嘗試設置EditText小部件時發生java.lang.NullPointerException

Question

當我執行下面的代碼時，第41行（setText）出現java.lang.NullPointerException錯誤。

我看過EditText框被設置爲此行之前，我不明白我做錯了什麼。

我沒有經驗與日食或Java，但我正在努力學習。請幫忙！

代碼：

package Psychrometric.Calculator; 

import android.app.Activity; 
import android.app.AlertDialog; 
import android.util.Log; 
import android.view.View; 
import android.view.View.OnClickListener; 
import android.widget.Button; 
import android.widget.EditText; 
import android.widget.TextView; 
import android.text.TextUtils; 
import android.os.Bundle; 

public class Calculator extends Activity { 

private EditText altitude; 
private EditText barometricPressure; 

/** Called when the activity is first created. */ 
@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
      setContentView(R.layout.main); 

    Button calcButton = (Button) findViewById(R.id.calcButton); 

    calcButton.setOnClickListener(new OnClickListener() { 
      public void onClick(View arg0) {   

      altitude = (EditText) findViewById(R.id.altitude); 


      if (altitude.getText().length() != 0) 
      { 

       double altitudeDouble = new Double(altitude.getText().toString());  

       double barometricPressureDouble = 29.92 * java.lang.Math.pow((1 - 0.0000068753 * altitudeDouble), 5.259); 

       altitude.setText(Double.toString(altitudeDouble)); 
       barometricPressure.setText(Double.toString(barometricPressureDouble)); 

      } 

    } 

    }); 

    } 

    } 

XML: 
<?xml version="1.0" encoding="utf-8"?> 

<ScrollView 
xmlns:android="http://schemas.android.com/apk/res/android" 
android:id="@+id/ScrollView01" 
android:layout_width="wrap_content" 
android:layout_height="wrap_content" 
> 

<TableLayout 
android:id="@+id/widget27" 
android:layout_width="320px" 
android:layout_height="825px" 
xmlns:android="http://schemas.android.com/apk/res/android" 
android:orientation="vertical" 
android:stretchColumns="1" 
> 

<TableRow 
android:id="@+id/firstRow" 
android:layout_width="fill_parent" 
android:layout_height="75px" 
xmlns:android="http://schemas.android.com/apk/res/android" 
android:orientation="horizontal" 
> 

<Button 
android:id="@+id/calcButton" 
android:layout_width="185px" 
android:layout_height="75px" 
android:text="Calculate" 
> 
</Button> 
</TableRow> 

<TableRow 
android:id="@+id/widget45" 
android:layout_width="fill_parent" 
android:layout_height="50px" 
xmlns:android="http://schemas.android.com/apk/res/android" 
android:orientation="horizontal" 
> 


<TextView 
android:id="@+id/widget221" 
android:layout_width="wrap_content" 
android:layout_height="50px" 
android:text="Altitude" 
> 
</TextView> 
<EditText 
android:id="@+id/altitude" 
android:layout_width="wrap_content" 
android:layout_height="50px" 
android:textSize="18sp" 
android:numeric="decimal" 
> 
</EditText> 
<TextView 
android:id="@+id/widget454" 
android:layout_width="wrap_content" 
android:layout_height="50px" 
android:text="ft" 
> 
</TextView> 
</TableRow> 
<TableRow 
android:id="@+id/widget32" 
android:layout_width="fill_parent" 
android:layout_height="50px" 
xmlns:android="http://schemas.android.com/apk/res/android" 
android:orientation="horizontal" 
> 
<TextView 
android:id="@+id/widget222" 
android:layout_width="wrap_content" 
android:layout_height="50px" 
android:text="Barometric Pressure" 
> 
</TextView> 
<EditText 
android:id="@+id/barometricPressure" 
android:layout_width="wrap_content" 
android:layout_height="50px" 
android:textSize="18sp" 
android:numeric="decimal" 
> 
</EditText> 
<TextView 
android:id="@+id/widget455" 
android:layout_width="wrap_content" 
android:layout_height="50px" 
android:text="in. Hg" 
> 
</TextView> 
</TableRow> 
</TableLayout> 
</ScrollView> 

LogCat: 
02-11 12:49:02.334: DEBUG/AndroidRuntime(11161): Shutting down VM 

02-11 12:49:02.334: WARN/dalvikvm(11161): threadid=1: thread exiting with uncaught exception (group=0x400259f8) 

02-11 12:49:02.394: ERROR/AndroidRuntime(11161): FATAL EXCEPTION: main 

02-11 12:49:02.394: ERROR/AndroidRuntime(11161): java.lang.NullPointerException 

02-11 12:49:02.394: ERROR/AndroidRuntime(11161):  at Psychrometric.Calculator.Calculator$1.onClick(Calculator.java:41) 

02-11 12:49:02.394: ERROR/AndroidRuntime(11161):  at android.view.View.performClick(View.java:2408) 

02-11 12:49:02.394: ERROR/AndroidRuntime(11161):  at android.view.View$PerformClick.run(View.java:8817) 

02-11 12:49:02.394: ERROR/AndroidRuntime(11161):  at android.os.Handler.handleCallback(Handler.java:587) 

02-11 12:49:02.394: ERROR/AndroidRuntime(11161):  at android.os.Handler.dispatchMessage(Handler.java:92) 

02-11 12:49:02.394: ERROR/AndroidRuntime(11161):  at android.os.Looper.loop(Looper.java:144) 

02-11 12:49:02.394: ERROR/AndroidRuntime(11161):  at android.app.ActivityThread.main(ActivityThread.java:4937) 

02-11 12:49:02.394: ERROR/AndroidRuntime(11161):  at java.lang.reflect.Method.invokeNative(Native Method) 

02-11 12:49:02.394: ERROR/AndroidRuntime(11161):  at java.lang.reflect.Method.invoke(Method.java:521) 

02-11 12:49:02.394: ERROR/AndroidRuntime(11161):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868) 

02-11 12:49:02.394: ERROR/AndroidRuntime(11161):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626) 

02-11 12:49:02.394: ERROR/AndroidRuntime(11161):  at dalvik.system.NativeStart.main(Native Method) 

02-11 12:49:02.404: WARN/ActivityManager(220): Force finishing activity Psychrometric.Calculator/.Calculator 

Answer 1

你還沒有分配給barometricPressure - 它仍然是零。

爲altitude你有

altitude = (EditText) findViewById(R.id.altitude); 

想必你需要barometricPressure過類似的東西。