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android - ROOM 数据库的空指针异常

问题描述

我正在使用 android studio 开发一个 android 应用程序我的应用程序有 ROOM 数据库当我存储在数据库中时出现运行时错误，我无法解决它

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;


}


}

这插入到数据库代码中

 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();



                            }
                        } );

这就是错误

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)

标签： androidandroid-room

解决方案

你是空的，如果你在你的方法中创建一个参数而不是将它存储在你的类中Context会更好。ContextgetInstance()CartDatabase

试试下面的代码

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;
}

然后在你的onClick()

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

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