import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

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

        Button btnFirst = findViewById(R.id.btn_first);

//      With Lambda Expressions
        btnFirst.setOnClickListener(view -> Toast.makeText(MainActivity.this, "First Button Clicked", Toast.LENGTH_SHORT).show());

    }
}