PDF小丑突出显示多个搜索词失败，因为PDF包含图像，彩色文本，复杂图

Question

I´m using PDFClown to highlight the multiple search word in a PDF Document. In many pdf documents which contains colorful images , Complex diagrams, colored text PDFClown throws exception there and cannot highlight the matching words. The code mentioned is working fine for normal or simple Pdf's.

Here is the PDF I used for testing https://drive.google.com/file/d/0B-nuOO6Zsa4rXy1DS2JjX1RnYmM/view?usp=sharing

    public void searchWordInPdf(DocumentMetadata documentMetadata , String searchWord,
                                HttpServletResponse response)throws IOException{
        try {
            byte[] bytes = null;
            org.pdfclown.files.File file =null;
            if (documentMetadata.getProject().getFtpId() != null && documentMetadata.getProject().getFtpId() > 0) {
                FtpServer ftpServer = ftpServerService.getFtpServer(documentMetadata.getProject().getFtpId());

                ByteArrayOutputStream bos = new ByteArrayOutputStream();
                retrievePdfFile(ftpServer, bos, documentMetadata.getFilePath());
                bytes = bos.toByteArray();
                 file = new org.pdfclown.files.File(bytes);
            }else{
                 file = new org.pdfclown.files.File(documentMetadata.getFilePath());
            }
            List<String> matchList = new ArrayList<String>();
            //Pattern regex = Pattern.compile("[^\\s\"']+|\"([^\"]*)\"|'([^']*)'");
            Pattern regex = Pattern.compile("[^\\s\"']+|\"([^\"]*)\"|'([^']*)'");
            Matcher regexMatcher = regex.matcher(searchWord);
            while (regexMatcher.find()) {
                if (regexMatcher.group(1) != null) {
                    // Add double-quoted string without the quotes
                    matchList.add(regexMatcher.group(1));
                } else if (regexMatcher.group(2) != null) {
                    // Add single-quoted string without the quotes
                    matchList.add(regexMatcher.group(2));
                } else {
                    // Add unquoted word
                    matchList.add(regexMatcher.group());
                }
            }

            for (String key : matchList){
            Pattern pattern = Pattern.compile(key, Pattern.CASE_INSENSITIVE);
            // 2. Iterating through the document pages...
            TextExtractor textExtractor = new TextExtractor(true, true);
            for (final Page page : file.getDocument().getPages()) {
                System.out.println("\nScanning page " + (page.getIndex() + 1) + "...\n");

                // 2.1. Extract the page text!
                Map<Rectangle2D, List<ITextString>> textStrings = textExtractor.extract(page);
                // 2.2. Find the text pattern matches!
                final Matcher matcher = pattern.matcher(TextExtractor.toString(textStrings));

                // 2.3. Highlight the text pattern matches!
                textExtractor.filter(
                    textStrings,
                    new TextExtractor.IIntervalFilter() {
                        @Override
                        public boolean hasNext() {
                            if (matcher.find()) {
                                //count++;
                                return true;
                            }
                            return false;
                        }

                        @Override
                        public Interval<Integer> next() {
                            return new Interval<Integer>(matcher.start(), matcher.end());
                        }

                        @Override
                        public void process(
                            Interval<Integer> interval,
                            ITextString match
                        ) {
                            Rectangle2D textBox = null;
                            // Defining the highlight box of the text pattern match...
                            List<Quad> highlightQuads = new ArrayList<Quad>();
                            {
                        /*
                            NOTE: A text pattern match may be split across multiple contiguous lines,
                             so we have to define a distinct highlight box for each text chunk.
                        */

                                for (TextChar textChar : match.getTextChars()) {
                                    Rectangle2D textCharBox = textChar.getBox();
                                    if (textBox == null) {
                                        textBox = (Rectangle2D) textCharBox.clone();
                                    } else {
                                        if (textCharBox.getY() > textBox.getMaxY()) {
                                            highlightQuads.add(Quad.get(textBox));
                                            textBox = (Rectangle2D) textCharBox.clone();
                                        } else {
                                            textBox.add(textCharBox);
                                        }
                                    }
                                }
                                highlightQuads.add(Quad.get(textBox));
                            }
                            // Highlight the text pattern match!
                            new TextMarkup(page, highlightQuads, null, MarkupTypeEnum.Highlight);
                        }

                        @Override
                        public void remove() {
                            throw new UnsupportedOperationException();
                        }
                    }
                );
            }
        }
                String contentType = getContentType(documentMetadata.getFileName());
                if (contentType == null) {
                    contentType = "binary/octet-stream";
                }
            response.setStatus(HttpStatus.OK.value());
            ByteArrayOutputStream output = new ByteArrayOutputStream();
            if(output != null){
                file.save(output, SerializationModeEnum.Standard );
                bytes =  org.springframework.security.crypto.codec.Base64.encode(output.toByteArray());
                response.addHeader("Content-Disposition", "attachment; filename=" + documentMetadata.getFileName());
                response.addHeader("Content-Type", contentType);
                response.getOutputStream().write(bytes);
            }

