title: java.lang.UnsupportedOperationException adding to a list
body: <p>I have some really strange problem. I want to call the method called <code>create</code> but unfortunately the server throws error, but I don't know why.
  Here is my sample REST controller</p>
  
  <pre><code>import org.springframework.http.MediaType;
  import org.springframework.web.bind.annotation.*;
  import pl.toby.model.User;
  
  import java.util.Arrays;
  import java.util.List;
  import java.util.stream.Collectors;
  
  @RestController
  @RequestMapping("/index")
  public class HelloController {
  
  private List&lt;User&gt; users;
  
  public HelloController() {
      users = Arrays.asList(
              new User(1, "Jan", "Kowalski"),
              new User(2, "Anna", "Nowak"),
              new User(3, "Andrzej", "Przybylski"),
              new User(4, "Maciej", "Adamczyk"),
              new User(5, "Jacek", "Król")
      );
  }
  
  // here is method, that I can create user
  // i call it with /index/create request URI
  @RequestMapping(value = "/create", method = RequestMethod.POST, produces = MediaType.APPLICATION_JSON_UTF8_VALUE)
  public List&lt;User&gt; create(@RequestBody User user) {
      users.add(user);
  
      return users;
  }
  </code></pre>
  
  <p>}</p>
  
  <p>And I cannot add user, because server throws the "Internal server error", but I don't know, why.</p>
  
  <pre><code>{
    "timestamp": 1493548738275,
    "status": 500,
    "error": "Internal Server Error",
    "exception": "java.lang.UnsupportedOperationException",
    "message": "No message available",
    "path": "/index/create"
  }
  </code></pre>
  
  <p>EDIT</p>
  
  <p>Ops. I forgot it. Here is my Tomcat error Log</p>
  
  <pre><code>2017-04-30 12:53:42.033 ERROR 26448 --- [nio-8020-exec-4] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.UnsupportedOperationException] with root cause
  
  java.lang.UnsupportedOperationException: null
      at java.util.AbstractList.add(AbstractList.java:148) ~[na:1.8.0_102-ea]
      at java.util.AbstractList.add(AbstractList.java:108) ~[na:1.8.0_102-ea]
      at pl.toby.HelloController.create(HelloController.java:40) ~[classes/:na]
      at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_102-ea]
      at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_102-ea]
      at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_102-ea]
      at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_102-ea]
      at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:205) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:133) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:97) ~[spring-webmvc-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827) ~[spring-webmvc-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738) ~[spring-webmvc-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85) ~[spring-webmvc-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:963) ~[spring-webmvc-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:897) ~[spring-webmvc-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970) ~[spring-webmvc-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:872) ~[spring-webmvc-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at javax.servlet.http.HttpServlet.service(HttpServlet.java:661) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846) ~[spring-webmvc-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at javax.servlet.http.HttpServlet.service(HttpServlet.java:742) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) ~[tomcat-embed-websocket-8.5.14.jar:8.5.14]
      at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.springframework.boot.web.filter.ApplicationContextHeaderFilter.doFilterInternal(ApplicationContextHeaderFilter.java:55) ~[spring-boot-1.5.3.RELEASE.jar:1.5.3.RELEASE]
      at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.springframework.boot.actuate.trace.WebRequestTraceFilter.doFilterInternal(WebRequestTraceFilter.java:110) ~[spring-boot-actuator-1.5.3.RELEASE.jar:1.5.3.RELEASE]
      at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:105) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:81) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:197) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.springframework.boot.actuate.autoconfigure.MetricsFilter.doFilterInternal(MetricsFilter.java:106) ~[spring-boot-actuator-1.5.3.RELEASE.jar:1.5.3.RELEASE]
      at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.3.8.RELEASE.jar:4.3.8.RELEASE]
      at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198) ~[tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) [tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:478) [tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140) [tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:80) [tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) [tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) [tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:799) [tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:861) [tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1455) [tomcat-embed-core-8.5.14.jar:8.5.14]
      at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-8.5.14.jar:8.5.14]
      at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [na:1.8.0_102-ea]
      at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [na:1.8.0_102-ea]
      at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.5.14.jar:8.5.14]
      at java.lang.Thread.run(Thread.java:745) [na:1.8.0_102-ea]
  </code></pre>