XML code :-

<Button
        android:id="@+id/button"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="250dp"
        android:text="+" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="50dp"
        android:paddingLeft="200dp"
        android:textSize="25sp"
        android:text="0" />

Java code :-

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

public class MainActivity extends AppCompatActivity {
    Button btn;
    TextView txt;

    protected int a = 1;

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

        btn = (Button) findViewById(R.id.button);
        txt = (TextView) findViewById(R.id.textView);

    }

    public void display(final int n) {
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                txt.setText(n);
            }
        });
    }