Java ADT简单应用程序-致命异常

我刚开始使用Android和Java，我刚刚制作了一个简单的应用程序，其中包含两个文本视图、一个按钮和一个edittext

当我运行应用程序时，我收到一个错误消息：不幸的是，FirstApp已停止。 我熟悉编程，不知道我是否错过了一些库，或者ADT是否配置错误

我尝试过清理和重建，右键单击PackageExplorer和Android工具->添加支持库中的项目，重新启动emulator，重新启动ADT并重新启动计算机。我想不出这是怎么回事

你们谁能看出这里有什么不对劲吗

MainActivity.java导入

import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.*;
import android.os.Build;

MainActivity.java

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);


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

    btnCalc.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            EditText number = (EditText)findViewById(R.id.num);
            TextView display = (TextView)findViewById(R.id.display);

            double num = Double.parseDouble(number.getText().toString());
            num = num * 5;
            display.setText(num + "");
        }
    });




    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
    }
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {

    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

fragment_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.danlevi.testproject.MainActivity$PlaceholderFragment" >

<TextView
    android:id="@+id/title"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:layout_centerHorizontal="true"
    android:text="@string/lblTitle" />

<EditText
    android:id="@+id/num"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/textView1"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="15dp"
    android:ems="10"
    android:inputType="numberDecimal|numberSigned" >

    <requestFocus />
</EditText>

<Button
    android:id="@+id/btnCalc"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/num"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="14dp"
    android:text="@string/btnCalc" />

<TextView
    android:id="@+id/display"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentLeft="true"
    android:layout_below="@+id/btnCalc"
    android:layout_marginTop="16dp" />

</RelativeLayout>

LogCat

05-16 12:33:37.412: D/AndroidRuntime(1579): Shutting down VM
05-16 12:33:37.412: W/dalvikvm(1579): threadid=1: thread exiting with uncaught exception (group=0xb2b08ba8)
05-16 12:33:37.432: E/AndroidRuntime(1579): FATAL EXCEPTION: main
05-16 12:33:37.432: E/AndroidRuntime(1579): Process: com.danlevi.testproject, PID: 1579
05-16 12:33:37.432: E/AndroidRuntime(1579): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.danlevi.testproject/com.danlevi.testproject.MainActivity}: java.lang.NullPointerException
05-16 12:33:37.432: E/AndroidRuntime(1579):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
05-16 12:33:37.432: E/AndroidRuntime(1579):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
05-16 12:33:37.432: E/AndroidRuntime(1579):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
05-16 12:33:37.432: E/AndroidRuntime(1579):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
05-16 12:33:37.432: E/AndroidRuntime(1579):     at android.os.Handler.dispatchMessage(Handler.java:102)
05-16 12:33:37.432: E/AndroidRuntime(1579):     at android.os.Looper.loop(Looper.java:136)
05-16 12:33:37.432: E/AndroidRuntime(1579):     at android.app.ActivityThread.main(ActivityThread.java:5017)
05-16 12:33:37.432: E/AndroidRuntime(1579):     at java.lang.reflect.Method.invokeNative(Native Method)
05-16 12:33:37.432: E/AndroidRuntime(1579):     at java.lang.reflect.Method.invoke(Method.java:515)
05-16 12:33:37.432: E/AndroidRuntime(1579):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
05-16 12:33:37.432: E/AndroidRuntime(1579):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
05-16 12:33:37.432: E/AndroidRuntime(1579):     at dalvik.system.NativeStart.main(Native Method)
05-16 12:33:37.432: E/AndroidRuntime(1579): Caused by: java.lang.NullPointerException
05-16 12:33:37.432: E/AndroidRuntime(1579):     at com.danlevi.testproject.MainActivity.onCreate(MainActivity.java:25)
05-16 12:33:37.432: E/AndroidRuntime(1579):     at android.app.Activity.performCreate(Activity.java:5231)
05-16 12:33:37.432: E/AndroidRuntime(1579):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
05-16 12:33:37.432: E/AndroidRuntime(1579):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
05-16 12:33:37.432: E/AndroidRuntime(1579):     ... 11 more
05-16 12:33:44.352: I/Process(1579): Sending signal. PID: 1579 SIG: 9

---

这是因为findViewById（）在活动主布局中搜索，而按钮位于片段的布局片段主布局中

在片段的

onCreateView（）

方法中移动该代码段：

//...

View rootView = inflater.inflate(R.layout.fragment_main, container, false);
Button buttonClick = (Button)rootView.findViewById(R.id.button);
buttonClick.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
        onButtonClick((Button) view);
    }
});

请注意，现在您可以通过

rootView视图

访问它：

Button buttonClick = (Button)rootView.findViewById(R.id.button);

否则，您将再次获得

NullPointerException

在代码中，移动

changer = (Button) findViewById(R.id.change1);
changeme = (TextView) findViewById (R.id.changeme1);
changer.setOnClickListener(new View.OnClickListener() {

@Override
public void onClick(View arg0) {
// TODO Auto-generated method stub
changeme.setText("Changed");
}
});

---

将fragment\u main中的所有内容复制到activity\u main.xml。这将解决问题。这是最简单的解决方案。但正确的方法是在我的答案中指定。首先尝试我的注释，如果这有助于尝试我的答案…或替换

setContentView（R.layout.activity_main）

as

setContentView（R.layout.fragment_main）

您应该能够通过使用调试器自己找到错误。如果这有助于您接受我的答案……请也给我一个投票。@Dan LeviTomtaThank您！我将XML移到activity_main.XML，这就成功了。接受您的答案。

changer = (Button) findViewById(R.id.change1);
changeme = (TextView) findViewById (R.id.changeme1);
changer.setOnClickListener(new View.OnClickListener() {

@Override
public void onClick(View arg0) {
// TODO Auto-generated method stub
changeme.setText("Changed");
}
});