Using the Google Gemini Large Language Model APIs in Java

 1 package com.markwatson.gemini;
 2 
 3 import org.json.JSONObject;
 4 
 5 import java.io.BufferedReader;
 6 import java.io.IOException;
 7 import java.io.InputStreamReader;
 8 import java.io.OutputStream;
 9 import java.net.HttpURLConnection;
10 import java.net.URI;
11 import java.net.URL;
12 import java.net.URLConnection;
13 import java.nio.file.Files;
14 import java.nio.file.Path;
15 import java.nio.file.Paths;
16 
17 public class GeminiCompletions {
18 
19     public static String model = "gemini-3-flash-preview";
20 
21     public static void main(String[] args) throws Exception {
22         String prompt = "How much is 11 + 22?";
23         String completion = getCompletion(prompt);
24         System.out.println("completion: " + completion);
25     }
26 
27     public static String getCompletion(String prompt) throws Exception {
28         String jsonBody = "{\"contents\":[{\"parts\":[{\"text\":\"" + prompt + "\"}]}]}";
29         return executeRequest(jsonBody);
30     }
31 
32     public static String getCompletionWithSearch(String prompt) throws Exception {
33         String jsonBody = "{\"contents\":[{\"parts\":[{\"text\":\"" + prompt
34                 + "\"}]}],\"tools\":[{\"google_search\":{}}]}";
35         return executeRequest(jsonBody);
36     }
37 
38     private static String executeRequest(String jsonBody) throws Exception {
39         String apiKey = System.getenv("GOOGLE_API_KEY");
40         if (apiKey == null || apiKey.isEmpty()) {
41             throw new IOException("GOOGLE_API_KEY environment variable not set.");
42         }
43         URI uri = new URI(
44                 "https://generativelanguage.googleapis.com/v1beta/models/" + model + ":generateContent?key=" + apiKey);
45         URL url = uri.toURL();
46 
47         HttpURLConnection connection = (HttpURLConnection) url.openConnection();
48         connection.setRequestMethod("POST");
49         connection.setRequestProperty("Content-Type", "application/json");
50         connection.setDoOutput(true);
51         try (OutputStream os = connection.getOutputStream()) {
52             byte[] input = jsonBody.getBytes("utf-8");
53             os.write(input, 0, input.length);
54         }
55 
56         StringBuilder response = new StringBuilder();
57         try (BufferedReader br = new BufferedReader(
58                 new InputStreamReader(connection.getInputStream(), "utf-8"))) {
59             String responseLine = null;
60             while ((responseLine = br.readLine()) != null) {
61                 response.append(responseLine.trim());
62             }
63         }
64 
65         connection.disconnect();
66         JSONObject jsonObject = new JSONObject(response.toString());
67         return jsonObject.getJSONArray("candidates").getJSONObject(0).getJSONObject("content").getJSONArray("parts")
68                 .getJSONObject(0).getString("text");
69     }
70     
71     
72     /***
73      * Utilities for using the Gemini API
74      */
75 
76     // read the contents of a file path into a Java string
77     public static String readFileToString(String filePath) throws IOException {
78         Path path = Paths.get(filePath);
79         return new String(Files.readAllBytes(path)).replace("\"", "\\\"");
80     }
81 
82     public static String replaceSubstring(String originalString, String substringToReplace, String replacementString) {
83         return originalString.replace(substringToReplace, replacementString);
84     }
85     public static String promptVar(String prompt0, String varName, String varValue) {
86         String prompt = replaceSubstring(prompt0, varName, varValue);
87         return replaceSubstring(prompt, varName, varValue);
88     }
89 }

 1    public void testCompletion() throws Exception {
 2         String r = GeminiCompletions
 3                 .getCompletion("Translate the following English text to French: 'Hello, how are you?'");
 4         System.out.println("completion: " + r);
 5         assertTrue(true);
 6     }
 7 
 8     public void testTwoShotTemplate() throws Exception {
 9         String input_text = "Mark Johnson enjoys living in Berkeley California at 102 Dunston Street and use mjess@foobar.com for contacting him.";
10         String prompt0 = GeminiCompletions.readFileToString("../prompts/two-shot-2-var.txt");
11         System.out.println("prompt0: " + prompt0);
12         String prompt = GeminiCompletions.promptVar(prompt0, "{input_text}", input_text);
13         System.out.println("prompt: " + prompt);
14         String r = GeminiCompletions.getCompletion(prompt);
15         System.out.println("two shot extraction completion: " + r);
16         assertTrue(true);
17     }
18 
19     public void testSummarization() throws Exception {
20         String input_text = "Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass one-thousandth that of the Sun, but two-and-a-half times that of all the other planets in the Solar System combined. Jupiter is one of the brightest objects visible to the naked eye in the night sky, and has been known to ancient civilizations since before recorded history. It is named after the Roman god Jupiter.[19] When viewed from Earth, Jupiter can be bright enough for its reflected light to cast visible shadows,[ and is on average the third-brightest natural object in the night sky after the Moon and Venus.";
21         String prompt0 = GeminiCompletions.readFileToString("../prompts/summarization_prompt.txt");
22         System.out.println("prompt0: " + prompt0);
23         String prompt = GeminiCompletions.promptVar(prompt0, "{input_text}", input_text);
24         System.out.println("prompt: " + prompt);
25         String r = GeminiCompletions.getCompletion(prompt);
26         System.out.println("summarization completion: " + r);
27         assertTrue(true);
28     }
29 
30     public void testCompletionWithSearch() throws Exception {
31         String prompt = "What is the current stock price of Google?";
32         String r = GeminiCompletions.getCompletionWithSearch(prompt);
33         System.out.println("Search completion: " + r);
34         assertNotNull(r);
35         assertFalse(r.isEmpty());
36     }

