MainActivity.java

import android.content.Intent;
import android.content.pm.PackageManager;
import android.net.Uri;
import android.os.Bundle;
import android.view.Gravity;
import android.widget.Toast;

import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // open whatsapp through intent
        openWhatsApp("+919865426582");

    }


    private void openWhatsApp(String contactNumber) {

        String url = "https://api.whatsapp.com/send?phone=" + contactNumber;

        Intent intent = new Intent(Intent.ACTION_VIEW);
        intent.setData(Uri.parse(url));
        intent.setPackage("com.whatsapp");
        startActivity(intent);
    }
}