public class mainactivityactivty extends AppCompatActivity {

    private TextView textView123;
    //private Button button;


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

        //TextView textView123 = (TextView) findViewById(R.id.textView);
        textView123 = (TextView) findViewById(R.id.textView);

        Button button = (Button) findViewById(R.id.button);
        //button = (Button) findViewById(R.id.button);

        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {


                textView123.setText("12345");
            }
        });

    }
}