当我尝试将新的 List 对象添加到预定义的 List 对象时出现错误答案

【问题标题】：Getting error when I am trying to add a new List object to a pre-defined List object当我尝试将新的 List 对象添加到预定义的 List 对象时出现错误
【发布时间】：2020-07-28 14:26:03
【问题描述】：

我正在使用 Java 8 开发 Spring Boot 版本 2，当我尝试在“createTopic”方法中将新的 List 对象添加到预定义的 List 对象（即 topicList）时出现错误。

TopicService.java

public class TopicService {

    private List<TopicObject>  topicList = Arrays.asList(
            new TopicObject("Java", "Java Course", "Spring boot in java"),
            new TopicObject("Maths", "Mathematics class","12th maths")
    );



    public String createTopic(TopicObject topicObj){

         topicList.add(new TopicObject("Java1", "Java Course1", "Spring boot in java1"));

        return "Request successful"  ;
    }
}

TopicObject.java

package io.javabrains.springBootStarter.controller;

import com.fasterxml.jackson.annotation.JsonProperty;

public class TopicObject {

    @JsonProperty("id")
    private String id;

    @JsonProperty("name")
    private String name;

    @JsonProperty("description")
    private String description;

    public TopicObject() {
    }

    public TopicObject(String id, String name, String description) {
        this.id = id;
        this.name = name;
        this.description = description;
    }


    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 getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

}

错误

2020-04-15 19:52:27.172 错误 15312 --- [nio-8080-exec-2] o.a.c.c.C.[.[.[/].[dispatcherServlet] ：Servlet.service() for 带有路径 [] 的上下文中的 servlet [dispatcherServlet] 引发异常 [请求处理失败；嵌套异常是 java.lang.UnsupportedOperationException] 的根本原因

java.lang.UnsupportedOperationException: null at java.util.AbstractList.add(AbstractList.java:148) ~[na:1.8.0_221] 在 java.util.AbstractList.add(AbstractList.java:108) ~[na:1.8.0_221] 在 io.javabrains.springBootStarter.Service.TopicService.createTopic(TopicService.java:29) 〜[类/：na]在 io.javabrains.springBootStarter.controller.TopicController.createTopic(TopicController.java:31) ~[classes/:na] 在 sun.reflect.NativeMethodAccessorImpl.invoke0(Native 方法）~[na:1.8.0_221] 在 sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_221] 在 sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_221] 在 java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_221] 在 org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:209) ~[spring-web-5.0.13.RELEASE.jar:5.0.13.RELEASE] 在 org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:136) ~[spring-web-5.0.13.RELEASE.jar:5.0.13.RELEASE] 在 org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:102) ~[spring-webmvc-5.0.13.RELEASE.jar:5.0.13.RELEASE] 在 org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:894) ~[spring-webmvc-5.0.13.RELEASE.jar:5.0.13.RELEASE] 在 org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:800) ~[spring-webmvc-5.0.13.RELEASE.jar:5.0.13.RELEASE] 在 org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.0.13.RELEASE.jar:5.0.13.RELEASE] 在 org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:991) ~[spring-webmvc-5.0.13.RELEASE.jar:5.0.13.RELEASE] 在 org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:925) ~[spring-webmvc-5.0.13.RELEASE.jar:5.0.13.RELEASE] 在 org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:981) ~[spring-webmvc-5.0.13.RELEASE.jar:5.0.13.RELEASE] 在 org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:884) ~[spring-webmvc-5.0.13.RELEASE.jar:5.0.13.RELEASE] 在 javax.servlet.http.HttpServlet.service(HttpServlet.java:661) ~[tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:858) ~[spring-webmvc-5.0.13.RELEASE.jar:5.0.13.RELEASE] 在 javax.servlet.http.HttpServlet.service(HttpServlet.java:742) ~[tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) ~[tomcat-embed-websocket-8.5.39.jar:8.5.39] 在 org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) ~[spring-web-5.0.13.RELEASE.jar:5.0.13.RELEASE] 在 org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.13.RELEASE.jar:5.0.13.RELEASE] 在 org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:109) ~[spring-web-5.0.13.RELEASE.jar:5.0.13.RELEASE] 在 org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.13.RELEASE.jar:5.0.13.RELEASE] 在 org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:93) ~[spring-web-5.0.13.RELEASE.jar:5.0.13.RELEASE] 在 org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.13.RELEASE.jar:5.0.13.RELEASE] 在 org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200) ~[spring-web-5.0.13.RELEASE.jar:5.0.13.RELEASE] 在 org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.13.RELEASE.jar:5.0.13.RELEASE] 在 org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.catalina.core.StandardWrapperValve.invoke（StandardWrapperValve.java:199） ~[tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) [tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:493) [tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:137) [tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81) [tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) [tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.catalina.connector.CoyoteAdapter.service（CoyoteAdapter.java:343） [tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:798) [tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:806) [tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1498) [tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-8.5.39.jar:8.5.39] 在 java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [na:1.8.0_221] 在 java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [na:1.8.0_221] 在 org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.5.39.jar:8.5.39] 在 java.lang.Thread.run(Thread.java:748) [na:1.8.0_221]

>不知道用这种方式能不能把新对象加入到“topicList”中。有人可以帮忙吗？

【问题讨论】：

标签： java list java-8 unsupportedoperation

【解决方案1】：

Arrays.asList() 不支持添加新元素。所以初始化新的 ArrayList 并添加新元素。所以不是

private List<TopicObject>  topicList = Arrays.asList(
            new TopicObject("Java", "Java Course", "Spring boot in java"),
            new TopicObject("Maths", "Mathematics class","12th maths")
    );

改成

private List<TopicObject>  topicList = new ArrayList<>(Arrays.asList(
        new TopicObject("Java", "Java Course", "Spring boot in java"),
        new TopicObject("Maths", "Mathematics class", "12th maths")
));

【讨论】：

【解决方案2】：

Arrays.asLit 返回一个不可修改的List 对象。如果您打算向其中添加元素，则需要使用可修改的List 类型，例如ArrayList：

private List<TopicObject>  topicList = new ArrayList<>(Arrays.asList(
        new TopicObject("Java", "Java Course", "Spring boot in java"),
        new TopicObject("Maths", "Mathematics class", "12th maths")
));

【讨论】：