为什么我会收到死密码警告？

Question

Before entering the data into the sqlite database, I would want to check whether the item id exist. However, I am getting dead code from these lines. What is the reason for it?

Dead code:

if (checkQuery == null) {
    ContentValues values = new ContentValues();
    values.put(KEY_IID, item.getIID()); // mysql item id
    values.put(KEY_NAME, item.getName()); // item name
    values.put(KEY_PRICE, item.getPrice()); // item price
    values.put(KEY_CREATED_AT, item.getDate()); // Created At
    values.put(KEY_TYPE, item.getType()); // type

    // Inserting Row
    db.insert(TABLE_ITEM, null, values);
    db.close(); // Closing database connection
    } 

Database handler class:

public class DatabaseHandler extends SQLiteOpenHelper {

// All Static variables
// Database Version
private static final int DATABASE_VERSION = 2;

// Database Name
private static final String DATABASE_NAME = "itemManager";

// table name
public static final String TABLE_ITEM = "item";

// item Table Columns names
private static final String KEY_ID = "id";
private static final String KEY_IID = "iid";
private static final String KEY_NAME = "name";
private static final String KEY_PRICE = "price";
private static final String KEY_CREATED_AT = "created_at";
private static final String KEY_TYPE = "type";

public DatabaseHandler(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
}

// Creating Tables
@Override
public void onCreate(SQLiteDatabase db) {
    String CREATE_ITEM_TABLE = "CREATE TABLE " + TABLE_ITEM + "("
            + KEY_ID + " INTEGER PRIMARY KEY autoincrement,"
            + KEY_IID + " INTEGER"
            + KEY_NAME + " TEXT,"
            + KEY_PRICE + " TEXT,"
            + KEY_CREATED_AT + " TEXT,"
            + KEY_TYPE + " TEXT" + ")";
    db.execSQL(CREATE_ITEM_TABLE);
}

// Upgrading database
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    // Drop older table if existed
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_ITEM);

    // Create tables again
    onCreate(db);
}

/**
 * Storing item details in database
 * */
public void addItem(Items item) {
    String checkQuery = "SELECT iid FROM item WHERE iid = KEY_IID";
    SQLiteDatabase db = this.getWritableDatabase();

    if (checkQuery == null) {
    ContentValues values = new ContentValues();
    values.put(KEY_IID, item.getIID()); // mysql item id
    values.put(KEY_NAME, item.getName()); // item name
    values.put(KEY_PRICE, item.getPrice()); // item price
    values.put(KEY_CREATED_AT, item.getDate()); // Created At
    values.put(KEY_TYPE, item.getType()); // type

    // Inserting Row
    db.insert(TABLE_ITEM, null, values);
    db.close(); // Closing database connection
    } else {
        Log.d("notice", "item already in watchlist");
        db.close();
    }  
}

/**
 * Getting item data from database
 * */
public List<Items> getAllItems(){
    List<Items> itemList = new ArrayList<Items>();
    String selectQuery = "SELECT  * FROM " + TABLE_ITEM;

    SQLiteDatabase db = this.getWritableDatabase();
    Cursor cursor = db.rawQuery(selectQuery, null);
    // Move to first row
    if (cursor.moveToFirst()) {
        do {
            Items item = new Items();
            item.setID(Integer.parseInt(cursor.getString(0)));
            item.setName(cursor.getString(1));
            item.setPrice(cursor.getString(2));
            item.setDate(cursor.getString(3));
            item.setType(cursor.getString(3));
            // Adding item to list
            itemList.add(item);
        } while (cursor.moveToNext());
    }

    // return item list
    return itemList;
}

/**
 * return true if rows are there in table
 * */
public int getRowCount() {
    String countQuery = "SELECT  * FROM " + TABLE_ITEM;
    SQLiteDatabase db = this.getReadableDatabase();
    Cursor cursor = db.rawQuery(countQuery, null);
    int rowCount = cursor.getCount();
    db.close();
    cursor.close();

    // return row count
    return rowCount;
}

 // Updating item
public int updateItem(Items item) {
    SQLiteDatabase db = this.getWritableDatabase();

    ContentValues values = new ContentValues();
    values.put(KEY_NAME, item.getName());
    values.put(KEY_PRICE, item.getPrice());
    values.put(KEY_CREATED_AT, item.getDate());
    values.put(KEY_TYPE, item.getType());

    // updating row
    return db.update(TABLE_ITEM, values, KEY_ID + " = ?",
            new String[] { String.valueOf(item.getID()) });
}

// Deleting item
public void deleteItem(Items item) {
    SQLiteDatabase db = this.getWritableDatabase();
    db.delete(TABLE_ITEM, KEY_ID + " = ?",
            new String[] { String.valueOf(item.getID()) });
    db.close();
}
}

Answer 1

String checkQuery = "SELECT iid FROM item WHERE iid = KEY_IID";
SQLiteDatabase db = this.getWritableDatabase();

if (checkQuery == null) {

checkQuery can never be null, since you initialize it 2 lines above the if-check... therefore the code within the if-clause will never be called.

Answer 2

String checkQuery = "SELECT iid FROM item WHERE iid = KEY_IID";
SQLiteDatabase db = this.getWritableDatabase();

if (checkQuery == null) {

There is no way that checkQuery will be null here. You just initialized it to a literal and it cannot be changed since then. Hence the dead code warning.

Answer 3

You're setting checkQuery explicitly to be a non-null value, therefore checkQuery is certain to not be null at your if statement. As the if will always be false, all the code inside it will be considered dead because it cannot ever execute.

Answer 4

因为你在if(checkQuery == null)行之上初始化了checkQuery 2行...

Answer 5

You are checking whether the String checkQuery is null, but you are assigning a value to it two lines above. So the ode in the if block will never be executed.

if (checkQuery == null) {

Answer 6

String checkQuery = "SELECT iid FROM item WHERE iid = KEY_IID";
    SQLiteDatabase db = this.getWritableDatabase();

    if (checkQuery == null) {
    ContentValues values = new ContentValues();

Yes,it is a dead code, you are assigning somehting in checkQuery and then asking to check if is null, how can it be null when you have already assigned value to it in the line just two lines above it.