java - 如何清除散列映射<string,string> 值？

Question

我想在将值分配给来自 db 的 HashMap 之前清除所有 HashMap 值。例如，hashmap<string,string> demo是我的 hashmap，因为我从 db 添加了以下值。hashmap 的结果响应是 {A=1, B=2, C=3, D=4, E=5} .After view loads 我想在上面的 hashmap {A=test, B=demo, C=sample, D=empty, E=null} 中插入这个。为此，我如何在插入第二组值之前清除第一个值。我也尝试使用 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。提出一些解决方案来解决问题。

这是从数据库中检索数据的代码。

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 HashMap 。

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() 对我来说总是很好用。

静止

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

将自动覆盖键“A”的现有值。

如果你还想清除值，

你可以使用

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

这将在 hMap 中初始化一个新的 hashmap。