我正在尝试编写一个简单的计算器。我有一个SimpleActivity类，如下所示：
package org.muzzy.calc;
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.webkit.WebChromeClient;
import android.webkit.WebView;
import android.widget.Button;
public class SimpleActivity extends Activity implements OnClickListener {
/** Called when the activity is first created. */
private WebView numer;
private Button guzik;
private StringBuilder wys;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
numer = (WebView) findViewById(R.id.webview);
numer.getSettings().setJavaScriptEnabled(true);
numer.setWebChromeClient(new WebChromeClient());
guzik = (Button) findViewById(R.id.button0);
guzik.setOnClickListener(this);
guzik = (Button) findViewById(R.id.button1);
guzik.setOnClickListener(this);
guzik = (Button) findViewById(R.id.button2);
guzik.setOnClickListener(this);
guzik = (Button) findViewById(R.id.button3);
guzik.setOnClickListener(this);
guzik = (Button) findViewById(R.id.button4);
guzik.setOnClickListener(this);
guzik = (Button) findViewById(R.id.button5);
guzik.setOnClickListener(this);
guzik = (Button) findViewById(R.id.button6);
guzik.setOnClickListener(this);
guzik = (Button) findViewById(R.id.button7);
guzik.setOnClickListener(this);
guzik = (Button) findViewById(R.id.button8);
guzik.setOnClickListener(this);
guzik = (Button) findViewById(R.id.button9);
guzik.setOnClickListener(this);
guzik = (Button) findViewById(R.id.plus);
guzik.setOnClickListener(this);
guzik = (Button) findViewById(R.id.minus);
guzik.setOnClickListener(this);
guzik = (Button) findViewById(R.id.multi);
guzik.setOnClickListener(this);
guzik = (Button) findViewById(R.id.divide);
guzik.setOnClickListener(this);
guzik = (Button) findViewById(R.id.leftP);
guzik.setOnClickListener(this);
guzik = (Button) findViewById(R.id.rightP);
guzik.setOnClickListener(this);
}
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
StringBuilder wyr = new StringBuilder();
switch (v.getId()) {
case R.id.bs:
if (wyr.length() > 0)
wyr.deleteCharAt(wyr.length() - 1);
break;
case R.id.ac:
if (wyr.length() > 0)
wyr.delete(0, wyr.length());
break;
default:
wyr.append(((Button) v).getText());
}
wys.append("<html><body>");
wys.append("<script type=\"text/javascript\">document.write('");
wys.append(wyr.toString());
wys.append("');");
wys.append("document.write('<br />=' + eval(\"");
wys.append(wyr.toString());
wys.append("\"));</script>");
wys.append("</body></html>");
numer.loadData(wys.toString(), "application/xhtml", "UTF-8");
}
}
我正在从另一个Activity类开始这个类：
Intent simpleIntent = new Intent(this, SimpleActivity.class);
startActivity(simpleIntent);
当我尝试在VM Android 2.3.3上运行该项目时，我遇到了这样的错误：
03-07 21:52:36.624: D/AndroidRuntime(411): Shutting down VM
03-07 21:52:36.624: W/dalvikvm(411): threadid=1: thread exiting with uncaught exception (group=0x40015560)
03-07 21:52:36.634: E/AndroidRuntime(411): FATAL EXCEPTION: main
03-07 21:52:36.634: E/AndroidRuntime(411): java.lang.RuntimeException: Unable to start activity ComponentInfo{org.muzzy.calc/org.muzzy.calc.SimpleActivity}: java.lang.NullPointerException
03-07 21:52:36.634: E/AndroidRuntime(411): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
03-07 21:52:36.634: E/AndroidRuntime(411): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
03-07 21:52:36.634: E/AndroidRuntime(411): at android.app.ActivityThread.access$1500(ActivityThread.java:117)
03-07 21:52:36.634: E/AndroidRuntime(411): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
03-07 21:52:36.634: E/AndroidRuntime(411): at android.os.Handler.dispatchMessage(Handler.java:99)
03-07 21:52:36.634: E/AndroidRuntime(411): at android.os.Looper.loop(Looper.java:123)
