如何清除哈希映射<string，string>值？

Question

我想清除所有哈希映射值，然后将值分配给db的哈希映射。例如，hashmap<string,string> demo是我的哈希映射，因为我已经从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}。查看加载我想在上面的hashmap中插入这个{A=test, B=demo, C=sample, D=empty, E=null}。对于这个问题，我怎样才能在插入第二组值之前清除第一个值。并且我也尝试过使用demo.clear()它不工作。如何清除第一个列表值之前添加第二个。建议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; 

} 

在本演示只有到来为我解释above.And响应那么我这样做

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

并将值传递通过这种方式来哈希映射。

demo = dbhelper.getdata(); 

而且我logcat的例外：

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()总是为我工作得很好。

不过

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

将自动覆盖关键 “A” 现有的值。

如果你仍然想清除值，

您可以使用

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

将在hMap初始化一个新的HashMap中。