带列表的TreeMap

Question

“”“

@Override
    public TreeMap<String, List<String>> getCityWithPincode() {
        List<User> listOfUser = findAllUser();

        TreeMap<String, List<String>> tm = new TreeMap<>();

        for(User user : listOfUser) {
            
            if(tm.isEmpty()) {
                tm.put(user.getCity(), Arrays.asList(user.getPincode()));
            }
            else {
                
                if(tm.containsKey(user.getCity())) {
                    List<String> list = tm.get(user.getCity());
                    
                    list.add(user.getPincode());
                    
                    tm.put(user.getCity(), list);
                }
                else {
                    tm.put(user.getCity(), Arrays.asList(user.getPincode()));
                }
                
            }
            
        }

        return tm;
    }

“”“

我试着收集城市与那里的城市，并在一个城市有许多平码的权利，所以我已经创建了树状地图收集这些细节和关键是一个城市和价值是一个小码列表，但这个不起作用.

错误是：

{
  "timestamp": "2022-11-27T06:37:21.712+00:00",
  "status": 500,
  "error": "Internal Server Error",
/*
* 提示：该行代码过长，系统自动注释不进行高亮。一键复制会移除系统注释 
* "trace": "java.lang.UnsupportedOperationException\r\n\tat java.base/java.util.AbstractList.add(AbstractList.java:153)\r\n\tat java.base/java.util.AbstractList.add(AbstractList.java:111)\r\n\tat com.admin.panel.service.UserServiceImpl.getCityWithPincode(UserServiceImpl.java:182)\r\n\tat com.admin.panel.controller.UserController.getCityWithPincode(UserController.java:167)\r\n\tat java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)\r\n\tat java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)\r\n\tat java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)\r\n\tat java.base/java.lang.reflect.Method.invoke(Method.java:568)\r\n\tat org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:205)\r\n\tat org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:150)\r\n\tat org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:117)\r\n\tat org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:895)\r\n\tat org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:808)\r\n\tat org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87)\r\n\tat org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1071)\r\n\tat org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:964)\r\n\tat org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006)\r\n\tat org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898)\r\n\tat javax.servlet.http.HttpServlet.service(HttpServlet.java:670)\r\n\tat org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883)\r\n\tat javax.servlet.http.HttpServlet.service(HttpServlet.java:779)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:227)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)\r\n\tat org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)\r\n\tat org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100)\r\n\tat org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)\r\n\tat org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93)\r\n\tat org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)\r\n\tat org.springframework.boot.actuate.metrics.web.servlet.WebMvcMetricsFilter.doFilterInternal(WebMvcMetricsFilter.java:96)\r\n\tat org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)\r\n\tat org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201)\r\n\tat org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)\r\n\tat org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:197)\r\n\tat org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97)\r\n\tat org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541)\r\n\tat org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:135)\r\n\tat org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)\r\n\tat org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78)\r\n\tat org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:360)\r\n\tat org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:399)\r\n\tat org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)\r\n\tat org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:893)\r\n\tat org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1789)\r\n\tat org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)\r\n\tat org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191)\r\n\tat org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659)\r\n\tat org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)\r\n\tat java.base/java.lang.Thread.run(Thread.java:833)\r\n",
*/
  "message": "No message available",
  "path": "/user/citywithpincode/cal"
}

Answer 1

您应该使用new ArrayList<>()而不是Arrays.asList()。不能将元素添加到Arrays.asList()返回的列表中。

您可以使用Map.computeIfAbsent()简化代码。

public TreeMap<String, List<String>> getCityWithPincode() {
    List<User> listOfUser = findAllUser();

    TreeMap<String, List<String>> tm = new TreeMap<>();

    for(User user : listOfUser) {
        tm.computeIfAbsent(user.getCity(), k -> new ArrayList<>()).add(user.getPincode());
    }
    return tm;
}

Answer 2

查看堆栈跟踪，AbstractList上的AbstractList方法抛出一个UnsupportedOperationException。

您不能将一个新元素添加到Arrays.asList()返回的列表中，因为它由一个数组支持，该数组封装在调用asList()时作为参数提供的元素周围，因此不支持结构修改。

要么创建一个您选择的空列表并添加到其中，要么用一个可变列表包装Arrays.asList调用，比如一个ArrayList

tm.put(user.getCity(), new ArrayList<>(Arrays.asList(user.getPincode())));

Answer 3

基于Stream的解决方案：

用户列表( User::getCity

• TreeMap::new

• )用户列表

• 使用Collectors.groupingBy()作为密钥提取器，Supplier<Map>作为地图工厂，Collectors.mapping作为下游收集器：
• 获取用户所在的城市中的一个关键的
• -创建排序地图的实例<代码>H 214<代码>H 115帮助将用户映射到其弹码并存储列表：Collectors.mapping(User::getPinCode, Collectors.toList())

@Override
public TreeMap<String, List<String>> getCityWithPincode() {
    return findAllUser()
            .stream()
            .collect(Collectors.groupingBy(
                User::getCity,   // key - city name
                TreeMap::new,    // create TreeMap instance
                Collectors.mapping(
                    User::getPinCode, Collectors.toList()
                )               // List<String> pin codes
            ));
}