检查字符串时出现空指针异常

【问题标题】：null pointer exception while checking string检查字符串时出现空指针异常
【发布时间】：2016-07-28 03:56:27
【问题描述】：

在我的应用程序中，我想通过检查条件来执行操作。我有两个字符串 odate 和 indate。这些字符串从数据库中获取数据。如果两个字符串中都没有数据，我想显示一个警报对话框。如果 indate 或 odate 有任何价值，我想转到下一个活动。当我尝试检查“如果 indate 和 odate 是否等于 null”执行功能时，会出现问题。但是当它实际上为 null 时，它会显示一个空指针异常，而不是进入 else 子句。 这是我的代码

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.menu_iem);


getdate();
invorder();
orderheader();

}
  private void orderheader() {
    // TODO Auto-generated method stub
      try{
      DatabaseHelper dbHelper = new DatabaseHelper(this.getApplicationContext());
        newDB = dbHelper.getWritableDatabase();
        Cursor c = newDB.rawQuery("select order_date from order_header where cust_code="+code, null);
        while (c.moveToNext()){

             odate = c.getString(c.getColumnIndex("order_date"));
                Log.v("orderheader", odate);

            }}
     catch (SQLiteException se ) {
        Log.e(getClass().getSimpleName(), "Could not create or Open the database");
       }        
}
private void invorder() {
    // TODO Auto-generated method stub
    try{
     DatabaseHelper dbHelper = new DatabaseHelper(this.getApplicationContext());
        newDB = dbHelper.getWritableDatabase();
        Cursor c = newDB.rawQuery("select inv_date from inv_header where cust_code="+code, null);
    while (c.moveToNext()){

             indate = c.getString(c.getColumnIndex("inv_date"));
                Log.v("invheader", indate);

            }
     catch (SQLiteException se ) {
        Log.e(getClass().getSimpleName(), "Could not create or Open the database");
       }    
        }
public void onBackPressed(){ 
    gettime();
    String m = "";
    if(odate.matches("") && indate.matches("")){

        DatabaseHelper databasecontroller = new     DatabaseHelper(Menu_iem.this);
         databas.put("rscode","0");
         databasecontroller.enterreason(databas);
        Intent inn= new Intent(getApplicationContext(),Home.class);
         startActivity(inn);
    }
    else{
    spinneralert();
    }
 DatabaseHelper db = new DatabaseHelper(getApplicationContext());  
        List<String> labels = db.getAllLabels();  

        // Creating adapter for spinner  
      final  ArrayAdapter<String> dataAdapter = new ArrayAdapter<String>(this,android.R.layout.simple_spinner_item, labels);  
        final Spinner sp = new Spinner(Menu_iem.this);
        sp.setLayoutParams(new  LinearLayout.LayoutParams(LayoutParams.WRAP_CONTENT,LayoutParams.WRAP_CONTENT));
        sp.setAdapter(dataAdapter);
        // attaching data adapter to spinner  
        String str = "PLEASE SELECT YOUR REASON ";
        sp.setAdapter(dataAdapter);  
        sp.setOnItemSelectedListener(this);  
        AlertDialog.Builder builder = new AlertDialog.Builder(Menu_iem.this);
        builder.setTitle(str);
        builder.setView(sp);
        builder.setPositiveButton("YES", new DialogInterface.OnClickListener() {

            @Override
            public void onClick(DialogInterface dialog, int which) {
                // TODO Auto-generated method stub
                 DatabaseHelper databasecontroller = new DatabaseHelper(Menu_iem.this);
                 databas.put("rscode",rc);
                 databasecontroller.enterreason(databas);
                Intent inn= new Intent(getApplicationContext(),Home.class);
                 startActivity(inn);
            }
        });
        builder.setNegativeButton("CANCEL",new DialogInterface.OnClickListener() {

            @Override
            public void onClick(DialogInterface dialog, int which) {
                // TODO Auto-generated method stub
                dialog.cancel();
            }
        });

        builder.create().show();
    }
@Override
public void onItemSelected(AdapterView<?> parent, View view, int position,
        long id) {
    // TODO Auto-generated method stub
     String label = parent.getItemAtPosition(position).toString();  
    try{
         DatabaseHelper dbHelper = new    DatabaseHelper(this.getApplicationContext());
    newDB = dbHelper.getWritableDatabase();
    Cursor c = newDB.rawQuery("select reason_code from reason_master where reason_name =" +label, null);


        while (c.moveToNext()){

         rc  = c.getString(c.getColumnIndex("reason_code"));

        }
}catch (SQLiteException se ) {
Log.e(getClass().getSimpleName(), "Could not create or Open the database");
   }

    }
@Override
public void onNothingSelected(AdapterView<?> parent) {
        }  
}
}

