Please be patient as this is my first time learning about how to use web services. 

I am trying to build a simple system that when given a parameter (String id), the web service returns a JSON object. I am somewhat following this codebase as a guide <https://o7planning.org/en/11207/simple-crud-example-with-java-restful-web-service>

Problem: When I run the service and go the URL <http://localhost:8080/RESTfulServiceLIT_war_exploded/users/>{id}, I receive a NullPointer exception in the glassfish log. It should simply display the JSON object relative to the user object returned from the id.

Here is the UserDAO class code

```
import java.util.*;

public class UserDAO {
    private static final Map<String, User> userMap = new HashMap<String, User>();

    static {
        initUsers();
    }

    private static void initUsers() {
        User u1 = new User("K1", "Student 1", "Student");
        User u2 = new User("K2", "Student 2", "Student");
        User u3 = new User("K3", "Student 3", "Student");
        User u4 = new User("S1", "Staff 1", "Staff");
        User u5 = new User("S2", "Staff 2", "Staff");
        User u6 = new User("S3", "Staff 3", "Staff");

        userMap.put(u1.getId(), u1);
        userMap.put(u2.getId(), u2);
        userMap.put(u3.getId(), u3);
        userMap.put(u4.getId(), u4);
        userMap.put(u5.getId(), u5);
        userMap.put(u6.getId(), u6);
    }

    public static User getUser(String id) {
        return userMap.get(id);
    }

    public static List<User> getAllUsers() {
        Collection<User> c = userMap.values();
        List<User> list = new ArrayList<User>();
        list.addAll(c);
        return list;
    }

    List<User> list;

}

```

Here is the Main class that has the rest service

```
import javax.print.attribute.standard.Media;
import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.PathParam;
import javax.ws.rs.Produces;
import java.util.ArrayList;
import java.util.List;
import javax.ws.rs.core.MediaType;

// The Java class will be hosted at the URI path "/helloworld"
@Path("/users")

public class Main {

    @GET
    @Produces({MediaType.APPLICATION_JSON, MediaType.APPLICATION_XML})
    @Path("/{id}")
    public User getUser(@PathParam("id") String id) {
        return UserDAO.getUser(id);
    }

//    @GET
//    @Produces({MediaType.APPLICATION_JSON, MediaType.APPLICATION_XML})
//    @Path{"/all"}
//    public List<User> getUsers_JSON(){
//        List<User> listOfUsers = UserDAO.getAllUsers();
//        return listOfUsers;
//    }

}

```

Here is the server log

```
[2018-05-22T20:09:17.937+0100] [glassfish 4.1] [WARNING] [] [javax.enterprise.web] [tid: _ThreadID=35 _ThreadName=http-listener-1(3)] [timeMillis: 1527016157937] [levelValue: 900] [[
  StandardWrapperValve[MyApplication]: Servlet.service() for servlet MyApplication threw exception
java.lang.NullPointerException
    at org.eclipse.persistence.jaxb.rs.MOXyJsonProvider.writeTo(MOXyJsonProvider.java:944)
    at org.glassfish.jersey.message.internal.WriterInterceptorExecutor$TerminalWriterInterceptor.invokeWriteTo(WriterInterceptorExecutor.java:265)
    at org.glassfish.jersey.message.internal.WriterInterceptorExecutor$TerminalWriterInterceptor.aroundWriteTo(WriterInterceptorExecutor.java:250)
    at org.glassfish.jersey.message.internal.WriterInterceptorExecutor.proceed(WriterInterceptorExecutor.java:162)
    at org.glassfish.jersey.server.internal.JsonWithPaddingInterceptor.aroundWriteTo(JsonWithPaddingInterceptor.java:106)
    at org.glassfish.jersey.message.internal.WriterInterceptorExecutor.proceed(WriterInterceptorExecutor.java:162)
    at org.glassfish.jersey.server.internal.MappableExceptionWrapperInterceptor.aroundWriteTo(MappableExceptionWrapperInterceptor.java:86)
    at org.glassfish.jersey.message.internal.WriterInterceptorExecutor.proceed(WriterInterceptorExecutor.java:162)
    at org.glassfish.jersey.message.internal.MessageBodyFactory.writeTo(MessageBodyFactory.java:1130)
    at org.glassfish.jersey.server.ServerRuntime$Responder.writeResponse(ServerRuntime.java:683)
    at org.glassfish.jersey.server.ServerRuntime$Responder.processResponse(ServerRuntime.java:424)
    at org.glassfish.jersey.server.ServerRuntime$Responder.process(ServerRuntime.java:414)
    at org.glassfish.jersey.server.ServerRuntime$2.run(ServerRuntime.java:312)
    at org.glassfish.jersey.internal.Errors$1.call(Errors.java:271)
    at org.glassfish.jersey.internal.Errors$1.call(Errors.java:267)
    at org.glassfish.jersey.internal.Errors.process(Errors.java:315)
    at org.glassfish.jersey.internal.Errors.process(Errors.java:297)
    at org.glassfish.jersey.internal.Errors.process(Errors.java:267)
    at org.glassfish.jersey.process.internal.RequestScope.runInScope(RequestScope.java:317)
    at org.glassfish.jersey.server.ServerRuntime.process(ServerRuntime.java:292)
    at org.glassfish.jersey.server.ApplicationHandler.handle(ApplicationHandler.java:1139)
    at org.glassfish.jersey.servlet.WebComponent.service(WebComponent.java:460)
    at org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:386)
    at org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:334)
    at org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:221)
    at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1682)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:318)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:160)
    at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:734)
    at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:673)
    at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:99)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:174)
    at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:416)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:283)
    at com.sun.enterprise.v3.services.impl.ContainerMapper$HttpHandlerCallable.call(ContainerMapper.java:459)
    at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:167)
    at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java:206)
    at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java:180)
    at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java:235)
    at org.glassfish.grizzly.filterchain.ExecutorResolver$9.execute(ExecutorResolver.java:119)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeFilter(DefaultFilterChain.java:283)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeChainPart(DefaultFilterChain.java:200)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.execute(DefaultFilterChain.java:132)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.process(DefaultFilterChain.java:111)
    at org.glassfish.grizzly.ProcessorExecutor.execute(ProcessorExecutor.java:77)
    at org.glassfish.grizzly.nio.transport.TCPNIOTransport.fireIOEvent(TCPNIOTransport.java:536)
    at org.glassfish.grizzly.strategies.AbstractIOStrategy.fireIOEvent(AbstractIOStrategy.java:112)
    at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.run0(WorkerThreadIOStrategy.java:117)
    at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.access$100(WorkerThreadIOStrategy.java:56)
    at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy$WorkerThreadRunnable.run(WorkerThreadIOStrategy.java:137)
    at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:591)
    at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java:571)
    at java.lang.Thread.run(Thread.java:748)
]]

```

User class 

```
public class User {

    private String id;
    private String name;
    private String type;

    User(){}

    User(String id, String name, String type){
        this.setId(id);
        this.setName(name);
        this.setType(type);
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }
}

```