03-07 21:52:36.634: E/AndroidRuntime(411): at android.app.ActivityThread.main(ActivityThread.java:3683)
03-07 21:52:36.634: E/AndroidRuntime(411): at java.lang.reflect.Method.invokeNative(Native Method)
03-07 21:52:36.634: E/AndroidRuntime(411): at java.lang.reflect.Method.invoke(Method.java:507)
03-07 21:52:36.634: E/AndroidRuntime(411): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
03-07 21:52:36.634: E/AndroidRuntime(411): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
03-07 21:52:36.634: E/AndroidRuntime(411): at dalvik.system.NativeStart.main(Native Method)
03-07 21:52:36.634: E/AndroidRuntime(411): Caused by: java.lang.NullPointerException
03-07 21:52:36.634: E/AndroidRuntime(411): at org.muzzy.calc.SimpleActivity.onCreate(SimpleActivity.java:25)
03-07 21:52:36.634: E/AndroidRuntime(411): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
03-07 21:52:36.634: E/AndroidRuntime(411): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
03-07 21:52:36.634: E/AndroidRuntime(411): ... 11 more
simpleActivity的my xml文件如下所示：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >
<WebView
android:id="@+id/webview"
android:layout_width="fill_parent"
android:layout_height="50dip"
android:visibility="visible" />
<TableLayout
android:id="@+id/tableLayout1"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:stretchColumns="0,1,2,3" >
<TableRow
android:id="@+id/tableRow1"
android:layout_width="wrap_content"
android:layout_height="wrap_content" >
<Button
android:id="@+id/button1"
android:text="@string/b1" />
<Button
android:id="@+id/button2"
android:text="@string/b2" />
<Button
android:id="@+id/button3"
android:text="@string/b3" />
<Button
android:id="@+id/plus"
android:text="@string/plus" />
</TableRow>
<TableRow
android:id="@+id/tableRow2"
android:layout_width="wrap_content"
android:layout_height="wrap_content" >
<Button
android:id="@+id/button4"
android:text="@string/b4" />
<Button
android:id="@+id/button5"
android:text="@string/b5" />
<Button
android:id="@+id/button6"
android:text="@string/b6" />
<Button
android:id="@+id/minus"
android:text="@string/minus" />
</TableRow>
<TableRow
android:id="@+id/tableRow3"
android:layout_width="match_parent"
android:layout_height="wrap_content" >
<Button
android:id="@+id/button7"
android:text="@string/b7" />
<Button
android:id="@+id/button8"
android:text="@string/b8" />
<Button
android:id="@+id/button9"
android:text="@string/b9" />
<Button
android:id="@+id/multi"
android:text="@string/multi" />
</TableRow>
<TableRow
android:id="@+id/tableRow4"
android:layout_width="match_parent"
android:layout_height="wrap_content" >
<Button
android:id="@+id/leftP"
android:text="@string/leftParen" />
<Button
android:id="@+id/button0"
android:text="@string/b0" />
<Button
android:id="@+id/rightP"
android:text="@string/rightParen" />
<Button
android:id="@+id/divide"
android:text="@string/divide" />
</TableRow>
<TableRow
android:id="@+id/tableRow5"
android:layout_width="match_parent"
android:layout_height="wrap_content" >
<Button
android:id="@+id/bs"
android:text="@string/backspace" />
<Button
android:id="@+id/dot"
android:text="@string/dot" />
<Button
android:id="@+id/ac"
android:text="@string/allclear" />
<Button
android:id="@+id/equals"
android:text="@string/equals" />
</TableRow>
</TableLayout>
</LinearLayout>
我在很多论坛上阅读了很多教程和主题但仍然没有。我做错了什么？
答案 0 :(得分：1)
在大多数情况下，这是一个简单的错误。我的SimpleActivity布局不是主要的，而是简单的。主布局中没有webview，因此它不起作用并返回空指针异常。
答案 1 :(得分：0)
双击此行的Log Cat窗口： 03-07 21：52：36.634：E / AndroidRuntime（411）：at org.muzzy.calc.SimpleActivity.onCreate（SimpleActivity.java:25）
这会将您带到导致问题的源行。
你忘了发表声明 wys = new StringBuilder（）;
忘了清除一个新按钮