java.lang.IllegalStateException in NCXChecker

[tofi86]

Happens with test epub Issue21.epub from the testsuite within this repository with epubcheck 4.0.1:

→ java -Duser.language=en -jar epubcheck.jar src/test/resources/20/epub/Issue21.epub 
Validating using EPUB version 2.0.1 rules.
WARNING(OPF-055): ../src/test/resources/20/epub/Issue21.epub/OEBPS/content.opf(4,13): title tag is empty.
ERROR(OPF-054): ../src/test/resources/20/epub/Issue21.epub/OEBPS/content.opf(9,12): Date value '' is not valid as per http://www.w3.org/TR/NOTE-datetime:zero-length string.
ERROR(OPF-050): ../src/test/resources/20/epub/Issue21.epub/OEBPS/content.opf(22,75): TOC attribute references resource with non-NCX mime type; 'application/x-dtbncx+xml' is expected.
java.lang.IllegalStateException
    at com.google.common.base.Preconditions.checkState(Preconditions.java:133)
    at com.adobe.epubcheck.ncx.NCXChecker.<init>(NCXChecker.java:44)
    at com.adobe.epubcheck.ncx.NCXCheckerFactory.newInstance(NCXCheckerFactory.java:35)
    at com.adobe.epubcheck.opf.OPFChecker.checkItemContent(OPFChecker.java:420)
    at com.adobe.epubcheck.opf.OPFChecker.runChecks(OPFChecker.java:157)
    at com.adobe.epubcheck.ocf.OCFChecker.runChecks(OCFChecker.java:300)
    at com.adobe.epubcheck.api.EpubCheck.doValidate(EpubCheck.java:215)
    at com.adobe.epubcheck.tool.EpubChecker.validateFile(EpubChecker.java:160)
    at com.adobe.epubcheck.tool.EpubChecker.processFile(EpubChecker.java:494)
    at com.adobe.epubcheck.tool.EpubChecker.run(EpubChecker.java:254)
    at com.adobe.epubcheck.tool.Checker.main(Checker.java:31)
epubcheck completed

[santoch]

Related to this bug:
There are many cases where the Preconditions.checkState method is called.
These are essentially asserts that halt the execution of the process if the condition is not met.
I think we should consider changing these to non-halting checks with error messages when they are due to mis-configured epub content. I would think that the null checks are ok, as long as the nulls are NOT expected (ie, looking something up but failing to find it may be an expected result, so we would NOT want to assert and halt in that case)

Here's the list of assertions besides the without the null checks:

./src/main/java/com/adobe/epubcheck/ocf/OCFChecker.java:104: Preconditions.checkState(context.ocf.isPresent());
./src/main/java/com/adobe/epubcheck/nav/NavChecker.java:77: Preconditions.checkState("application/xhtml+xml".equals(context.mimeType));
./src/main/java/com/adobe/epubcheck/dtbook/DTBookChecker.java:44: Preconditions.checkState("application/x-dtbook+xml".equals(context.mimeType));
./src/main/java/com/adobe/epubcheck/opf/MetadataSet.java:473: Preconditions.checkArgument(meta.getId().isPresent());
./src/main/java/com/adobe/epubcheck/opf/OPFHandler.java:678: Preconditions.checkState(items == null);
./src/main/java/com/adobe/epubcheck/opf/ResourceCollection.java:233: Preconditions.checkState(roles != null);
./src/main/java/com/adobe/epubcheck/ncx/NCXChecker.java:44: Preconditions.checkState("application/x-dtbncx+xml".equals(context.mimeType));
./src/main/java/com/adobe/epubcheck/overlay/OverlayChecker.java:45: Preconditions.checkState("application/smil+xml".equals(context.mimeType));
./src/main/java/com/adobe/epubcheck/dict/SearchKeyMapChecker.java:45: Preconditions.checkState("application/vnd.epub.search-key-map+xml".equals(context.mimeType));

NotNull checks:
./src/main/java/com/adobe/epubcheck/ocf/OCFPackage.java:35: Preconditions.checkNotNull(reporter);
./src/main/java/com/adobe/epubcheck/ocf/OCFPackage.java:52: Preconditions.checkNotNull(reporter);
./src/main/java/com/adobe/epubcheck/api/EPUBLocation.java:45: Preconditions.checkNotNull(path);
./src/main/java/com/adobe/epubcheck/api/FeatureReport.java:95: Preconditions.checkNotNull(name);
./src/main/java/com/adobe/epubcheck/api/FeatureReport.java:96: Preconditions.checkNotNull(location);
./src/main/java/com/adobe/epubcheck/dtbook/DTBookHandler.java:150: return new URI(Preconditions.checkNotNull(uri).trim());
./src/main/java/com/adobe/epubcheck/vocab/Property.java:74: Preconditions.checkNotNull(clazz);
./src/main/java/com/adobe/epubcheck/vocab/VocabUtil.java:104: Preconditions.checkNotNull(vocabs);
./src/main/java/com/adobe/epubcheck/vocab/VocabUtil.java:105: Preconditions.checkNotNull(report);
./src/main/java/com/adobe/epubcheck/vocab/VocabUtil.java:106: Preconditions.checkNotNull(location);
./src/main/java/com/adobe/epubcheck/vocab/EnumVocab.java:114: Preconditions.checkNotNull(property);
./src/main/java/com/adobe/epubcheck/ops/OPSHandler.java:421: return new URI(Preconditions.checkNotNull(uri).trim());
./src/main/java/com/adobe/epubcheck/opf/MetadataSet.java:230: Preconditions.checkNotNull(metas);
./src/main/java/com/adobe/epubcheck/opf/MetadataSet.java:231: Preconditions.checkNotNull(property);
./src/main/java/com/adobe/epubcheck/opf/MetadataSet.java:232: Preconditions.checkNotNull(value);
./src/main/java/com/adobe/epubcheck/opf/MetadataSet.java:265: Preconditions.checkNotNull(metas);
./src/main/java/com/adobe/epubcheck/opf/MetadataSet.java:266: Preconditions.checkNotNull(property);
./src/main/java/com/adobe/epubcheck/opf/MetadataSet.java:267: Preconditions.checkNotNull(value);
./src/main/java/com/adobe/epubcheck/opf/MetadataSet.java:295: Preconditions.checkNotNull(property);
./src/main/java/com/adobe/epubcheck/opf/OPFItem.java:302: this.id = Preconditions.checkNotNull(id).trim();
./src/main/java/com/adobe/epubcheck/opf/OPFItem.java:303: this.path = Preconditions.checkNotNull(path).trim();
./src/main/java/com/adobe/epubcheck/opf/ValidationContext.java:214: properties.add(Preconditions.checkNotNull(property));
./src/main/java/com/adobe/epubcheck/opf/XRefChecker.java:185: Resource res = Preconditions.checkNotNull(resources.get(path));
./src/main/java/com/adobe/epubcheck/opf/OPFItems.java:78: this.items = ImmutableList.copyOf(Preconditions.checkNotNull(items));
./src/main/java/com/adobe/epubcheck/opf/OPFItems.java:113: return new OPFItems(Iterables.transform(Preconditions.checkNotNull(itemBuilders),
./src/main/java/com/adobe/epubcheck/opf/OPFItems.java:121: }), Preconditions.checkNotNull(spineIDs));