當我嘗試將新列表 object 添加到預定義列表 object 時出現錯誤

Question

我正在使用 Spring 啟動版本 2，使用 Java 8 並且當我嘗試將新列表 object 添加到預定義的列表對象時（即，主題列表出現錯誤）。

**

主題服務.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;
    }

}

**

錯誤

**

2020-04-15 19:52:27.172 ERROR 15312 --- [nio-8080-exec-2] oac.c.C.[.[.[/].[dispatcherServlet]: Servlet.service() for servlet [帶有路徑 [] 的上下文中的 dispatcherServlet] 引發異常 [請求處理失敗； 嵌套異常是 java.lang.UnsupportedOperationException] 根本原因

java.lang.UnsupportedOperationException: null at java.util.AbstractList.add(AbstractList.java:148) ~[na:1.8.0_221] at java.util.AbstractList.add(AbstractList.java:108) ~[na:1.8 .0_221] at io.javabrains.springBootStarter.Service.TopicService.createTopic(TopicService.java:29) ~[classes/:na] at io.javabrains.springBootStarter.controller.TopicController.createTopic(TopicController.java:31) ~[ classes/:na] 在 sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_221] 在 sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_221] 在 sun .reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMe thodAccessorImpl.java:43) ~[na:1.8.0_221] at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_221] at org.springframework.web.method.support. InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:209) ~[spring-web-5.0.13.RELEASE.jar:5.0.13.RELEASE] at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:136 ) ~[spring-web-5.0.13.RELEASE.jar:5.0.13.RELEASE] at 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.sprin gframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:894) ~[spring-webmvc-5.0.13.RELEASE.jar:5.0.13.RELEASE] at org.springframework.web.servlet .mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:800) ~[spring-webmvc-5.0.13.RELEASE.jar:5.0.13.RELEASE] at org.springframework.web.servlet.mvc.method. AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.0.13.RELEASE.jar:5.0.13.RELEASE] at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:991) ~ [spring-webmvc-5.0.13.RE LEASE.jar:5.0.13.RELEASE] at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:925) ~[spring-webmvc-5.0.13.RELEASE.jar:5.0.13.RELEASE] at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:981) ~[spring-webmvc-5.0.13.RELEASE.jar:5.0.13.RELEASE] at org.springframework.web.servlet.FrameworkServlet.doPost (FrameworkServlet.java:884) ~[spring-webmvc-5.0.13.RELEASE.jar:5.0.13.RELEASE] at javax.servlet.http.HttpServlet.service(HttpServlet.java:661) ~[tomcat-embed- core-8.5.39.jar:8.5.39] 在 org.springframework.Z2567A5EC9705EB7AC2C984033 E06189DZ.servlet.FrameworkServlet.service(FrameworkServlet.java:858) ~[spring-webmvc-5.0.13.RELEASE.jar:5.0.13.RELEASE] at javax.servlet.http.HttpServlet.service(HttpServlet.java:742 ) ~[tomcat-embed-core-8.5.39.jar:8.5.39] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-8.5.39. jar:8.5.39] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.39.jar:8.5.39] at org.apache.tomcat. websocket.server.WsFilter.doFilter(WsFilter.Z93F725A07423FE1C889F448B33D2 1F46Z:52) ~[tomcat-embed-websocket-8.5.39.jar:8.5.39] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5 .39.jar:8.5.39] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.39.jar:8.5.39] at org.springframework .web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) ~[spring-web-5.0.13.RELEASE.jar:5.0.13.RELEASE] at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter. java:107) ~[spring-web-5.0.13.RELEASE.jar:5.0。 13.RELEASE] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.39.jar:8.5.39] at org.apache.catalina.core. ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.39.jar:8.5.39] at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:109) ~[spring -web-5.0.13.RELEASE.jar:5.0.13.RELEASE] at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.13.RELEASE.jar: 5.0.13.RELEASE] 在 org.apache.catalina.c ore.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.39.jar:8.5.39] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~ [tomcat-embed-core-8.5.39.jar:8.5.39] at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:93) ~[spring-web-5.0.13.RELEASE.jar: 5.0.13.RELEASE] at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.13.RELEASE.jar:5.0.13.RELEASE] at org.apache. catalina.core.ApplicationFilterChain.internalDoFilter（ApplicationFilterChain.Z9 3F725A07423FE1C889F448B33D21F46Z:193) ~[tomcat-embed-core-8.5.39.jar:8.5.39] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5 .39.jar:8.5.39] at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200) ~[spring-web-5.0.13.RELEASE.jar:5.0.13.RELEASE] at org .springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.13.RELEASE.jar:5.0.13.RELEASE] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter( ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.39.Z 68995FCBF432492D15484D04A9D2AC40Z:8.5.39] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.39.jar:8.5.39] at org.apache.catalina. core.StandardWrapperValve.invoke(StandardWrapperValve.java:199) ~[tomcat-embed-core-8.5.39.jar:8.5.39] at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) [ tomcat-embed-core-8.5.39.jar:8.5.39] at 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(StandardHos tValve.java:137) [tomcat-embed-core-8.5.39.jar:8.5.39] at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81) [tomcat-embed-core-8.5 .39.jar:8.5.39] at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) [tomcat-embed-core-8.5.39.jar:8.5.39] at org.apache. catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343) [tomcat-embed-core-8.5.39.jar:8.5.39] at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:798) [tomcat-embed-core-8.5.39.jar:8.5.39] 在 org.apache.coyote.Abstra ctProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-8.5.39.jar:8.5.39] at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:806) [tomcat-embed -core-8.5.39.jar:8.5.39] at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1498) [tomcat-embed-core-8.5.39.jar:8.5 .39] at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-8.5.39.jar:8.5.39] at java.util.concurrent.ThreadPoolExecutor. runWorker(ThreadPoolExecutor.java:1149) [na :1.8.0_221] at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [na:1.8.0_221] at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread. java:61) [tomcat-embed-core-8.5.39.jar:8.5.39] at java.lang.Thread.run(Thread.java:748) [na:1.8.0_221]

>不知道用這種方式能不能把新的object加入到“topicList”中。 有人可以幫忙嗎？

Answer 1

Arrays.asLit返回一個不可修改的List object。 如果您打算向其中添加元素，則需要使用可修改的List類型，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")
));

Answer 2

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

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")
));