build: add Checkstyle reporter for unused imports and trailing whitespace

client/transport/spi/src/main/java/org/a2aproject/sdk/client/transport/spi/interceptors/auth/AuthInterceptor.java

@@ -1,7 +1,6 @@
 package org.a2aproject.sdk.client.transport.spi.interceptors.auth;
 
 import java.util.HashMap;
-import java.util.List;
 import java.util.Locale;
 import java.util.Map;
 import java.util.Set;

http-client/src/test/java/org/a2aproject/sdk/client/http/JdkA2AHttpClientTest.java

@@ -1,7 +1,6 @@
 package org.a2aproject.sdk.client.http;
 
 import org.junit.jupiter.api.AfterEach;
-import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.mockserver.integration.ClientAndServer;
 

jsonrpc-common/src/test/java/org/a2aproject/sdk/jsonrpc/common/json/TaskSerializationTest.java

@@ -7,7 +7,6 @@
 import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
-import java.io.IOException;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.time.OffsetDateTime;

pom.xml

@@ -48,6 +48,9 @@
         <maven-surefire-plugin.version>3.5.6</maven-surefire-plugin.version>
         <maven-javadoc-plugin.version>3.12.0</maven-javadoc-plugin.version>
         <maven-gpg-plugin.version>3.2.8</maven-gpg-plugin.version>
+        <maven-checkstyle-plugin.version>3.5.0</maven-checkstyle-plugin.version>
+        <checkstyle.failOnViolation>false</checkstyle.failOnViolation>
+        <checkstyle.version>9.3</checkstyle.version>
         <gmavenplus-plugin.version>5.1.0</gmavenplus-plugin.version>
         <sonatype-central-publishing-plugin.version>0.11.0</sonatype-central-publishing-plugin.version>
         <groovy.version>5.1.2</groovy.version>
@@ -591,6 +594,36 @@
                 <groupId>org.sonatype.central</groupId>
                 <artifactId>central-publishing-maven-plugin</artifactId>
             </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-checkstyle-plugin</artifactId>
+                <version>${maven-checkstyle-plugin.version}</version>
+                <dependencies>
+                    <dependency>
+                        <groupId>com.puppycrawl.tools</groupId>
+                        <artifactId>checkstyle</artifactId>
+                        <version>${checkstyle.version}</version>
+                    </dependency>
+                </dependencies>
+                <executions>
+                    <execution>
+                        <id>checkstyle-validation</id>
+                        <phase>validate</phase>
+                        <inherited>true</inherited>
+                        <configuration>
+                            <skip>false</skip>
+                            <configLocation>src/checkstyle/checkstyle.xml</configLocation>
+                            <includeTestSourceDirectory>true</includeTestSourceDirectory>
+                            <consoleOutput>true</consoleOutput>
+                            <failsOnError>false</failsOnError>
+                            <failOnViolation>${checkstyle.failOnViolation}</failOnViolation>
+                        </configuration>
+                        <goals>
+                            <goal>check</goal>
+                        </goals>
+                    </execution>
+                </executions>
+            </plugin>
         </plugins>
     </build>
 

spec-grpc/src/main/java/org/a2aproject/sdk/grpc/mapper/AuthorizationCodeOAuthFlowMapper.java

@@ -2,7 +2,6 @@
 
 import org.mapstruct.CollectionMappingStrategy;
 import org.mapstruct.Mapper;
-import org.mapstruct.Mapping;
 
 /**
  * Mapper between {@link org.a2aproject.sdk.spec.AuthorizationCodeOAuthFlow} and {@link org.a2aproject.sdk.grpc.AuthorizationCodeOAuthFlow}.

spec-grpc/src/test/java/org/a2aproject/sdk/grpc/utils/JSONRPCUtilsTest.java

@@ -34,7 +34,6 @@
 import org.a2aproject.sdk.jsonrpc.common.wrappers.GetTaskResponse;
 import org.a2aproject.sdk.jsonrpc.common.wrappers.SendMessageRequest;
 import org.a2aproject.sdk.spec.DataPart;
-import org.a2aproject.sdk.spec.GetExtendedAgentCardParams;
 import org.a2aproject.sdk.spec.InvalidParamsError;
 import org.a2aproject.sdk.spec.JSONParseError;
 import org.a2aproject.sdk.spec.Message;

spec/src/main/java/org/a2aproject/sdk/spec/ListTasksParams.java

@@ -1,6 +1,5 @@
 package org.a2aproject.sdk.spec;
 
-import org.a2aproject.sdk.util.Assert;
 import java.time.Instant;
 
 import org.a2aproject.sdk.spec.util.Utils;

src/checkstyle/checkstyle.xml

@@ -0,0 +1,21 @@
+<?xml version="1.0"?>
+<!DOCTYPE module PUBLIC
+    "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
+    "https://checkstyle.org/dtds/configuration_1_3.dtd">
+
+<module name="Checker">
+
+    <module name="TreeWalker">
+
+        <!-- Report trailing whitespace. -->
+        <module name="RegexpSinglelineJava">
+            <property name="format" value="[ \t]+$"/>
+            <property name="message" value="Line has trailing whitespace."/>
+        </module>
+
+        <!-- Report imports that are not used. -->
+        <module name="UnusedImports"/>
+
+    </module>
+
+</module>