java - 不幸的是 MyFirstApp 已经停止

Question

当我为 Button 使用代码时出现问题，我指的是 The new Boston tutorials。我完全按照指示进行操作，但在 logcat 中出现错误。

   package com.example.myfirstapp;

    import android.support.v7.app.ActionBarActivity;
    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.Button;
    import android.widget.TextView;


    public class MainActivity extends ActionBarActivity {

         int counter=0;
        TextView display;
        Button add;
        Button sub;
        @Override
        protected void onCreate(Bundle savedInstanceState) {

            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);

             add=(Button) findViewById(R.id.bAdd);
            sub=(Button) findViewById(R.id.bSub);
            display=(TextView) findViewById(R.id.tvDisplay);
            add.setOnClickListener(new View.OnClickListener() {

                @Override
                public void onClick(View arg0) {
                    // TODO Auto-generated method stub
                    counter++;
                    display.setText("Your Total is " +counter);

                }
            });     

            sub.setOnClickListener(new View.OnClickListener() {

                @Override
                public void onClick(View arg0) {
                    // TODO Auto-generated method stub
                    counter--;
                    display.setText("Your Total is " +counter);
                }
            }); 

            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;
        }

        @Override
        public boolean onOptionsItemSelected(MenuItem item) {
            // Handle action bar item clicks here. The action bar will
            // automatically handle clicks on the Home/Up button, so long
            // as you specify a parent activity in AndroidManifest.xml.
            int id = item.getItemId();
            if (id == R.id.action_settings) {
                return true;
            }
            return super.onOptionsItemSelected(item);
        }

        /**
         * A placeholder fragment containing a simple view.
         */
        public static class PlaceholderFragment extends Fragment {

            public PlaceholderFragment() {
            }

            @Override
            public View onCreateView(LayoutInflater inflater, ViewGroup container,
                    Bundle savedInstanceState) {
                View rootView = inflater.inflate(R.layout.fragment_main, container,
                        false);
                return rootView;
            }
        }

    }

这是我的 list 文件

 <?xml version="1.0" encoding="utf-8"?>
    <manifest xmlns:android="http://schemas.android.com/apk/res/android"
        package="com.example.myfirstapp"
        android:versionCode="1"
        android:versionName="1.0" >

        <uses-sdk
            android:minSdkVersion="8"
            android:targetSdkVersion="19" />

        <application
            android:allowBackup="true"
            android:icon="@drawable/ic_launcher"
            android:label="@string/app_name"
            android:theme="@style/AppTheme" >
            <activity
                android:name=".MainActivity"
                android:label="@string/app_name" >
                <intent-filter>
                    <action android:name="android.intent.action.MAIN" />
                      <category android:name="android.intent.category.DEFAULT" />
                    <category android:name="android.intent.category.LAUNCHER" />
                </intent-filter>
            </activity>
        </application>

    </manifest>

Logcat 文件显示这些错误

: FATAL EXCEPTION: main

Process: com.example.myfirstapp, PID: 1873

E/AndroidRuntime(1873): java.lang.RuntimeException: Unable to  instantiate activity ComponentInfo{com.example.myfirstapp/com.example.myfirstapp.MainActivity}: java.lang.NullPointerException

 E/AndroidRuntime(1873): at     android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2121)

E/AndroidRuntime(1873):at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)

 E/AndroidRuntime(1873):at android.app.ActivityThread.access$800(ActivityThread.java:135)

 E/AndroidRuntime(1873): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)

E/AndroidRuntime(1873): at android.os.Handler.dispatchMessage(Handler.java:102)

 E/AndroidRuntime(1873): at android.os.Looper.loop(Looper.java:136)

 E/AndroidRuntime(1873): at android.app.ActivityThread.main(ActivityThread.java:5017)

 E/AndroidRuntime(1873): at java.lang.reflect.Method.invokeNative(Native Method)

 E/AndroidRuntime(1873): at java.lang.reflect.Method.invoke(Method.java:515)

 E/AndroidRuntime(1873): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)

 E/AndroidRuntime(1873): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)

 E/AndroidRuntime(1873): at dalvik.system.NativeStart.main(Native Method)


 E/AndroidRuntime(1873): Caused by: java.lang.NullPointerException

 E/AndroidRuntime(1873): at android.view.ViewConfiguration.get(ViewConfiguration.java:325)


 E/AndroidRuntime(1873): at android.view.View.<init>(View.java:3448)


 E/AndroidRuntime(1873): at android.view.View.<init>(View.java:3505)


 E/AndroidRuntime(1873): at android.widget.TextView.<init>(TextView.java:623)


 E/AndroidRuntime(1873): at android.widget.Button.<init>(Button.java:107)


 E/AndroidRuntime(1873): at android.widget.Button.<init>(Button.java:103)


 E/AndroidRuntime(1873): at android.widget.Button.<init>(Button.java:99)


 E/AndroidRuntime(1873): at com.example.myfirstapp.MainActivity.<init>(MainActivity.java:18)


 E/AndroidRuntime(1873): at java.lang.Class.newInstanceImpl(Native Method)


 E/AndroidRuntime(1873): at java.lang.Class.newInstance(Class.java:1208)


 E/AndroidRuntime(1873): at android.app.Instrumentation.newActivity(Instrumentation.java:1061)


 E/AndroidRuntime(1873): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2112)


 E/AndroidRuntime(1873):    ... 11 more


: I/Process(1873): Sending signal. PID: 1873 SIG: 9

这是布局xml文件

<LinearLayout 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:orientation="vertical"
    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.example.myfirstapp.MainActivity$PlaceholderFragment" >

    <TextView

        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="@string/hello_world"
        android:textSize="45sp" 
        android:layout_gravity="center"
        android:gravity="center"
        android:id="@+id/tvDisplay"
        />

    <Button
        android:layout_width="250dp"
        android:layout_height="wrap_content"
        android:text="@string/add_button" 
        android:gravity="center"
        android:layout_gravity="center"
        android:id="@+id/bAdd"

        />

     <Button
         android:id="@+id/bSub"
         android:layout_width="250dp"
         android:layout_height="wrap_content"
         android:layout_gravity="center"
         android:gravity="center"
         android:text="@string/sub_button" />

</LinearLayout>

请帮助我，请不要给我投反对票，因为我是新手，正在学习。

Answer 1

fragment 持有布局。您必须在 onCreateView()

中从膨胀的 view 获取按钮和 textview 的引用

public static class PlaceholderFragment extends Fragment {

int counter=0;
TextView display;
Button add;
Button sub;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
    Bundle savedInstanceState) {

    View rootView = inflater.inflate(R.layout.fragment_main, container,
                        false);

    add = (Button) rootView.findViewById(R.id.bAdd);
    sub = (Button) rootView.findViewById(R.id.bSub);
    display = (TextView) rootView.findViewById(R.id.tvDisplay);

    add.setOnClickListener(new View.OnClickListener() {

       @Override
       public void onClick(View arg0) {
           // TODO Auto-generated method stub
           counter++;
           display.setText("Your Total is " +counter);

       }
    });     

    sub.setOnClickListener(new View.OnClickListener() {

       @Override
       public void onClick(View arg0) {
           // TODO Auto-generated method stub
           counter--;
           display.setText("Your Total is " +counter);
       }

    });

    return rootView;
}


}