Sample.java

import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.net.URI;


import java.net.URLEncoder;
import java.io.UnsupportedEncodingException;


public class Sample {

    public static void main(String ...args) {
        try {
            HttpClient client = HttpClient.newHttpClient();
            // Note: non utf-8 characters needs to be URL encoded 
            // Ref: https://help.salesforce.com/articleView?id=000316466&language=en_US&type=1&mode=1
            String password = URLEncoder.encode("g#$6-8YHVj", "UTF-8" );

            HttpRequest request = HttpRequest.newBuilder()
                                                .uri(URI.create("https://test.salesforce.com/services/oauth2/token"))
                                                .header("Content-Type", "application/x-www-form-urlencoded")
                                                .POST(HttpRequest.BodyPublishers.ofString("grant_type=password" + "&client_id=3MVG95AcBeaB55lUkwfHx7LMDAyKq1KBRiIG2ZcmRhtXFdRWRqUpjbYVqgbtK7l.Y8yVX3IOvnlx89P03uYRY" + "&client_secret=7F7819F37750B0F4B7C5C37C0194BD2757491775EE1CA39F1AF7CD970C763487" + "&username=test-0ad7fasoqwwc@example.com" + "&password="+password))
                                                .build();
    
            client.sendAsync(request, HttpResponse.BodyHandlers.ofString())
                    .thenApply(HttpResponse::body)
                    .thenAccept(System.out::println)
                    .join();
            
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        }
    }
}