在Android Studio中使用Spinner添加图像

Question

I'm trying to load image in MainActivity based on selected Spinner Element.

So if my spinner consist two channel names in layout, and these names are stored in array channels = ["skysports", "premierlive"]

I have defined my ImageView:

ImageView logo = (ImageView) findViewById(R.id.logo);

While creating Spinner I'm using this channels array and from there I'm calling

getLogo(channels[i]); 

where channels[i] is current selected Spinner element.

Finally, I'm trying to implement this image switch with switch-case inside getLogo:

public void getLogo(String channel){
        switch(channel){
            case ("skysports"):
                logo.setImageResource(R.drawable.skysports);
                break;
            case ("premierlive"):
                logo.setImageResource(R.drawable.premierlive);
                break;
        }
    }

All this gives me

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageView.setImageResource(int)' on a null object reference

What shall I do? Tnx

EDIT:

activity_main:

    <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:paddingLeft="@dimen/activity_horizontal_margin"
        android:paddingRight="@dimen/activity_horizontal_margin"
        android:paddingTop="@dimen/activity_vertical_margin"
        android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity">

     <Spinner
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:id="@+id/spinnerMain"
            android:paddingLeft="30dp"
            android:paddingRight="10dp"
            android:paddingTop="2dp"
            android:paddingBottom="2dp"
            android:clickable="true"
            android:touchscreenBlocksFocus="true"
            android:layout_alignParentTop="true"
            android:visibility="visible"
            android:textSize="@android:dimen/app_icon_size"
            android:layout_centerVertical="true"
            android:layout_alignParentLeft="true"
            android:layout_alignParentStart="true"

        <ImageView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:id="@+id/logo"
            android:layout_centerHorizontal="true"
            android:layout_marginTop="20dp" />

</RelativeLayout>

and MainActivity:

   public class MainActivity extends ActionBarActivity implements View.OnClickListener {
        String[] channels;
        String channel;
        Spinner sp;
        ImageView logo;


protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ImageView logo = (ImageView) findViewById(R.id.logo);
         sp = (Spinner) findViewById(R.id.spinnerMain);
        addSpinner();

 public void addSpinner(){
        channels = getChannels();
        ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, R.layout.spinner_item, channels);
        sp.setAdapter(adapter);
        sp.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
            public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
                int index = parent.getSelectedItemPosition();
                getLogo(channels[index]);    
               }
         });
   }    
}

}

Answer 1

I ser in your activity a private field logo and in the onCreate method a local variable called logo which is the variable initialized. In onCreate method you need to initialize the private field to be used in other methods of your class, rather than have

ImageView logo = (ImageView) ...

You should have

logo = (ImaView) ...