context parameter to your getInstance method like that...

public static CartDatabase getInstance(Context context) {
if (instance == null) {
    instance = Room.databaseBuilder(context, CartDatabase.class, "Cart").allowMainThreadQueries().build();
}
return instance;

}

And then need to call from activity like that...

CartDatabase.getInstance(getApplicationContext()).cartDAO().insertToCart(cart);

来源：https://stackoverflow.com/questions/58286636/null-pointer-exception-with-room-database