Question

我在完美工作的应用程序中添加了第二个活动，之后它在运行时已经崩溃了。我虽然做了所有必要的事情来使其工作，如清理项目，添加对清单文件等的引用，但它仍然无法正常工作。选择 imageButtonInfo 后，应开始新的活动。什么想法可能是错的？

MainActivity.java：

    package com.example.beatzlooper;

import java.io.IOException;

import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.app.Fragment;
import android.net.Uri;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.ImageButton;
import android.widget.TextView;
import android.os.Build;
import android.content.Intent;
import android.graphics.Color;
import android.media.MediaPlayer;

public class MainActivity extends ActionBarActivity implements OnClickListener{

    /* Declarations */
    MediaPlayer mp = new MediaPlayer();
    Beat beatOne = new Beat("Unknown", "1 asdasfsdfsdfsdfsfdad", R.raw.elements6); // (author, title, id)
    Beat beatTwo = new Beat("Unknown", "2 adasdavvvzzzzfff", R.raw.beat);
    TextView labelView = (TextView) findViewById(R.id.labelView);
    Intent intent = new Intent(MainActivity.this, InfoActivity.class);

    /* ----------------------------------------------------------- */

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

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

    /* Buttons */
    ImageButton btnStop = (ImageButton) findViewById(R.id.imageButtonStop);
    btnStop.setOnClickListener(this);
    ImageButton btnInfo = (ImageButton) findViewById(R.id.imageButtonInfo);
    btnInfo.setOnClickListener(this);    
    ImageButton btnExit = (ImageButton) findViewById(R.id.imageButtonExit);
    btnExit.setOnClickListener(this);
    Button btnOne = (Button) findViewById(R.id.button1);
    btnOne.setOnClickListener(this);
    Button btnTwo = (Button) findViewById(R.id.button2);
    btnTwo.setOnClickListener(this);
    /* ----------------------------------------------------------- */
    }

    public void onClick(View v) {

        switch(v.getId()) {

            case R.id.button1:
                mp.stop();
                mp.release();
                mp = null;
                mp = MediaPlayer.create(this, beatOne.id());
                mp.setLooping(true);
                labelView.setText(1 + ". " + beatTwo.author() + " - " + beatOne.title());
                labelView.setTextColor(Color.parseColor("#FFFFFF"));
                mp.start();
                break;
            case R.id.button2:
                mp.stop();
                mp.release();
                mp = null;
                mp = MediaPlayer.create(this, beatTwo.id());
                mp.setLooping(true);
                labelView.setText(2 + ". " + beatTwo.author() + " - " + beatTwo.title());
                labelView.setTextColor(Color.parseColor("#FFFFFF"));
                mp.start();
                break;
            case R.id.imageButtonStop:
                mp.stop();
                mp.release();
                mp = null;
                break;
            case R.id.imageButtonInfo:
                mp.stop();
                mp.release();
                mp = null;
                MainActivity.this.startActivity(intent);
                break;
            case R.id.imageButtonExit:
                android.os.Process.killProcess(android.os.Process.myPid());
                break;
            default:
                return;
            }
    }           

    @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;
        }
    }
}

InfoActivity.java：

package com.example.beatzlooper;

import com.example.beatzlooper.R;
import com.example.beatzlooper.R.id;
import com.example.beatzlooper.R.layout;
import com.example.beatzlooper.R.menu;
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.os.Build;

public class InfoActivity extends MainActivity {

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

//      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.info, 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_info, container,
                    false);
            return rootView;
        }
    }
}

的AndroidManifest.xml：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.beatzlooper"
    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:configChanges="orientation|screenSize"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name=".InfoActivity"
            android:configChanges="orientation|screenSize"
            android:label="@string/app_name_info" >
        </activity>
    </application>

</manifest>

activity_main.xml中：

   <?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/RelativeLayout1"
    android:layout_width="match_parent"
    android:layout_height="fill_parent"
    android:background="@drawable/bandanabackground"
    android:columnCount="2"
    android:orientation="horizontal" >


    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/button1"
        android:layout_alignBottom="@+id/button1"
        android:layout_alignParentRight="true"
        android:layout_marginRight="35dp"
        android:text="2" />

    <ImageButton
        android:id="@+id/imageButtonInfo"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="30dp"
        android:background="@drawable/buttonstyle"
        android:padding="10dp"
        android:src="@drawable/info_invert" />

    <ImageButton
        android:id="@+id/imageButtonExit"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/button2"
        android:layout_alignTop="@+id/imageButtonInfo"
        android:background="@drawable/buttonstyle"
        android:padding="10dp"
        android:src="@drawable/exit_invert" />

    <ImageButton
        android:id="@+id/imageButtonStop"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignRight="@+id/button1"
        android:layout_alignTop="@+id/imageButtonInfo"
        android:background="@drawable/buttonstyle"
        android:padding="10dp"
        android:src="@drawable/stop_invert" />

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_below="@+id/labelView"
        android:layout_marginLeft="37dp"
        android:layout_marginTop="42dp"
        android:text="1" />

    <TextView
        android:id="@+id/labelView"
        android:layout_width="16dp"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/button1"
        android:layout_alignParentTop="true"
        android:layout_alignRight="@+id/imageButtonExit"
        android:layout_marginTop="39dp"
        android:gravity="center" />

