What is a lambda expression exactly?

 A lambda expression is a concise way of declaring an interface that contains only one method. For example, an anonymous OnClickListener could have only one method, namely, the onClick method. So you can replace this anonymous OnClickListener with a lambda expression.

import android.os.Bundle;

import android.support.v7.app.AppCompatActivity;

import android.widget.Button;

import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

Button button;

TextView textView;

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

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

button.setOnClickListener(view -> textView.setText(R.string.you_clicked));

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

}

}