MainActivity.java

public class MainActivity extends AppCompatActivity {

    TextView firstNumTextView;
    TextView secondNumTextView;
    TextView operatorTextView;

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

        firstNumTextView = findViewById(R.id.firstNumTextView);
        secondNumTextView = findViewById(R.id.secondNumTextView);
        operatorTextView = findViewById(R.id.operatorTextView);

        firstNumTextView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                
            }
        });

        secondNumTextView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                
            }
        });

        operatorTextView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
               
            }
        });
    }
}