我尝试了其他问题，但没有任何帮助。请帮忙解决这个问题。

编辑 这是我的错误日志

04-07 16:52:23.584: E/AndroidRuntime(4304): FATAL EXCEPTION: main
04-07 16:52:23.584: E/AndroidRuntime(4304): Process: com.example.sellatease,     PID: 4304
04-07 16:52:23.584: E/AndroidRuntime(4304): java.lang.NullPointerException:   Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()'     on a null object reference
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:394)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.widget.ArrayAdapter.getView(ArrayAdapter.java:362)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.widget.Spinner.measureContentWidth(Spinner.java:759)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.widget.Spinner.onMeasure(Spinner.java:510)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.view.View.measure(View.java:17547)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:436)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.view.View.measure(View.java:17547)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:436)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.view.View.measure(View.java:17547)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.view.View.measure(View.java:17547)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:436)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.view.View.measure(View.java:17547)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:436)
 04-07 16:52:23.584: E/AndroidRuntime(4304):    at android.view.View.measure(View.java:17547)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:436)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2615)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.view.View.measure(View.java:17547)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2015)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1148)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1379)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1061)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5885)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.view.Choreographer.doCallbacks(Choreographer.java:580)
 04-07 16:52:23.584: E/AndroidRuntime(4304):    at android.view.Choreographer.doFrame(Choreographer.java:550)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.os.Handler.handleCallback(Handler.java:739)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.os.Handler.dispatchMessage(Handler.java:95)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.os.Looper.loop(Looper.java:135)
04-07 16:52:23.584: E/AndroidRuntime(4304):     at android.app.ActivityThread.main(ActivityThread.java:5254)
 04-07 16:52:23.584: E/AndroidRuntime(4304):    at java.lang.reflect.Method.invoke(Native Method)
 04-07 16:52:23.584: E/AndroidRuntime(4304):    at java.lang.reflect.Method.invoke(Method.java:372)
 04-07 16:52:23.584: E/AndroidRuntime(4304):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
 04-07 16:52:23.584: E/AndroidRuntime(4304):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

【问题讨论】：

  • indate.matches("") 不喜欢这样if(indate==null)

标签： android nullpointerexception


【解决方案1】： 
if(odate.matches("") && indate.matches("")){

在这里，您将字符串与空匹配。首先确保这些字符串不为空，然后检查它是否有任何值。

【讨论】：

  • 我试过这个，但我收到错误“java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.matches(java.lang.String)' on an null object参考“
  • @Bivin crashOveride 添加了我在回答中的意思的示例代码。
【解决方案2】：

做

   if(!TextUtils.isEmpty(odate) || !TextUtils.isEmpty(indate) {
      // go to activity
   } else {
      // show dialog
   }

您可以阅读有关 TextUtils 的更多信息http://developer.android.com/reference/android/text/TextUtils.html

【讨论】：

  • 请注意，TextUtils.isEmpty(String) 会为纯空格字符串返回 false
  • 是的。只有空格的字符串被认为是空的
  • 我也用过这个，但是没有变化
  • 检查编辑。已更改代码以适合您的场景
  • 我尝试了编辑后的代码。使用它时，即使我在这些变量中有值，我也会得到错误。我已经用我的错误日志编辑了我的问题。你能看看吗？
【解决方案3】：

把if(odate.matches("") &amp;&amp; indate.matches("")){改成

if((odate == null || odate.trim.length() < 2) &&  (indate == null || indate.trim.length() < 2)){

当然，您可以将数字 2 更改为您需要的任何长度。

更新 1：

从您的错误日志中，我认为错误是变量“标签”在行中为空

final  ArrayAdapter<String> dataAdapter = new ArrayAdapter<String>(this,android.R.layout.simple_spinner_item, labels);

检查dbHelper函数是否返回null给标签，并在使用标签之前添加一个检查

If(labels !=null){//create adapter here}

【讨论】：

  • 我仍然收到空指针异常。else 子句没有执行
  • 我做了这个改变。但我仍然有同样的错误。为什么会这样？
  • @Bivin 看起来至少 List 'labels' 的一项为空。
  • 尝试调试并检查列表标签的所有值。或者你可以使用 for(String item:labels){ Log.w("Item", "Value : " + String.valueOf(item);} 就在空检查下面
【解决方案4】： 
if(odate.matches("") && indate.matches("")){
}

将上面的代码替换为

if(odate!=null && indate!=null){
     if(odate.matches("") && indate.matches("")){
      }
}else{
   // you getting null object 
}

它将首先检查您的字符串对象是否为空，如果它不为空，然后它将尝试调用数学函数。

【讨论】：

  • 它仍然在同一页面中
  • 因为你的对象是空的
  • 是的。当它为空时，我想显示一个警告对话框。否则移动到下一个操作。
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