HDFS-16720. Import detailed class instead of import * in hadoop-hdfs moulde

[ZanderXu]

Description of PR

During reading code in hadoop-hdfs module, I found that there is import * case in some classes, such as

// BlockPlacementPolicyDefault.java
import java.util.*;

I feel that we should import detailed class instead of *.

[slfan1989]

@ZanderXu This fix cannot be approved, see the discussion of this pr.

#4390

Tips from steveloughran

-1 to all needless import changes, even if cleanup. import changes are the nightmare of cherrypicks, which is why we so rarely clean them up, even ongoing patches. given a choice between "messy imports in existing code" and "backport hell", i will take the mess.

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 35s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 1s		No case conflicting files found.
+0 🆗	codespell	0m 0s		codespell was not available.
+0 🆗	detsecrets	0m 0s		detect-secrets was not available.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
-1 ❌	test4tests	0m 0s		The patch doesn't appear to include any new or modified tests. Please justify why no new tests are needed for this patch. Also please list what manual steps were performed to verify this patch.
			_ trunk Compile Tests _
+1 💚	mvninstall	38m 54s		trunk passed
+1 💚	compile	1m 46s		trunk passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 💚	compile	1m 32s		trunk passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	checkstyle	1m 24s		trunk passed
+1 💚	mvnsite	1m 44s		trunk passed
+1 💚	javadoc	1m 25s		trunk passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 💚	javadoc	1m 49s		trunk passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	spotbugs	3m 41s		trunk passed
+1 💚	shadedclient	23m 7s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	1m 21s		the patch passed
+1 💚	compile	1m 27s		the patch passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 💚	javac	1m 27s		the patch passed
+1 💚	compile	1m 20s		the patch passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	javac	1m 20s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	1m 2s		the patch passed
+1 💚	mvnsite	1m 25s		the patch passed
+1 💚	javadoc	0m 58s		the patch passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 💚	javadoc	1m 32s		the patch passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	spotbugs	3m 21s		the patch passed
+1 💚	shadedclient	22m 47s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	unit	236m 46s		hadoop-hdfs in the patch passed.
+1 💚	asflicense	1m 14s		The patch does not generate ASF License warnings.
		347m 32s

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4709/1/artifact/out/Dockerfile
GITHUB PR	#4709
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets
uname	Linux 757b060dc166 4.15.0-58-generic #64-Ubuntu SMP Tue Aug 6 11:12:41 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / 3816597
Default Java	Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
Multi-JDK versions	/usr/lib/jvm/java-11-openjdk-amd64:Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1 /usr/lib/jvm/java-8-openjdk-amd64:Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4709/1/testReport/
Max. process+thread count	3551 (vs. ulimit of 5500)
modules	C: hadoop-hdfs-project/hadoop-hdfs U: hadoop-hdfs-project/hadoop-hdfs
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4709/1/console
versions	git=2.25.1 maven=3.6.3 spotbugs=4.2.2
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[ZanderXu]

Thanks @slfan1989 for you review.