When catch doesn't actually catch anything

Bob Stout :

I had a program crash because of bad data stored in a database recently. This confused me, because I thought I had a catch to prevent this.

The intent of the following code is to compare employee badge numbers and sort them. If there's an error, return -1 and soldier on -- don't stop because one of several thousand badge numbers is wrong:

public int compare(Employee t, Employee t1) {
    Integer returnValue = -1;
    try {
        Integer tb = Integer.parseInt(t.getBadgeNumber());
        Integer t1b = Integer.parseInt(t1.getBadgeNumber());
        returnValue = tb.compareTo(t1b);
    } catch (Exception e) {
        returnValue = -1;//useless statement, I know.
    }
    return returnValue;
}

When the bad badge number hit (as t in this case), I got an "java.lang.IllegalArgumentException: Comparison method violates its general contract!" error instead of returning the -1 in the catch.

What don't I understand about the catch here?

The full stacktrace:

16-May-2018 14:28:53.496 SEVERE [http-nio-8084-exec-601] org.apache.catalina.core.StandardWrapperValve.invoke Servlet.service() for servlet [RequestServlet] in context with path [/AppearanceRequest] threw exception
 java.lang.IllegalArgumentException: Comparison method violates its general contract!
at java.util.TimSort.mergeHi(TimSort.java:868)
at java.util.TimSort.mergeAt(TimSort.java:485)
at java.util.TimSort.mergeForceCollapse(TimSort.java:426)
at java.util.TimSort.sort(TimSort.java:223)
at java.util.TimSort.sort(TimSort.java:173)
at java.util.Arrays.sort(Arrays.java:659)
at java.util.Collections.sort(Collections.java:217)
at org.bcso.com.appearancerequest.html.NotifierHTML.getHTML(NotifierHTML.java:363)
at org.bcso.com.appearancerequest.AppearanceRequestServlet.processRequest(AppearanceRequestServlet.java:96)
at org.bcso.com.appearancerequest.AppearanceRequestServlet.doGet(AppearanceRequestServlet.java:565)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:618)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:725)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:301)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:393)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:219)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:503)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:136)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:74)
at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:610)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:516)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1015)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:652)
at org.apache.coyote.http11.Http11NioProtocol$Http11ConnectionHandler.process(Http11NioProtocol.java:222)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1575)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1533)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)

The calling code:

    List<Employee> employeeList = DatabaseUtil.getEmployees();
    Collections.sort(employeeList, new BadgeComparator());

rgettman :

The exception (whatever it was) was caught by catch (Exception e). You didn't log this exception, so you don't know what it was. You should log it somehow so you know what really happened.

The problem occurs when you return -1. This allows for the possibility of inconsistent ordering, which Java's current sorting algorithm sometimes catches. In short, returning -1 on an error means that you are asserting that both a < b and b < a are true, because the exception will be caught in both cases. This is logically incorrect. The sorting algorithm detects this and throws the IllegalArgumentException. Note that the compare method is not in your stack trace; it's the call to Collections.sort.

In addition to logging the exception, handle it before you even get to the comparison step in your program. If you have to parse the string as an integer, do that when creating the Employee objects, so that the validation occurs before you even get to the sorting step in your program. A Comparator shouldn't have to validate data; it should only compare the data.