diff --git a/concurrency/src/main/java/io/mincong/concurrency/completablefuture/stackoverflow/So61957439.java b/concurrency/src/main/java/io/mincong/concurrency/completablefuture/stackoverflow/So61957439.java
index 05761377..06aed3a3 100644
--- a/concurrency/src/main/java/io/mincong/concurrency/completablefuture/stackoverflow/So61957439.java
+++ b/concurrency/src/main/java/io/mincong/concurrency/completablefuture/stackoverflow/So61957439.java
@@ -32,6 +32,7 @@
  * 08:20:10.165436 - [Result{name='C3'}, Result{name='C1'}, Result{name='C2'}]
  * </pre>
  */
+@SuppressWarnings("StreamToString")
 public class So61957439 {
 
   public static void main(String[] args) throws Exception {
diff --git a/date/src/test/java/io/mincongh/date/so42364818/DateTest.java b/date/src/test/java/io/mincongh/date/so42364818/DateTest.java
index 80035fcd..c6d1383e 100644
--- a/date/src/test/java/io/mincongh/date/so42364818/DateTest.java
+++ b/date/src/test/java/io/mincongh/date/so42364818/DateTest.java
@@ -20,7 +20,6 @@ void testDate() throws Exception {
     assertEquals(45, d.getMinute());
     assertEquals(30, d.getSecond());
 
-    d.format(DateTimeFormatter.ISO_LOCAL_DATE_TIME);
     assertEquals("2009-06-15T13:45:30", d.format(DateTimeFormatter.ISO_LOCAL_DATE_TIME));
   }
 }
diff --git a/hashcode/src/main/java/io/mincongh/hashcode/bad/AthleteOnlyOverrideEquals.java b/hashcode/src/main/java/io/mincongh/hashcode/bad/AthleteOnlyOverrideEquals.java
index 68eb46a1..f170cc32 100644
--- a/hashcode/src/main/java/io/mincongh/hashcode/bad/AthleteOnlyOverrideEquals.java
+++ b/hashcode/src/main/java/io/mincongh/hashcode/bad/AthleteOnlyOverrideEquals.java
@@ -6,6 +6,7 @@
  *
  * @author Mincong Huang
  */
+@SuppressWarnings("EqualsHashCode") // hashCode() unimplemented for demo purpose
 public class AthleteOnlyOverrideEquals {
 
   private String email;
diff --git a/hashcode/src/main/java/io/mincongh/hashcode/bad/PhoneNumberNoHash.java b/hashcode/src/main/java/io/mincongh/hashcode/bad/PhoneNumberNoHash.java
index 2103c3b0..3ef95296 100644
--- a/hashcode/src/main/java/io/mincongh/hashcode/bad/PhoneNumberNoHash.java
+++ b/hashcode/src/main/java/io/mincongh/hashcode/bad/PhoneNumberNoHash.java
@@ -1,6 +1,7 @@
 package io.mincongh.hashcode.bad;
 
 /** @author Mincong Huang */
+@SuppressWarnings("EqualsHashCode") // hashCode() unimplemented for demo purpose
 public class PhoneNumberNoHash {
 
   private final short areaCode;
diff --git a/immutables/pom.xml b/immutables/pom.xml
index c58b8639..ee0761e0 100644
--- a/immutables/pom.xml
+++ b/immutables/pom.xml
@@ -14,15 +14,10 @@
   <artifactId>java-examples-immutables</artifactId>
   <name>Java Examples - Immutables</name>
 
-  <properties>
-    <immutables.version>2.8.8</immutables.version>
-  </properties>
-
   <dependencies>
     <dependency>
       <groupId>org.immutables</groupId>
       <artifactId>value</artifactId>
-      <version>${immutables.version}</version>
       <scope>provided</scope>
     </dependency>
     <!--
diff --git a/java8/src/test/java/io/mincongh/java8/so2607289/ConvertArrayToListTest.java b/java8/src/test/java/io/mincongh/java8/so2607289/ConvertArrayToListTest.java
index f7dba0d9..bb9fb804 100644
--- a/java8/src/test/java/io/mincongh/java8/so2607289/ConvertArrayToListTest.java
+++ b/java8/src/test/java/io/mincongh/java8/so2607289/ConvertArrayToListTest.java
@@ -4,7 +4,7 @@
 import static org.junit.jupiter.api.Assertions.fail;
 
 import java.util.*;
-import java.util.function.Predicate;
+import java.util.function.Consumer;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 import org.junit.jupiter.api.Test;
@@ -71,13 +71,13 @@ private void assertListValues(List<Integer> list) {
     assertEquals(3, list.get(2).intValue());
   }
 
