当引用NullPointerException时totalPriceEditText

Question

我是Java和Android的新手，我想制作一个Android程序，让用户输入标签价格，并且程序能够显示最终价格。(税后为8%)我需要使用Netbean。没有红线，也没有错误消息。但是每次我运行它时，都会在仿真程序中显示“不幸的是，税收价格计算器必须停止”。请帮帮我。我很感激大家的回答。谢谢！

TaxCalculator.java：

package com.finalproject.taxcalculator;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.text.TextWatcher;
import android.text.Editable;

public class TaxCalculator extends Activity{

private static final String TAG_PRICE = "TAG_PRICE";
private static final String TOTAL_PRICE = "TOTAL_PRICE";
private static final double TAX_RATE = 0.08;//Tax rate in Philadelphia

private double tagPrice;//Tag price entered by the user
private double totalPrice;//Total prices calculated by the program

private EditText tagPriceEditText;//accepts input for tag prices
private EditText totalPriceEditText;//displays total prices after tax


/** Called when the activity is first created. */

@Override
public void onCreate(Bundle savedInstanceState)
{
    // constants used when saving/restoring state
    super.onCreate(savedInstanceState);// call superclass's version
    setContentView(R.layout.main);// inflate the GUI

    // check if app just started or is being restored from memory
    if ( savedInstanceState == null ) // the app just started running
       {
            tagPrice = 0.0; // initialize the tag price to zero

       } // end if
    else // app is being restored from memory, not executed from scratch
       {
         // initialize the tag price to saved amount
            tagPrice = savedInstanceState.getDouble(TAG_PRICE); 

       } // end else

         // get references to tag and total price edit text
            tagPriceEditText = (EditText)findViewById(R.id.tagPriceEditText);
            tagPriceEditText = (EditText)findViewById(R.id.totalPriceEditText);

         // tagPriceEditTextWatcher handles tagPriceEditText's onTextChanged event
            tagPriceEditText.addTextChangedListener(tagPriceEditTextWatcher);
}// end method onCreate


private void updateStandard() 
{
    // calculate the total price after the tax
    totalPrice = tagPrice * (1 + TAX_RATE);
    // set totalPriceEditText's text to total price
    totalPriceEditText.setText(String.format("%.02f", totalPrice));
} // end method updateStandard

// save values of tagPriceEditText 
@Override
protected void onSaveInstanceState(Bundle outState)
{
   super.onSaveInstanceState(outState);

   outState.putDouble(TAG_PRICE, tagPrice);


} // end method onSaveInstanceState

// event-handling object that responds to tagPriceEditText's events    
private TextWatcher tagPriceEditTextWatcher = new TextWatcher() 
{
  // called when the user enters a number
  @Override
  public void onTextChanged(CharSequence s, int start, 
     int before, int count) 
  {         
     // convert billEditText's text to a double
     try
     {
        tagPrice = Double.parseDouble(s.toString());
     } // end try
     catch (NumberFormatException e)
     {
        tagPrice = 0.0; // default if an exception occurs
     } // end catch 

     // update the tagPriceEditText
     updateStandard();
  } // end method onTextChanged

  @Override
  public void afterTextChanged(Editable s) 
  {
  } // end method afterTextChanged

  @Override
  public void beforeTextChanged(CharSequence s, int start, int count,
     int after) 
  {
  } // end method beforeTextChanged

}; // end tagPriceEditTextWatcher

} // end class TaxCaculator

来自main.xml：

 <?xml version="1.0" encoding="utf-8"?>
 <TableLayout xmlns:android="http://schemas.android.com/apk/res/android"
 android:layout_width="match_parent" android:layout_height="match_parent" 
 android:background="#FFF" android:id="@+id/tableLayout" 
 android:stretchColumns="1,2,3" android:padding="5dp">


  <!-- tagPriceInputRow -->
       <TableRow android:layout_height="wrap_content" 
       android:layout_width="match_parent" android:id="@+id/tagPriceInputRow">

     <EditText android:layout_width="wrap_content" 
           android:id="@+id/tagPriceEditText" 
           android:inputType="number"
           android:text="@string/tagPrice"
           android:layout_height="wrap_content" android:layout_span="3" 
           android:layout_weight="1">
     </EditText>
     </TableRow>

     <!-- totalPriceOutputRow -->
     <TableRow android:layout_height="wrap_content" 
      android:layout_width="match_parent" android:id="@+id/totalPriceOutputRow">

     <EditText android:layout_width="wrap_content" 
     android:id="@+id/totalPriceEditText" 
     android:text="@string/totalPrice"
     android:layout_height="wrap_content" android:layout_span="3" 
     android:inputType="numberDecimal" android:layout_weight="1">
     </EditText>
  </TableRow>
  </TableLayout>

来自strings.xml：

<?xml version="1.0" encoding="utf-8"?>
<resources>
     <string name="app_name">Tax Price Calculator</string>
     <string name="tagPrice">Tag Price</string>
     <string name="totalPrice">Total Price</string>   
</resources>

AndroidManifest.xml：

  <?xml version="1.0" encoding="utf-8"?>
  <manifest xmlns:android="http://schemas.android.com/apk/res/android"
  package="com.finalproject.taxcalculator"
  android:versionCode="1"
  android:versionName="1.0">
  <application android:label="@string/app_name" android:icon="@drawable/ic_launcher">
    <activity android:name="MainActivity"
              android:label="@string/app_name">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    </application>
  </manifest>

Answer 1

似乎在onCreate()内部的变量赋值中有一个错误，在引用totalPriceEditText时会导致NullPointerException。

tagPriceEditText = (EditText)findViewById(R.id.tagPriceEditText);
tagPriceEditText = (EditText)findViewById(R.id.totalPriceEditText); // <- wrong?

应改为

tagPriceEditText = (EditText)findViewById(R.id.tagPriceEditText);
totalPriceEditText= (EditText)findViewById(R.id.totalPriceEditText);

您的TaxCalculator活动还没有在AndroidManifest.xml中注册: 1)更改应用程序入口点的名称(如果没有MainActivity)，或者2)添加一个新的<activity>条目。

将应用程序的入口点更改为TaxCalculator的示例(替换当前的<activity>)

<activity android:name="TaxCalculator"
    android:label="@string/app_name">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>

奖励:如果您的布局已经是最终的，代码可以清理到这里。这只是对你当前布局的一个建议。我仍然不知道你是否需要TableLayout作为未来的用途。

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#FFF"
    android:orientation="vertical"
    android:padding="5dp" >
    <!-- tagPriceInputRow -->
    <EditText
        android:id="@+id/tagPriceEditText"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:inputType="number"
        android:text="@string/tagPrice" >
    </EditText>
    <!-- totalPriceOutputRow -->
    <EditText
        android:id="@+id/totalPriceEditText"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:inputType="numberDecimal"
        android:text="@string/totalPrice" >
    </EditText>
</LinearLayout>