集合排序引发异常

Question

我正在尝试对我的用户分数进行排序。它在本地运行得很好，但当我查看服务器日志时，它会抛出许多异常。所以我尝试将检查设为空并捕获异常，但exception仍然抛出异常。我不确定什么是例外。

Collections.sort(userScores, new Comparator<UserScore>() {
            @Override
            public int compare(UserScore o1, UserScore o2) {

                try {
                    if(o1 == null || o1.getScore() == null) {
                        return 1;
                    }
                    if(o2 == null || o2.getScore() == null) {
                        return -1;
                    }
                    if(o1.getScore().doubleValue() > o2.getScore().doubleValue())
                        return -1;
                    return 1;   
                }catch(Exception ex) {
                    logger.severe(ex.getMessage());
                    return 0;
                }

            }
        });

我的代码抛出，

java.lang.IllegalArgumentException: Comparison method violates its general contract!
        at java.util.TimSort.mergeLo(TimSort.java:777) ~[na:1.8.0_181]
        at java.util.TimSort.mergeAt(TimSort.java:514) ~[na:1.8.0_181]
        at java.util.TimSort.mergeCollapse(TimSort.java:441) ~[na:1.8.0_181]
        at java.util.TimSort.sort(TimSort.java:245) ~[na:1.8.0_181]
        at java.util.Arrays.sort(Arrays.java:1438) ~[na:1.8.0_181]
        at java.util.List.sort(List.java:478) ~[na:1.8.0_181]
        at java.util.Collections.sort(Collections.java:175) ~[na:1.8.0_181]
        at com.tll.treeofwords.game.service.GameService.generateRanking(GameService.java:179) ~[classes!/:0.0.1-SNAPSHOT]
        at com.tll.treeofwords.game.service.GameService.getCurrentGame(GameService.java:76) ~[classes!/:0.0.1-SNAPSHOT]
        at com.tll.treeofwords.game.service.GameService.getCurrentGame(GameService.java:59) ~[classes!/:0.0.1-SNAPSHOT]
        at com.tll.treeofwords.game.service.GameService$$FastClassBySpringCGLIB$$d5f3b9ef.invoke(<generated>) ~[classes!/:0.0.1-SNAPSHOT]
        at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:204) ~[spring-core-4.3.13.RELEASE.jar!/:4.3.13.RELEASE]
        at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.invokeJoinpoint(CglibAopProxy.java:738) ~[spring-aop-4.3.13.RELEASE.jar!/:4.3.13.RELEASE]
        at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:157) ~[spring-aop-4.3.13.RELEASE.jar!/:4.3.13.RELEASE]
        at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:99) ~[spring-tx-4.3.13.RELEASE.jar!/:4.3.13.RELEASE]
        at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:282) ~[spring-tx-4.3.13.RELEASE.jar!/:4.3.13.RELEASE]
        at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:96) ~[spring-tx-4.3.13.RELEASE.jar!/:4.3.13.RELEASE]
        at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179) ~[spring-aop-4.3.13.RELEASE.jar!/:4.3.13.RELEASE]
        at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:673) ~[spring-aop-4.3.13.RELEASE.jar!/:4.3.13.RELEASE]
        at com.tll.treeofwords.game.service.GameService$$EnhancerBySpringCGLIB$$38f6aa6d.getCurrentGame(<generated>) ~[classes!/:0.0.1-SNAPSHOT]
        at com.tll.treeofwords.game.controller.GameController.getGame(GameController.java:59) ~[classes!/:0.0.1-SNAPSHOT]
        at com.tll.treeofwords.game.controller.GameController$$FastClassBySpringCGLIB$$b9539785.invoke(<generated>) ~[classes!/:0.0.1-SNAPSHOT]
        at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:204) ~[spring-core-4.3.13.RELEASE.jar!/:4.3.13.RELEASE]
        at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:669) ~[spring-aop-4.3.13.RELEASE.jar!/:4.3.13.RELEASE]
        at com.tll.treeofwords.game.controller.GameController$$EnhancerBySpringCGLIB$$388a70ef.getGame(<generated>) ~[classes!/:0.0.1-SNAPSHOT]
        at sun.reflect.GeneratedMethodAccessor145.invoke(Unknown Source) ~[na:na]
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_181]
        at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_181]
        at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:205) ~[spring-web-4.3.13.RELEASE.jar!/:4.3.13.RELEASE]
        at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:133) ~[spring-web-4.3.13.RELEASE.jar!/:4.3.13.RELEASE]
        at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:97) ~[spring-webmvc-4.3.13.RELEASE.jar!/:4.3.13.RELEASE]
        at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827) ~[spring-webmvc-4.3.13.RELEASE.jar!/:4.3.13.RELEASE]

Answer 1

您的比较器不能正确处理将两个null视为相等的情况。如果您比较两个对象o1和o2，这两个对象都是null，您的代码将导致：

compare(o1,o2) == 1

还有也叫

compare(o2,o1) == 1

这应该是不可能的，因为o1不能同时比o2大和小。因此，您所看到的错误。

如果o1和o2 (或它们各自的getScore()方法)都为空，则需要修改代码以返回0。

Answer 2

无论传递给被覆盖的compare方法的参数的顺序如何，它都会给出相同的结果。假设我们有o1.getScore().doubleValue() == o2.getScore().doubleValue()。然后是compare(o1, o2) == 1 && compare(o2, o1) == 1。compare的约定是，对于每一对参数o1和o2，必须满足以下条件：compare(o1, o2) + compare(o2, o1) == 0。