        } catch (Exception e) {
            e.printStackTrace();
        }
    }

Here is the StackTrace

 java.lang.IllegalArgumentException: Comparison method violates its general contract!
at java.util.TimSort.mergeLo(TimSort.java:777)
at java.util.TimSort.mergeAt(TimSort.java:514)
at java.util.TimSort.mergeCollapse(TimSort.java:439)
at java.util.TimSort.sort(TimSort.java:245)
at java.util.Arrays.sort(Arrays.java:1512)
at java.util.ArrayList.sort(ArrayList.java:1454)
at java.util.Collections.sort(Collections.java:175)
at org.pdfclown.tools.TextExtractor.sort(TextExtractor.java:675)
at org.pdfclown.tools.TextExtractor.extract(TextExtractor.java:306)
at nu.optimise.projectweb.service.DocumentMetadataService.searchWordInPdf(DocumentMetadataService.java:2669)
at nu.optimise.projectweb.service.DocumentMetadataService$$FastClassBySpringCGLIB$$fc6434c2.invoke(<generated>)
at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:204)
at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.invokeJoinpoint(CglibAopProxy.java:720)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:157)
at org.springframework.aop.aspectj.MethodInvocationProceedingJoinPoint.proceed(MethodInvocationProceedingJoinPoint.java:85)
at nu.optimise.projectweb.aop.logging.LoggingAspect.logAround(LoggingAspect.java:51)
at sun.reflect.GeneratedMethodAccessor186.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.springframework.aop.aspectj.AbstractAspectJAdvice.invokeAdviceMethodWithGivenArgs(AbstractAspectJAdvice.java:620)
at org.springframework.aop.aspectj.AbstractAspectJAdvice.invokeAdviceMethod(AbstractAspectJAdvice.java:609)
at org.springframework.aop.aspectj.AspectJAroundAdvice.invoke(AspectJAroundAdvice.java:68)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
at org.springframework.aop.aspectj.AspectJAfterThrowingAdvice.invoke(AspectJAfterThrowingAdvice.java:59)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:99)
at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:281)
at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:96)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:92)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:655)
at nu.optimise.projectweb.service.DocumentMetadataService$$EnhancerBySpringCGLIB$$c3a15a18.searchWordInPdf(<generated>)
at nu.optimise.projectweb.web.rest.DocumentMetadataResource.searchContentPDF(DocumentMetadataResource.java:1026)
at nu.optimise.projectweb.web.rest.DocumentMetadataResource$$FastClassBySpringCGLIB$$bb12eea8.invoke(<generated>)
at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:204)
at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.invokeJoinpoint(CglibAopProxy.java:720)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:157)
at org.springframework.aop.aspectj.MethodInvocationProceedingJoinPoint.proceed(MethodInvocationProceedingJoinPoint.java:85)
at nu.optimise.projectweb.aop.logging.LoggingAspect.logAround(LoggingAspect.java:51)
at sun.reflect.GeneratedMethodAccessor186.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.springframework.aop.aspectj.AbstractAspectJAdvice.invokeAdviceMethodWithGivenArgs(AbstractAspectJAdvice.java:620)
at org.springframework.aop.aspectj.AbstractAspectJAdvice.invokeAdviceMethod(AbstractAspectJAdvice.java:609)
at org.springframework.aop.aspectj.AspectJAroundAdvice.invoke(AspectJAroundAdvice.java:68)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
at org.springframework.aop.aspectj.AspectJAfterThrowingAdvice.invoke(AspectJAfterThrowingAdvice.java:59)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:92)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
at com.ryantenney.metrics.spring.TimedMethodInterceptor.invoke(TimedMethodInterceptor.java:48)
at com.ryantenney.metrics.spring.TimedMethodInterceptor.invoke(TimedMethodInterceptor.java:34)
at com.ryantenney.metrics.spring.AbstractMetricMethodInterceptor.invoke(AbstractMetricMethodInterceptor.java:59)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:655)
at nu.optimise.projectweb.web.rest.DocumentMetadataResource$$EnhancerBySpringCGLIB$$bfe48b3d.searchContentPDF(<generated>)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:221)
at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:136)
at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:110)
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:832)
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:743)
at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:961)
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:895)
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:967)
at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:858)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:843)
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 com.codahale.metrics.servlet.AbstractInstrumentedFilter.doFilter(AbstractInstrumentedFilter.java:104)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.springframework.boot.actuate.autoconfigure.EndpointWebMvcAutoConfiguration$ApplicationContextHeaderFilter.doFilterInternal(EndpointWebMvcAutoConfiguration.java:281)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.springframework.boot.actuate.trace.WebRequestTraceFilter.doFilterInternal(WebRequestTraceFilter.java:115)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:317)
at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:127)
at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:91)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:331)
at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:115)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:331)
at org.springframework.security.web.session.SessionManagementFilter.doFilter(SessionManagementFilter.java:137)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:331)
at org.springframework.security.web.authentication.AnonymousAuthenticationFilter.doFilter(AnonymousAuthenticationFilter.java:112)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:331)
at org.springframework.security.web.servletapi.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:169)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:331)
at org.springframework.security.web.savedrequest.RequestCacheAwareFilter.doFilter(RequestCacheAwareFilter.java:63)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:331)
at nu.optimise.projectweb.security.jwt.JWTFilter.doFilter(JWTFilter.java:43)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:331)
at org.springframework.security.web.authentication.logout.LogoutFilter.doFilter(LogoutFilter.java:121)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:331)
at org.springframework.security.web.header.HeaderWriterFilter.doFilterInternal(HeaderWriterFilter.java:66)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:331)
at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:106)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:331)
at org.springframework.security.web.context.request.async.WebAsyncManagerIntegrationFilter.doFilterInternal(WebAsyncManagerIntegrationFilter.java:56)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:331)
at org.springframework.security.web.FilterChainProxy.doFilterInternal(FilterChainProxy.java:214)
at org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:177)
at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:346)
at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:262)
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:87)
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:77)
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.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:121)
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:106)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:528)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1099)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:670)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1520)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1476)
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)