 1 $ make
 2 mvn test -q # run test in quiet mode
 3 completion: Here are a few ways to say this in French, depending on the level of formality:
 4 
 5 *   **Formal/Standard:** *Bonjour, comment allez-vous ?*
 6 *   **Informal/Casual:** *Salut, ça va ?*
 7 *   **Neutral:** *Bonjour, comment ça va ?*
 8 
 9 Search completion: As of the market close on December 18, 2025, the stock prices for **Alphabet Inc. (Google)** are as follows:
10 
11 *   **Alphabet Inc. Class A (GOOGL):** ~$302.01
12 *   **Alphabet Inc. Class C (GOOG):** ~$303.29
13 
14 
15 prompt0: Given the two examples below, extract the names, addresses, and email addresses of individuals mentioned later as Process Text. Format the extracted information in JSON, with keys for \"name\", \"address\", and \"email\". If any information is missing, use \"null\" for that field. Be very concise in your output by providing only the output JSON.
16 
17 Example 1:
18 Text: \"John Doe lives at 1234 Maple Street, Springfield. His email is johndoe@example.com.\"
19 Output: 
20 {
21   \"name\": \"John Doe\",
22   \"address\": \"1234 Maple Street, Springfield\",
23   \"email\": \"johndoe@example.com\"
24 }
25 
26 Example 2:
27 Text: \"Jane Smith has recently moved to 5678 Oak Avenue, Anytown. She hasn't updated her email yet.\"
28 Output: 
29 {
30   \"name\": \"Jane Smith\",
31   \"address\": \"5678 Oak Avenue, Anytown\",
32   \"email\": null
33 }
34 
35 Process Text: \"{input_text}\"
36 Output:
37 
38 prompt: Given the two examples below, extract the names, addresses, and email addresses of individuals mentioned later as Process Text. Format the extracted information in JSON, with keys for \"name\", \"address\", and \"email\". If any information is missing, use \"null\" for that field. Be very concise in your output by providing only the output JSON.
39 
40 Example 1:
41 Text: \"John Doe lives at 1234 Maple Street, Springfield. His email is johndoe@example.com.\"
42 Output: 
43 {
44   \"name\": \"John Doe\",
45   \"address\": \"1234 Maple Street, Springfield\",
46   \"email\": \"johndoe@example.com\"
47 }
48 
49 Example 2:
50 Text: \"Jane Smith has recently moved to 5678 Oak Avenue, Anytown. She hasn't updated her email yet.\"
51 Output: 
52 {
53   \"name\": \"Jane Smith\",
54   \"address\": \"5678 Oak Avenue, Anytown\",
55   \"email\": null
56 }
57 
58 Process Text: \"Mark Johnson enjoys living in Berkeley California at 102 Dunston Street and use mjess@foobar.com for contacting him.\"
59 Output:
60 
61 two shot extraction completion: {
62   "name": "Mark Johnson",
63   "address": "102 Dunston Street, Berkeley California",
64   "email": "mjess@foobar.com"
65 }
66 prompt0: Summarize the following text: \"{input_text}\"
67 Output:
68 
69 prompt: Summarize the following text: \"Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass one-thousandth that of the Sun, but two-and-a-half times that of all the other planets in the Solar System combined. Jupiter is one of the brightest objects visible to the naked eye in the night sky, and has been known to ancient civilizations since before recorded history. It is named after the Roman god Jupiter.[19] When viewed from Earth, Jupiter can be bright enough for its reflected light to cast visible shadows,[ and is on average the third-brightest natural object in the night sky after the Moon and Venus.\"
70 Output:
71 
72 summarization completion: Jupiter is the fifth planet from the Sun and the largest in the Solar System. A gas giant named after the Roman god, it is more massive than all other planets combined and stands as the third-brightest natural object in the night sky.