Question

I am creating an app that it is throwing an error, activity is not started. My activity class is:

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.videos_layout);
    i = new FirstImage(this);
    c = (ViewFlipper) findViewById(R.id.viewFlipper3);
    back = (Button) findViewById(R.id.button1);
    next = (Button) findViewById(R.id.button2);
    t = (TextView) findViewById(R.id.textView1);
    if(count == 0)
        i.changeImage(R.drawable.human); 
    c.addView(i);
    c.setOnTouchListener(this);
    c.onTouchEvent(event);
    addListenerOnButton();
    }


    private void addListenerOnButton() {
        if (count == 0)
            back.setEnabled(false);
        next.setOnClickListener( new OnClickListener() {

            public void onClick(View arg0) {
                if(count==0){
                i.changeImage(R.drawable.hand);
                back.setEnabled(true);
                count++;

                addListenerOnButton();

                }
                else if (count==1){
                    i.changeImage(R.drawable.tissue);
                    count++;

                    addListenerOnButton();

                }

                else if (count==2){
                    image.setImageResource(R.drawable.cellconstfinal1);
                    count++;

                    addListenerOnButton();


                }
                else if (count==3){
                    //nucleus
                    image.setImageResource(R.drawable.cellconstfinal1);
                    count++;

                    addListenerOnButton();

                }
                else if (count==4){
                    //nchromosome
                    image.setImageResource(R.drawable.hand);
                    count++;

                    addListenerOnButton();

                }
                else
                {   //double helix
                    count++;
                    image.setImageResource(R.drawable.handash1);
                    next.setEnabled(false);

                    addListenerOnButton();

                }

            }
        });

        back.setOnClickListener(new OnClickListener() {


            public void onClick(View arg0) {
                if(count==1){
                    image.setImageResource(R.drawable.human);
                    back.setEnabled(false);
                    count--;

                    addListenerOnButton();

                    }
                    else if (count==2){
                        image.setImageResource(R.drawable.hand);
                        count--;

                        addListenerOnButton();

                    }

                    else if (count==3){
                        image.setImageResource(R.drawable.tissue);
                        count--;

                        addListenerOnButton();


                    }
                    else if (count==4){
                        image.setImageResource(R.drawable.cellconstfinal1);
                        count--;

                        addListenerOnButton();

                    }
                    else if (count==5){
                        //nucleus
                        image.setImageResource(R.drawable.tissue);
                        count--;

                        addListenerOnButton();


                    }
                    else {
                        //chromosome
                        //count==6
                        image.setImageResource(R.drawable.cellconstfinal1);
                        count--;

                        addListenerOnButton();
                        next.setEnabled(true);

                    }
                }
            });
    }




public void pageinfo(float a,float b){

        t.setText(Float.toString(a)+"x"+Float.toString(b)+"y");

    }



public boolean onTouch(View v, MotionEvent me) {
    // TODO Auto-generated method stub
    switch(me.getAction()){
    case MotionEvent.ACTION_DOWN:
        a=me.getX();
        b= me.getY();
        pageinfo(a,b);
        break;
    case MotionEvent.ACTION_MOVE:
        a=me.getX();
        b= me.getY();
        pageinfo(a,b);
        break;
    case MotionEvent.ACTION_UP:
        a=me.getX();
        b= me.getY();
        pageinfo(a,b);
        break;
    case MotionEvent.ACTION_OUTSIDE:
        a=me.getX();
        b= me.getY();
        pageinfo(a,b);
        break;
    default: return false;
    }
    return true;

}
}

My FirstImage class code:

public class FirstImage extends ImageView {

    float a=0; 
    float b=0;
    TextView t;
    ImageView i;

    public FirstImage(Context context) { 
        super(context); 
        t= (TextView) findViewById(R.id.textView1); 
        i= new ImageView (context); 
    } 

    public FirstImage(Context context, AttributeSet attrs) { 
        super(context, attrs);
   } 

    protected void changeImage(int id){ 
        i.setImageResource(id); 
    } 

The stacktrace with the exception:

  05-11 00:21:41.197: E/AndroidRuntime(1142): FATAL EXCEPTION: main
05-11 00:21:41.197: E/AndroidRuntime(1142): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.androidtablayout/com.example.androidtablayout.VideosActivity}: java.lang.NullPointerException
05-11 00:21:41.197: E/AndroidRuntime(1142):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
05-11 00:21:41.197: E/AndroidRuntime(1142):     at android.app.ActivityThread.startActivityNow(ActivityThread.java:2503)
05-11 00:21:41.197: E/AndroidRuntime(1142):     at android.app.LocalActivityManager.moveToState(LocalActivityManager.java:127)
05-11 00:21:41.197: E/AndroidRuntime(1142):     at android.app.LocalActivityManager.startActivity(LocalActivityManager.java:339)
05-11 00:21:41.197: E/AndroidRuntime(1142):     at android.widget.TabHost$IntentContentStrategy.getContentView(TabHost.java:651)
05-11 00:21:41.197: E/AndroidRuntime(1142):     at android.widget.TabHost.setCurrentTab(TabHost.java:323)
05-11 00:21:41.197: E/AndroidRuntime(1142):     at android.widget.TabHost$2.onTabSelectionChanged(TabHost.java:129)
05-11 00:21:41.197: E/AndroidRuntime(1142):     at android.widget.TabWidget$TabClickListener.onClick(TabWidget.java:453)
05-11 00:21:41.197: E/AndroidRuntime(1142):     at android.view.View.performClick(View.java:2408)
05-11 00:21:41.197: E/AndroidRuntime(1142):     at android.view.View$PerformClick.run(View.java:8816)
05-11 00:21:41.197: E/AndroidRuntime(1142):     at android.os.Handler.handleCallback(Handler.java:587)
05-11 00:21:41.197: E/AndroidRuntime(1142):     at android.os.Handler.dispatchMessage(Handler.java:92)
05-11 00:21:41.197: E/AndroidRuntime(1142):     at android.os.Looper.loop(Looper.java:123)
05-11 00:21:41.197: E/AndroidRuntime(1142):     at android.app.ActivityThread.main(ActivityThread.java:4627)
05-11 00:21:41.197: E/AndroidRuntime(1142):     at java.lang.reflect.Method.invokeNative(Native Method)
05-11 00:21:41.197: E/AndroidRuntime(1142):     at java.lang.reflect.Method.invoke(Method.java:521)
05-11 00:21:41.197: E/AndroidRuntime(1142):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
05-11 00:21:41.197: E/AndroidRuntime(1142):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
05-11 00:21:41.197: E/AndroidRuntime(1142):     at dalvik.system.NativeStart.main(Native Method)
05-11 00:21:41.197: E/AndroidRuntime(1142): Caused by: java.lang.NullPointerException
05-11 00:21:41.197: E/AndroidRuntime(1142):     at com.example.androidtablayout.VideosActivity.onCreate(VideosActivity.java:45)
05-11 00:21:41.197: E/AndroidRuntime(1142):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
05-11 00:21:41.197: E/AndroidRuntime(1142):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
05-11 00:21:41.197: E/AndroidRuntime(1142):     ... 18 more

Answer 1

You should always initialize the views in the onCreate method:

//...
FirstImage i;
//...

    @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
             setContentView(R.layout.videos_layout);
             i = new FirstImage(this);
    //...

Also in the FirstImage class initialize those Views in the constructor:

public FirstImage(Context context) {
    super(context);
   TextView t= (TextView) findViewById(R.id.textView1); // WHERE do you expect to find this view, in which layout?
   ImageView i= new ImageView (context); //NEVER pass a null Context
}

Edit:

My advice is to follow some tutorials for custom Views. You can’t add other Views to an ImageView subclass(and you shouldn’t). Even if you build the ImageView in the FirstImage the changeImage method will throw a NullPointerException the way your code is currently built.

A simple custom view:

 public class FirstImage extends LinearLayout {
    float a=0; 
    float b=0;

    TextView t;
    ImageView i;

    public FirstImage(Context context) { 
        super(context); 
        t = (TextView) findViewById(R.id.textView1); // What is with this TextView?!?
        i = new ImageView (context); 
        LinearLayout.LayoutParams lp = LinearLayout.LayoutParams(LinearLayout.LayoutParams.WRAP_CONTENT, LinearLayout.LayoutParams.WRAP_CONTENT);
        addView(i, lp);   
    } 

protected void changeImage(int id){ 
    i.setImageResource(id); 
}