Answer 1

This is a bug in PDF Clown: During text extraction it uses a custom Comparator implementation which does not completely follow the Comparator contract. In Java 7 and below this was ignored but in Java 8 this results in the exception at hand. If you instruct Java to use the old sorting algorithm, the program runs without exception.

The comparator

This is the faulty comparator

  /**
    Text string position comparator.
   */
  private static class TextStringPositionComparator
    implements Comparator<ITextString>
  {
    /**
      Gets whether the specified boxes lay on the same text line.
    */
    public static boolean isOnTheSameLine(
      Rectangle2D box1,
      Rectangle2D box2
      )
    {
      /*
        NOTE: In order to consider the two boxes being on the same line,
        we apply a simple rule of thumb: at least 25% of a box's height MUST
        lay on the horizontal projection of the other one.
      */
      double minHeight = Math.min(box1.getHeight(), box2.getHeight());
      double yThreshold = minHeight * .75;
      return ((box1.getY() > box2.getY() - yThreshold
          && box1.getY() < box2.getMaxY() + yThreshold - minHeight)
        || (box2.getY() > box1.getY() - yThreshold
          && box2.getY() < box1.getMaxY() + yThreshold - minHeight));
    }

    @Override
    public int compare(
      ITextString textString1,
      ITextString textString2
      )
    {
      Rectangle2D box1 = textString1.getBox();
      Rectangle2D box2 = textString2.getBox();
      if(isOnTheSameLine(box1,box2))
      {
        /*
          [FIX:55:0.1.3] In order not to violate the transitive condition, equivalence on x-axis
          MUST fall back on y-axis comparison.
        */
        int xCompare = Double.compare(box1.getX(), box2.getX());
        if(xCompare != 0)
          return xCompare;
      }
      return Double.compare(box1.getY(), box2.getY());
    }
  }

As indicated by the comment [FIX:55:0.1.3] ... , the author already had encountered issues with sorting. Unfortunately, though, he only fixed a single troublesome situation.

Obviously the isOnTheSameLine test used in compare can very often be the cause of non-transitivity, consider a situation with three ITextString instances A , B , and C :

(This can happen in regular text, eg in a line with first some text in subscript, then some in normal writing, then some in superscript.)

A and B would be considered to be on the same line and also B and C , but not A and C . Thus, the former two pairs will each be compared by x coordinates while the last one will be compared by y coordinates, resulting in a non-transitivity:

• A < B and
• B < C, but
• A > C (PDF Clown uses y coordinates increasing downwards).

The identity condition can also be violated, consider the case of two ITextString instances A and B , both having the same box, ie both having the same dimensions and being printed at the same position (eg to build a symbol out of overlapping letters). compare would return 0 which should only happen if comparing an objects with its equal ("should" because this merely is recommended, not strictly required).

Most often, though, the comparator does sort the text pieces as one considers correct.

A work-around

Before Java 8 built-in Java sorting algorithms did not test whether a Comparator implementation fulfilled the contract. The sort result might not be properly sorted but no exception was thrown by the sorting. (Some later called routines assuming the array to be sorted might fail abysmally, though.)

Java 8, though, uses a different default sorting algorithm which does some sanity checks recognizing some effects of unfulfilled Comparator contracts on the sorting process.

But by using the command line JRE parameter

-Djava.util.Arrays.useLegacyMergeSort=true

you can tell Java 8 to use the old sorting method which does not fail by exception.