</RelativeLayout>

activity_info.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:orientation="vertical" >

</RelativeLayout>

logcat的：

    05-24 17:53:24.309: D/AndroidRuntime(18837): Shutting down VM
05-24 17:53:24.309: W/dalvikvm(18837): threadid=1: thread exiting with uncaught exception (group=0x2b542210)
05-24 17:53:24.309: E/AndroidRuntime(18837): FATAL EXCEPTION: main
05-24 17:53:24.309: E/AndroidRuntime(18837): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.beatzlooper/com.example.beatzlooper.MainActivity}: java.lang.NullPointerException
05-24 17:53:24.309: E/AndroidRuntime(18837):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1891)
05-24 17:53:24.309: E/AndroidRuntime(18837):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1992)
05-24 17:53:24.309: E/AndroidRuntime(18837):    at android.app.ActivityThread.access$600(ActivityThread.java:127)
05-24 17:53:24.309: E/AndroidRuntime(18837):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1158)
05-24 17:53:24.309: E/AndroidRuntime(18837):    at android.os.Handler.dispatchMessage(Handler.java:99)
05-24 17:53:24.309: E/AndroidRuntime(18837):    at android.os.Looper.loop(Looper.java:137)
05-24 17:53:24.309: E/AndroidRuntime(18837):    at android.app.ActivityThread.main(ActivityThread.java:4441)
05-24 17:53:24.309: E/AndroidRuntime(18837):    at java.lang.reflect.Method.invokeNative(Native Method)
05-24 17:53:24.309: E/AndroidRuntime(18837):    at java.lang.reflect.Method.invoke(Method.java:511)
05-24 17:53:24.309: E/AndroidRuntime(18837):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
05-24 17:53:24.309: E/AndroidRuntime(18837):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
05-24 17:53:24.309: E/AndroidRuntime(18837):    at dalvik.system.NativeStart.main(Native Method)
05-24 17:53:24.309: E/AndroidRuntime(18837): Caused by: java.lang.NullPointerException
05-24 17:53:24.309: E/AndroidRuntime(18837):    at android.app.Activity.findViewById(Activity.java:1794)
05-24 17:53:24.309: E/AndroidRuntime(18837):    at com.example.beatzlooper.MainActivity.<init>(MainActivity.java:30)
05-24 17:53:24.309: E/AndroidRuntime(18837):    at java.lang.Class.newInstanceImpl(Native Method)
05-24 17:53:24.309: E/AndroidRuntime(18837):    at java.lang.Class.newInstance(Class.java:1319)
05-24 17:53:24.309: E/AndroidRuntime(18837):    at android.app.Instrumentation.newActivity(Instrumentation.java:1023)
05-24 17:53:24.309: E/AndroidRuntime(18837):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1882)
05-24 17:53:24.309: E/AndroidRuntime(18837):    ... 11 more

Answer 1

首先，第二个Activity声明如下：

public class InfoActivity extends MainActivity { }

它扩展了MainActivity并从此Activity扩展中导入所有内容。您应该使用自己的扩展程序创建第二个活动：

public class InfoActivity extends ActionBarActivity { }

请勿将其与其他活动相关联。

然后，您有初始化错误，请参阅：

at com.example.beatzlooper.MainActivity.<init>(MainActivity.java:30)

以下是出现问题的行：

TextView labelView = (TextView) findViewById(R.id.labelView);
Intent intent = new Intent(MainActivity.this, InfoActivity.class);

您应该创建var然后初始化它。同样我在评论中说过，您应该为var初始化Intent，但在onClick()内调用它。如下：

// create variables
Intent intent;
TextView labelView;

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

    labelView = (TextView) findViewById(R.id.labelView);
}

// initialize intent in onClick
public void onClick(View v) {
    switch(v.getId()) {
        case R.id.imageButtonInfo:
            intent = new Intent(MainActivity.this, InfoActivity.class);
            MainActivity.this.startActivity(intent);
            break;
    }
}

Answer 2

如果您只使用一次，请尝试尽可能地使您的变量或对象本地化。从顶部剪切这行代码 - Intent intent = new Intent（MainActivity.this，InfoActivity.class）;并在开始之前粘贴它（意图）; 。可能这会起作用。

还有一个问题，为什么你要将InfoActivity扩展到MainActivity，你的InfoActivity成为父类的子类，即主要活动。我第一次从你的代码中看到过这个东西，从来没有自己做过。可能这是崩溃的原因。尝试使用Activity或ActionBarActivity类扩展InfoActivity。

Answer 3

Intent intent = new Intent(MainActivity.this, InfoActivity.class);

在初始化对象期间，您无法访问静态此变量。您应该在onCreate（）方法中创建该意图。

at com.example.beatzlooper.MainActivity.<init>(MainActivity.java:30)

这一行表明问题出在对象的初始化中。您应该始终对跟踪进行分析，大多数情况下它会告诉您问题的原因。

<强>更新

您从代码中删除了关键部分：）

TextView labelView = (TextView) findViewById(R.id.labelView);

当然，在初始化之前，不应该访问该对象的公共方法。将其移动到onCreate（）也是。

第二个活动 - Android应用程序。无法实例化活动ComponentInfo

3 个答案: