Issue #3771: fixed in NPE in cache for HeaderCheck

src/main/java/com/puppycrawl/tools/checkstyle/checks/header/AbstractHeaderCheck.java

@@ -193,6 +193,15 @@ public abstract class AbstractHeaderCheck extends AbstractFileSetCheck
 
     @Override
     public Set<String> getExternalResourceLocations() {
-        return Collections.singleton(headerFile.toString());
+        final Set<String> result;
+
+        if (headerFile == null) {
+            result = Collections.emptySet();
+        }
+        else {
+            result = Collections.singleton(headerFile.toString());
+        }
+
+        return result;
     }
 }

src/test/java/com/puppycrawl/tools/checkstyle/checks/header/HeaderCheckTest.java

@@ -268,4 +268,25 @@ public class HeaderCheckTest extends BaseFileSetCheckTestSupport {
         verify(checker, getPath("InputHeader.java"), expected);
 
     }
+
+    @Test
+    public void testCacheHeaderWithoutFile() throws Exception {
+        final DefaultConfiguration checkConfig = createCheckConfig(HeaderCheck.class);
+        checkConfig.addAttribute("header", "Test");
+
+        final DefaultConfiguration checkerConfig = new DefaultConfiguration("checkstyle_checks");
+        checkerConfig.addChild(checkConfig);
+        checkerConfig.addAttribute("cacheFile", temporaryFolder.newFile().getPath());
+
+        final Checker checker = new Checker();
+        checker.setModuleClassLoader(Thread.currentThread().getContextClassLoader());
+        checker.configure(checkerConfig);
+        checker.addListener(new BriefUtLogger(stream));
+
+        final String[] expected = {
+            "1: " + getCheckMessage(MSG_MISMATCH, "Test"),
+        };
+
+        verify(checker, getPath("InputHeader.java"), expected);
+    }
 }