如何清除 hash map<string,string> 值？

Question

我想在将值从 db 分配给哈希映射之前清除所有哈希映射值。例如，hashmap<string,string> demo 是我的哈希映射，因为我从 db 添加了以下值。哈希映射的结果响应是{A=1, B=2, C=3, D=4, E=5}。视图加载后，我想在上面的哈希映射{A=test, B=demo, C=sample, D=empty, E=null} 中插入它。为此，我如何在插入第二组值之前清除第一个值。而且我也尝试过使用demo.clear() 它不起作用.如何在添加第二个值之前清除第一个值列表。建议一些解决方案来解决问题。

这是从 db 中检索数据的代码。

public HashMap<String, String> getdata() {
    try {
        HashMap<String, String> map = new HashMap<String, String>();
        Log.e("getdata", "Started");
        String selectQuery = "SELECT * FROM tablename";
        SQLiteDatabase database = this.getWritableDatabase();
        Cursor cursor = database.rawQuery(selectQuery, null);
        if (cursor.moveToFirst()) {
            do {
                map.put(cursor.getString(cursor.getColumnIndex("colmn1")),
                        cursor.getString(cursor.getColumnIndex("colmn2")));
            } while (cursor.moveToNext());
        }
        cursor.close();
        return map;

    } catch (Exception e) {
        e.printStackTrace();
        Log.e("getdata", "Ended");
    }
    return null;

}

在这个演示中，只有我上面解释的响应。然后我正在这样做

if(demo.isEmpty())
        {
//code for correct condition
        }
else
    {
     demo.clear();
    }

并通过这种方式将值传递给哈希映射。

demo = dbhelper.getdata();

还有我的 Logcat 异常：

09-06 12:48:53.656: W/dalvikvm(5071): threadid=1: thread exiting with uncaught exception (group=0x40fb0258)
09-06 12:48:53.660: W/System.err(5071): java.lang.RuntimeException: Unable to start activity ComponentInfo{packagename/packagename.listdata}: java.lang.NullPointerException
09-06 12:48:53.660: W/System.err(5071):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2077)
09-06 12:48:53.660: W/System.err(5071):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2104)
09-06 12:48:53.660: W/System.err(5071):     at android.app.ActivityThread.access$600(ActivityThread.java:134)
09-06 12:48:53.660: W/System.err(5071):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1247)
09-06 12:48:53.660: W/System.err(5071):     at android.os.Handler.dispatchMessage(Handler.java:99)
09-06 12:48:53.660: W/System.err(5071):     at android.os.Looper.loop(Looper.java:154)
09-06 12:48:53.660: W/System.err(5071):     at android.app.ActivityThread.main(ActivityThread.java:4624)
09-06 12:48:53.661: W/System.err(5071):     at java.lang.reflect.Method.invokeNative(Native Method)
09-06 12:48:53.661: W/System.err(5071):     at java.lang.reflect.Method.invoke(Method.java:511)
09-06 12:48:53.661: W/System.err(5071):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:809)
09-06 12:48:53.661: W/System.err(5071):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:576)
09-06 12:48:53.661: W/System.err(5071):     at dalvik.system.NativeStart.main(Native Method)
09-06 12:48:53.661: W/System.err(5071): Caused by: java.lang.NullPointerException
09-06 12:48:53.661: W/System.err(5071):     at packagename.listdata.onCreate(listdata.java:231)
09-06 12:48:53.661: W/System.err(5071):     at android.app.Activity.performCreate(Activity.java:4479)
09-06 12:48:53.661: W/System.err(5071):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1050)
09-06 12:48:53.661: W/System.err(5071):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2041)

Answer 1

hMap.clear() 对我来说总是很好。

还是

hMap.put("A","test");

将自动覆盖键“A”的现有值。

如果您仍希望清除值，

你可以使用

hMap = new HashMap<String,String>();

这将在hMap 中初始化一个新的hashmap。