java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.app.Activity.findViewById(int)' on a null object reference

Question

I always get an Error in my Android Project.

java.lang.NullPointerException: Attempt to invoke virtual method \'android.view.View android.app.Activi

Answer 1

It's very simple

public class MainActivity extends AppCompatActivity {



    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button btn1 = (Button) findViewById(R.id.button1);

        btn1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
              Intent intent = new Intent(MainActivity.this, YourNextClass.class);
                startActivity(intent);
            }
        });

    }
}

Answer 2

In addition to what @Selvin pointed out, it looks like this:

Button add = (Button) activity.findViewById(R.id.add);

...should be changed to this:

Button add = (Button) rootView.findViewById(R.id.add);

Answer 3

Use the following code:

 listView1.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

       //     selectedPosition = position;

            View newTypeView=listView1.getChildAt(position-listView1.getFirstVisiblePosition()).findViewById(R.id.RelativeLayout);

            if (newTypeView.getClass().getName().toString().contains("RelativeLayout")) {


                if (lastTouchedVw != null)
                    lastTouchedVw.setBackgroundColor(Color.parseColor("#ffffff"));
                 // mainImageView.setImageURI(Uri.parse(al.get(position).toString()));
                //bigImageRelativeLayoutVw.setBackgroundColor(Color.parseColor("#ffb000"));
                    newTypeView.setBackgroundResource(R.drawable.cellbg);

                    lastTouchedVw = newTypeView;
            }