Android Sudio - Basic Calculator - app shows unfortunately stopped : I am a newbie at android studio. Tried this code and builded and generated an unsigned apk. I tried in on Samsung galaxy trend GT-s7392 and Asus Zenfone 5, also tried in bluestacks. But when the app is opened , It shows - " Unfortunately Calculator has stopped.?"

Additionally, It never asks for for any permission upon installation. Should i need to  modify the android manifest.xml or any other file 

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<!-- language: lang-html -->

    package in.harisree.calculator;

    import android.os.Bundle;
    import android.support.v7.app.AppCompatActivity;
    import android.view.View;
    import android.view.View.OnClickListener;
    import android.widget.Button;
    import android.widget.EditText;
    import android.widget.TextView;

    public abstract class MainActivity extends AppCompatActivity implements OnClickListener {

        public Button btnAdd,btnSub,btnMul,btnDiv;
        public TextView tvResult;
        public EditText etNum1,etNum2;
        @Override

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

        }

        private void init() {
            btnAdd=(Button)findViewById(R.id.btnAdd);
            btnSub=(Button)findViewById(R.id.btnSub);
            btnMul=(Button)findViewById(R.id.btnMul);
            btnDiv=(Button)findViewById(R.id.btnDiv);
            etNum1=(EditText)findViewById(R.id.etNum1);
            etNum2=(EditText)findViewById(R.id.etNum2);
            tvResult=(TextView)findViewById(R.id.tvResult);

            btnAdd.setOnClickListener(this);
            btnSub.setOnClickListener(this);
            btnMul.setOnClickListener(this);
            btnDiv.setOnClickListener(this);
        }

        public void onClick(View view)
        {
            String a= String.valueOf(etNum1.getText());
            String b= String.valueOf(etNum1.getText());

            switch (view.getId())
            {
                case R.id.btnAdd:
                    tvResult.setText(String.valueOf(Integer.parseInt(a)+Integer.parseInt(b)));
                    break;
                case R.id.btnSub:
                    tvResult.setText(String.valueOf(Integer.parseInt(a)-Integer.parseInt(b)));
                    break;
                case R.id.btnMul:
                    tvResult.setText(String.valueOf(Integer.parseInt(a)*Integer.parseInt(b)));
                    break;
                case R.id.btnDiv:
                    tvResult.setText(String.valueOf(Integer.parseInt(a)/Integer.parseInt(b)));
                    break;
            }
        }
    }


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