ROOM数据库的空指针异常 [英] null pointer exception with ROOM database

问题描述

我正在使用android studio开发一个android应用程序，当我在数据库中存储运行时错误并且无法解决时，我的应用程序具有ROOM数据库

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

这是我的数据库代码

@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;


}


}

并将其插入数据库代码

 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();



                            }
                        } );

这是错误

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)

推荐答案

您的 Context 为null，最好在中创建 Context 参数> getInstance()方法，而不是将其存储在您的 CartDatabase 类中.

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.

尝试以下代码

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;
}

然后在您的 onClick()

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

这篇关于ROOM数据库的空指针异常的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！