安卓简单按钮程序中的java错误

Question

我是新来安卓的，所以请容忍我

当我试图在模拟器中运行我的程序时，我在安卓 studio中收到一个错误 我试图有一个按钮，播放按钮的声音。。。。。很简单吧

这是航海日志

Process: com.myApp.soundboard, PID: 1046
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.myApp.soundboard/com.myApp.soundboard.MainActivity}: java.lang.NullPointerException
        at 安卓.app.ActivityThread.performLaunchActivity(ActivityThread.java:2102)
        at 安卓.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2226)
        at 安卓.app.ActivityThread.access$700(ActivityThread.java:135)
        at 安卓.app.ActivityThread$H.handleMessage(ActivityThread.java:1397)
        at 安卓.os.Handler.dispatchMessage(Handler.java:102)
        at 安卓.os.Looper.loop(Looper.java:137)
        at 安卓.app.ActivityThread.main(ActivityThread.java:4998)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.安卓.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777)
        at com.安卓.internal.os.ZygoteInit.main(ZygoteInit.java:593)
        at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.NullPointerException
        at 安卓.content.ContextWrapper.getResources(ContextWrapper.java:89)
        at 安卓.view.ContextThemeWrapper.getResources(ContextThemeWrapper.java:78)
        at 安卓.media.MediaPlayer.create(MediaPlayer.java:844)
        at com.myApp.soundboard.MainActivity.<init>(MainActivity.java:19)
        at java.lang.Class.newInstanceImpl(Native Method)
        at java.lang.Class.newInstance(Class.java:1208)
        at 安卓.app.Instrumentation.newActivity(Instrumentation.java:1061)
        at 安卓.app.ActivityThread.performLaunchActivity(ActivityThread.java:2093)
        at 安卓.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2226)
        at 安卓.app.ActivityThread.access$700(ActivityThread.java:135)
        at 安卓.app.ActivityThread$H.handleMessage(ActivityThread.java:1397)
        at 安卓.os.Handler.dispatchMessage(Handler.java:102)
        at 安卓.os.Looper.loop(Looper.java:137)
        at 安卓.app.ActivityThread.main(ActivityThread.java:4998)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.安卓.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777)
        at com.安卓.internal.os.ZygoteInit.main(ZygoteInit.java:593)
        at dalvik.system.NativeStart.main(Native Method)

这是主程序的代码

 import 安卓.content.Intent;
    import 安卓.media.MediaPlayer;
    import 安卓.support.v7.app.ActionBarActivity;
    import 安卓.support.v7.app.ActionBar;
    import 安卓.support.v4.app.Fragment;
    import 安卓.os.Bundle;
    import 安卓.view.LayoutInflater;
    import 安卓.view.Menu;
    import 安卓.view.MenuItem;
    import 安卓.view.View;
    import 安卓.view.ViewGroup;
    import 安卓.os.Build;
    import 安卓.widget.Button;

    public class MainActivity extends ActionBarActivity {

    final MediaPlayer mp_imageButtonClick = MediaPlayer.create(this, R.raw.button7 );

    public void imageButtonOnClick(View v)
    {
        Button imageButton = (Button) v;
        ((Button) v).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view)
            {

                mp_imageButtonClick.start();

            }
        });

    }

    @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();
        }
    }


    @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.
        switch (item.getItemId()) {
            case 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;
        }
    }

    }

任何想法都认为它是由空指针异常引起的，对于知道自己在做什么的人来说可能很简单

Answer 1

移动这个

 final MediaPlayer mp_imageButtonClick = MediaPlayer.create(this, R.raw.button7 );

在onCreate

MediaPlayer mp_imageButtonClick;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mp_imageButtonClick = MediaPlayer.create(this, R.raw.button7 ); 

也换成

public void imageButtonOnClick(View v)
{

      mp_imageButtonClick.start();  

}

如果你有

 android:onClick:"imageButtonClick" 

用于xml中的按钮

Answer 2

final MediaPlayer mp_imageButtonClick = MediaPlayer.create(this, R.raw.button7 );

在创建对象之前，此行使用的是“This”引用。由于您尚未创建对象，“this”关键字不引用任何内容，因此您会得到一个空指针值。如果将这一行移到onCreate（）方法中，就应该可以了

Answer 3

在系统完全初始化之前，您正在使用此（MainActivity），这会导致MediaPlayer崩溃。create试图将活动用作上下文

您可以通过使用onCreate回调来确保活动已完全创建：

首先声明MediaPlayer变量：

private MediaPlayer mp_imageButtonClick;

然后将调用的其余部分移动到onCreate：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mp_imageButtonClick = MediaPlayer.create(this, R.raw.button7 ); 
}