[Java][Client] Generate servers for okhttp-gson (#14179)

* [Java][Client] Generate servers for okhttp-gson

* Update sample tests

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/ApiClient.java

@@ -66,6 +66,58 @@ import org.openapitools.client.auth.OAuthFlow;
 public class ApiClient {
 
     private String basePath = "http://petstore.swagger.io:80/v2";
+    protected List<ServerConfiguration> servers = new ArrayList<ServerConfiguration>(Arrays.asList(
+    new ServerConfiguration(
+      "http://{server}.swagger.io:{port}/v2",
+      "petstore server",
+      new HashMap<String, ServerVariable>() {{
+        put("server", new ServerVariable(
+          "No description provided",
+          "petstore",
+          new HashSet<String>(
+            Arrays.asList(
+              "petstore",
+              "qa-petstore",
+              "dev-petstore"
+            )
+          )
+        ));
+        put("port", new ServerVariable(
+          "No description provided",
+          "80",
+          new HashSet<String>(
+            Arrays.asList(
+              "80",
+              "8080"
+            )
+          )
+        ));
+      }}
+    ),
+    new ServerConfiguration(
+      "https://localhost:8080/{version}",
+      "The local server",
+      new HashMap<String, ServerVariable>() {{
+        put("version", new ServerVariable(
+          "No description provided",
+          "v2",
+          new HashSet<String>(
+            Arrays.asList(
+              "v1",
+              "v2"
+            )
+          )
+        ));
+      }}
+    ),
+    new ServerConfiguration(
+      "https://127.0.0.1/no_variable",
+      "The local server without variables",
+      new HashMap<String, ServerVariable>()
+    )
+  ));
+    protected Integer serverIndex = 0;
+    protected Map<String, String> serverVariables = null;
     private boolean debugging = false;
     private Map<String, String> defaultHeaderMap = new HashMap<String, String>();
     private Map<String, String> defaultCookieMap = new HashMap<String, String>();
@@ -242,6 +294,33 @@ public class ApiClient {
         return this;
     }
 
+    public List<ServerConfiguration> getServers() {
+        return servers;
+    }
+
+    public ApiClient setServers(List<ServerConfiguration> servers) {
+        this.servers = servers;
+        return this;
+    }
+
+    public Integer getServerIndex() {
+        return serverIndex;
+    }
+
+    public ApiClient setServerIndex(Integer serverIndex) {
+        this.serverIndex = serverIndex;
+        return this;
+    }
+
+    public Map<String, String> getServerVariables() {
+        return serverVariables;
+    }
+
+    public ApiClient setServerVariables(Map<String, String> serverVariables) {
+        this.serverVariables = serverVariables;
+        return this;
+    }
+
     /**
      * Get HTTP client
      *
@@ -1294,7 +1373,18 @@ public class ApiClient {
         if (baseUrl != null) {
             url.append(baseUrl).append(path);
         } else {
-            url.append(basePath).append(path);
+            String baseURL;
+            if (serverIndex != null) {
+                if (serverIndex < 0 || serverIndex >= servers.size()) {
+                    throw new ArrayIndexOutOfBoundsException(String.format(
+                    "Invalid index %d when selecting the host settings. Must be less than %d", serverIndex, servers.size()
+                    ));
+                }
+                baseURL = servers.get(serverIndex).URL(serverVariables);
+            } else {
+                baseURL = basePath;
+            }
+            url.append(baseURL).append(path);
         }
 
         if (queryParams != null && !queryParams.isEmpty()) {