Question

我正在制作一个将食品杂货添加到列表的应用程序，我有一个包含所有属性的Product类，因此我试图根据其ID从数据库表中获取产品并返回它，但它保持告诉我，只要我调用函数，我的应用就会停止运行，这是我的代码

public Product GetProductById(String id)
{
    SQLiteDatabase db=this.ourHelper.getReadableDatabase();
    Product p=null;
    Cursor cr;
    cr = db.query(PRODUCTS_TABLE, null, PRODUCT_ID+"="+id, null, null, null, null);
    cr.moveToFirst();
    if(!cr.isAfterLast()) {
        p=new Product();
        p.setId(cr.getInt(0));
        p.setName(cr.getString(1));
        p.setQuantity(cr.getInt(2));
        p.setPrice(cr.getDouble(3));
        p.setAllergens(cr.getString(4));
        p.setPurchased(cr.getInt(5));
        p.setImage(cr.getBlob(6));
        p.setPriority(cr.getInt(7));
    }
    return p;
}

此代码不起作用，我不知道错误在哪里。这是我的数据库列（名称和类型...）：

public static final String DB_NAME="db_groceries";
public static final int DB_VER=1;
public static final String PRODUCT_NAME = "name";
public static final String PRODUCT_QUANTITY = "quantity";
public static final String PRODUCT_PRICE = "price";
public static final String PRODUCT_ALLERGENS = "allergens";
public static final String PRODUCT_PRIORITY = "priority";
public static final String PRODUCT_ID = "_id";
public static final String PRODUCT_IMAGE = "image";
public static final String PRODUCTS_TABLE = "groceries";
public static final String PRODUCTS_PURCHASED = "purchased";
public static final String SCRIPT_CREATE_PRODUCTS_TABLE = "create table "
        + PRODUCTS_TABLE + "(" + PRODUCT_ID +
        " integer primary key autoincrement," +
        PRODUCT_NAME + " text not null," +
        PRODUCT_QUANTITY + " integer not null," +
        PRODUCT_PRICE + " real not null," +
        PRODUCT_ALLERGENS + " text not null, " +
        PRODUCTS_PURCHASED + " integer not null, " +
        PRODUCT_IMAGE + " blob," +
        PRODUCT_PRIORITY + " integer not null)";

你能弄清楚我的错误在哪里吗？

Answer 1

我相信您的问题是，在调用GetProductById方法之前，您尚未实例化 ourHelper 。

如果还没有，那么您将获得类似于：-

的空指针异常

11-01 19:54:35.925 1420-1420/axtest.axtest E/AndroidRuntime: FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to start activity ComponentInfo{axtest.axtest/axtest.axtest.MainActivity}: java.lang.NullPointerException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
        at android.app.ActivityThread.access$600(ActivityThread.java:130)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:4745)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
        at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
        at axtest.axtest.MainActivity.GetProductById(MainActivity.java:134)
        at axtest.axtest.MainActivity.onCreate(MainActivity.java:35)
        at android.app.Activity.performCreate(Activity.java:5008)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084) 
        at android.app.ActivityThread.access$600(ActivityThread.java:130) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195) 
        at android.os.Handler.dispatchMessage(Handler.java:99) 
        at android.os.Looper.loop(Looper.java:137) 
        at android.app.ActivityThread.main(ActivityThread.java:4745) 
        at java.lang.reflect.Method.invokeNative(Native Method) 
        at java.lang.reflect.Method.invoke(Method.java:511) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
        at dalvik.system.NativeStart.main(Native Method)

要解决此问题，您需要按照

的方式实例化ourHelper

**`ourHelper = new DatabaseHelper(this);`**

请注意Databasehelper可能有所不同（您尚未说出类名是什么），并且签名也可能有所不同（尽管签名通常如上所述，但它取决于Database helper中的代码）

否则，您的代码可以正常工作。但是，也可能是您更改了表的结构但未删除数据库。要排除这种情况，请删除该应用程序的数据，然后重新运行该应用程序。

如何根据ID

1 个答案: