Adding 2 number calculator

Question

Im new to android studio and this is my first project. I have created a simple calculator to add 2 numbers theres no error in code but whenever im trying to run the app it always crashes

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


public class MainActivity extends AppCompatActivity {
    EditText et1,et2;
    Button bu1,bu2,bu3,bu4;
    double val,val2,result;
    TextView v1;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
         et1=(EditText)findViewById(R.id.editText);
        et2=(EditText)findViewById(R.id.editText2);
        v1=(TextView)findViewById(R.id.t3);


        bu1=findViewById(R.id.b1);
        bu2=findViewById(R.id.b2);
        bu3=findViewById(R.id.b3);
        bu4=findViewById(R.id.b4);
        val = Double.parseDouble(et1.getText().toString());
        val2 = Double.parseDouble(et2.getText().toString());
        bu1.setOnClickListener(new View.OnClickListener(){
            @Override
            public void onClick(View view) {
                result=val+val2;

                v1.setText(Double.toString(result));
            }
        });

    }