java - 尝试在空对象引用上调用虚拟方法

Question

我有一个 asyncTask，在 DoInBackground 方法中我调用一个 DBHelper 类，该类应该返回 SQLiteDB 中的每一行。但是，当检索值的 DBHelper 类方法被触发时，我收到错误:

07-20 10:33:33.869: E/AndroidRuntime(22238): FATAL EXCEPTION: AsyncTask #5 07-20 10:33:33.869: E/AndroidRuntime(22238): Process: com.test.test, PID: 22238 07-20 10:33:33.869: E/AndroidRuntime(22238): java.lang.RuntimeException: An error occured while executing doInBackground() 07-20 10:33:33.869: E/AndroidRuntime(22238): at android.os.AsyncTask$3.done(AsyncTask.java:300) 07-20 10:33:33.869: E/AndroidRuntime(22238): at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355) 07-20 10:33:33.869: E/AndroidRuntime(22238): at java.util.concurrent.FutureTask.setException(FutureTask.java:222) 07-20 10:33:33.869: E/AndroidRuntime(22238): at java.util.concurrent.FutureTask.run(FutureTask.java:242) 07-20 10:33:33.869: E/AndroidRuntime(22238): at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231) 07-20 10:33:33.869: E/AndroidRuntime(22238): at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112) 07-20 10:33:33.869: E/AndroidRuntime(22238): at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587) 07-20 10:33:33.869: E/AndroidRuntime(22238): at java.lang.Thread.run(Thread.java:818) 07-20 10:33:33.869: E/AndroidRuntime(22238): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void test.ItemFolderP.setId(java.lang.String)' on a null object reference 07-20 10:33:33.869: E/AndroidRuntime(22238): at test.test.returnValues(testt.java:59) 07-20 10:33:33.869: E/AndroidRuntime(22238): at test.test$GetFProducts.doInBackground(test.java:107) 07-20 10:33:33.869: E/AndroidRuntime(22238): at test.test$GetFProducts.doInBackground(test.java:1) 07-20 10:33:33.869: E/AndroidRuntime(22238): at android.os.AsyncTask$2.call(AsyncTask.java:288) 07-20 10:33:33.869: E/AndroidRuntime(22238): at java.util.concurrent.FutureTask.run(FutureTask.java:237) 07-20 10:33:33.869: E/AndroidRuntime(22238): ... 4 more

这是DBHelper的代码:

public class DBHelper{  

    private ItemFolderP item; //here I declare my item

这是检索值的方法

public ItemFolderP returnValues(int index){

    Cursor mRow= database.rawQuery("select * from " + CM_TABLE + " LIMIT 0, 4", null);
    if (null!=mRow) {
        mRow.moveToNext();
        item.setId(Integer.toString(mRow.getInt(mRow.getColumnIndex("Id"))));
        item.setTitle(mRow.getString(mRow.getColumnIndex("test")));
        item.setCode(mRow.getString(mRow.getColumnIndex("code"))); 
        item.setPrice(mRow.getString(mRow.getColumnIndex("price")));
        item.setImageUrl(mRow.getString(mRow.getColumnIndex("url")));
    }

    return item;
}

这是项目类别:

public class ItemFolderP {
    public String id;
    public String title;
    public String codice;
    public String price;
    public String imageUrl;


public ItemFolderP(String id, String title, String codice, String price, String imageUrl) {
    super();
    this.title = title;
    this.id = id;
    this.price = price;
    this.imageUrl = imageUrl;

}

//titolo
public String getTitle() {
    return title;
}
public void setTitle(String title) {
    this.title = title;
}

//id
public String getId() {
    return id;
}
public void setId(String id) {
    this.id = id;
}

//codice
public String getCode() {
    return title;
}
public void setCode(String codice) {
    this.title = codice;
}

//image Url
public String getImageUrl() {
    return imageUrl;
}
public void setImageUrl(String imageUrl) {
    this.imageUrl = imageUrl;
}

//id
public String getPrice() {
    return price;
}
public void setPrice(String price) {
    this.price = price;
}

}

我觉得该错误可能是由于我没有将上下文传递给该方法而引起的，但我不明白如何修复它。有人可以帮助我吗？

Answer 1

您没有在代码中的任何位置为 item 赋予值，因此它始终为 null。您应该替换此 block :

item.setId(Integer.toString(mRow.getInt(mRow.getColumnIndex("Id"))));
item.setTitle(mRow.getString(mRow.getColumnIndex("test")));
item.setCode(mRow.getString(mRow.getColumnIndex("code"))); 
item.setPrice(mRow.getString(mRow.getColumnIndex("price")));
item.setImageUrl(mRow.getString(mRow.getColumnIndex("url")));

与

item = new ItemFolderP(
    Integer.toString(mRow.getInt(mRow.getColumnIndex("Id"))),
    mRow.getString(mRow.getColumnIndex("test")),
    mRow.getString(mRow.getColumnIndex("code")),
    mRow.getString(mRow.getColumnIndex("price")),
    mRow.getString(mRow.getColumnIndex("url")));

此外，我认为 item 不应该是私有(private)字段，因为它仅在 returnValues 方法中使用。一个局部变量就足够了。