protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        b1 = findViewById(R.id.button1); b1.setOnClickListener(this);
        b2 = findViewById(R.id.button2); b2.setOnClickListener(this);
        b3 = findViewById(R.id.button3); b3.setOnClickListener(this);
        b4 = findViewById(R.id.button4); b4.setOnClickListener(this);

        o1=findViewById(R.id.firstV);
        o2=findViewById(R.id.secondV);

        t = findViewById(R.id.txv);
}

public void onClick(View v){
        int result=0;
        if (v.getId() == R.id.button1) { result = Integer.valueOf(o1.getText().toString()) + Integer.valueOf(o2.getText().toString()); }
        if (v.getId() == R.id.button2) { result = Integer.valueOf(o1.getText().toString()) - Integer.valueOf(o2.getText().toString()); }
        if (v.getId() == R.id.button3) { result = Integer.valueOf(o1.getText().toString()) * Integer.valueOf(o2.getText().toString()); }
        if (v.getId() == R.id.button4) { result = Integer.valueOf(o1.getText().toString()) / Integer.valueOf(o2.getText().toString()); }
        t.setText("" + result);
}