Using java.util.List.sort with java.time.LocalDateTime compareTo method

Question

I have a List of users. Each user has a property called LastExtractJob, which is a java.time.LocalDateTime. I want to sort the user List based on LastExtractJob.

Here is my code

allUsers.sort((o1, o2) -> {
    final LocalDateTime o1DT = o1.getLastExtractJob();
    final LocalDateTime o2DT = o2.getLastExtractJob();
    if ((o1DT != null) && (o2DT != null)) {
        return o2DT.compareTo(o1DT);
    }
    return 0;
});

This has been working well for a while, but a new item in my user List is now causing this error to be thrown:

java.lang.IllegalArgumentException: Comparison method violates its general contract!

My user List contains many LastExtractJob null values, which I think is OK as LocalDateTime implements comparable. The objects being compared when the error is thrown are both null.

Can anyone spot the problem here?

EDIT: Full stacktrace:

java.lang.IllegalArgumentException: Comparison method violates its general contract!
at java.util.TimSort.mergeHi(TimSort.java:899)
at java.util.TimSort.mergeAt(TimSort.java:516)
at java.util.TimSort.mergeForceCollapse(TimSort.java:457)
at java.util.TimSort.sort(TimSort.java:254)
at java.util.Arrays.sort(Arrays.java:1512)
at java.util.ArrayList.sort(ArrayList.java:1454)
at com.company.app.AdminServlet.handleViewClientListState(AdminServlet.java:448)
at com.company.app.AdminServlet.doGet(AdminServlet.java:110)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:109)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:81)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.springframework.boot.web.servlet.support.ErrorPageFilter.doFilter(ErrorPageFilter.java:117)
at org.springframework.boot.web.servlet.support.ErrorPageFilter.access$000(ErrorPageFilter.java:61)
at org.springframework.boot.web.servlet.support.ErrorPageFilter$1.doFilterInternal(ErrorPageFilter.java:92)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
at org.springframework.boot.web.servlet.support.ErrorPageFilter.doFilter(ErrorPageFilter.java:110)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:94)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:504)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
at org.apache.catalina.valves.RemoteIpValve.invoke(RemoteIpValve.java:676)
at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:620)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:502)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1132)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:684)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1533)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1489)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:748)

Reopened, as the other Q&A doesn’t even remotely tell how to solve this specific problem. Just having the same exception message, is not sufficient. — Holger, Jul 10 '18 at 16:45

score 3 · Accepted Answer · answered Jul 10 '18 at 16:37

3

You are returning zero, meaning both elements are equal, not only when both elements are null, but also when only one element is null. You should decide, whether null elements ought to be smaller or bigger, and return a value reflecting that order. Or use

allUsers.sort(Comparator.nullsFirst(Comparator.comparing(o -> o.getLastExtractJob()));

instead of your manual implementation. Use nullsLast, if you want to place null at the end of the list.

answered Jul 10 '18 at 16:37

Holger

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@MartínStraus if it looks correct, then upvote the answer ;-). btw Holger this is the first time in my StackOverflow experience that I've seen your answer downvoted... corrected that for you as I don't see a reason why it was downvoted +1 ;-). – Ousmane D. Jul 10 '18 at 16:49
You're right, this was the problem. I tested for when one value only was null and returned the appropriate value. This fixed the problem - thanks! – F_SO_K Jul 10 '18 at 16:50
@Aominè I won't upvote unless i've _checked_ it's correct, which I didn't. That's why I said that it "looks" correct. – Martín Straus Jul 10 '18 at 16:59

Using java.util.List.sort with java.time.LocalDateTime compareTo method

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