Update code-change-guide.md

contributor-docs/code-change-guide.md

@@ -115,8 +115,6 @@ To run a Gradle task, use the command `./gradlew -p <PROJECT_PATH> <TASK>` or th
 ./gradlew :sdks:java:harness:test
 ```
 
-**It is recommended to run `./gradlew clean` if you run into some strange errors such as `java.lang.NoClassDefFoundError`.**
-
 #### Beam-specific Gradle project configuration
 
 For Apache Beam, one plugin manages everything: `buildSrc/src/main/groovy/org/apache/beam/gradle/BeamModulePlugin`.
@@ -626,6 +624,11 @@ Tips for using the Dataflow runner:
 
 ## Appendix
 
+### Common Issues
+
+* If you run into some strange errors such as `java.lang.NoClassDefFoundError`, run `./gradlew clean` first
+* To run one single Java test with gradle, use `--tests` to filter, for example, `./gradlew :it:google-cloud-platform:WordCountIntegrationTest --tests "org.apache.beam.it.gcp.WordCountIT.testWordCountDataflow"`
+
 ### Directories of snapshot builds
 
 * https://repository.apache.org/content/groups/snapshots/org/apache/beam/ Java SDK build (nightly)