How to exit Application when click on Back Button Android?

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This article is for Beginner and people who want to learn Android .This article is about How to exit Application when click on Back Button Android?
    import android.support.v7.app.AppCompatActivity;
    import android.os.Bundle;
    import android.view.View;
    import android.widget.Button;
    import android.widget.ImageView;
    import android.widget.Toast;


public class MainActivity extends AppCompatActivity {




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



        }
    @Override
    public void onBackPressed()
    {
        finish();
    }

    }