-  private <T> void assertOperationSupported(List<T> list, Predicate<List<T>> predicate) {
-    predicate.test(list);
+  private <T> void assertOperationSupported(List<T> list, Consumer<List<T>> action) {
+    action.accept(list);
   }
 
-  private <T> void assertOperationUnsupported(List<T> list, Predicate<List<T>> predicate) {
+  private <T> void assertOperationUnsupported(List<T> list, Consumer<List<T>> action) {
     try {
-      predicate.test(list);
+      action.accept(list);
       fail();
     } catch (UnsupportedOperationException e) {
       // Ok
diff --git a/jmh/pom.xml b/jmh/pom.xml
index 3498999d..ace0ac40 100644
--- a/jmh/pom.xml
+++ b/jmh/pom.xml
@@ -16,7 +16,6 @@
 
   <properties>
     <commons.codec.version>1.15</commons.codec.version>
-    <jmh.version>1.26</jmh.version>
   </properties>
 
   <dependencies>
@@ -32,12 +31,6 @@
       <version>${jmh.version}</version>
       <scope>test</scope>
     </dependency>
-    <dependency>
-      <groupId>org.openjdk.jmh</groupId>
-      <artifactId>jmh-generator-annprocess</artifactId>
-      <version>${jmh.version}</version>
-      <scope>test</scope>
-    </dependency>
     <dependency>
       <groupId>org.junit.jupiter</groupId>
       <artifactId>junit-jupiter-api</artifactId>
diff --git a/oca/src/test/java/io/mincong/ocajp/chapter3/ReviewQuestionTest.java b/oca/src/test/java/io/mincong/ocajp/chapter3/ReviewQuestionTest.java
index 7c8b8892..438598d1 100644
--- a/oca/src/test/java/io/mincong/ocajp/chapter3/ReviewQuestionTest.java
+++ b/oca/src/test/java/io/mincong/ocajp/chapter3/ReviewQuestionTest.java
@@ -56,6 +56,7 @@ public void testLegalArrayDeclaration() {
 
   /** Question 31. */
   @Test
+  @SuppressWarnings("ReturnValueIgnored")
   public void testLocalDateImmutability() {
     LocalDate date = LocalDate.of(2018, Month.APRIL, 30);
     date.plusDays(2);
diff --git a/ocp/src/test/java/io/mincong/ocpjp/autoboxing/WrapperClassTest.java b/ocp/src/test/java/io/mincong/ocpjp/autoboxing/WrapperClassTest.java
index e1e55740..fe186c17 100644
--- a/ocp/src/test/java/io/mincong/ocpjp/autoboxing/WrapperClassTest.java
+++ b/ocp/src/test/java/io/mincong/ocpjp/autoboxing/WrapperClassTest.java
@@ -62,6 +62,7 @@ public void comparingObjects() throws Exception {
      * Cannot use a `Float` object to retrieve the value that was
      * added to a `HashMap` using a `Double` instance.
      */
+    @SuppressWarnings("CollectionIncompatibleType")
     String value2 = map.get(new Float(2.0));
     assertThat(value2).isNull();
   }
diff --git a/ocp/src/test/java/io/mincong/ocpjp/collection/HashSetTest.java b/ocp/src/test/java/io/mincong/ocpjp/collection/HashSetTest.java
index 4baa058c..a8993378 100644
--- a/ocp/src/test/java/io/mincong/ocpjp/collection/HashSetTest.java
+++ b/ocp/src/test/java/io/mincong/ocpjp/collection/HashSetTest.java
@@ -146,6 +146,7 @@ public int hashCode() {
   }
 
   /** Only {@link #equals(Object)} is overridden. */
+  @SuppressWarnings("EqualsHashCode")
   private static class Person_E extends Person {
 
     Person_E(String name) {
diff --git a/ocp/src/test/java/io/mincong/ocpjp/date/Java8DateTest.java b/ocp/src/test/java/io/mincong/ocpjp/date/Java8DateTest.java
index 825838dd..4546be2f 100644
--- a/ocp/src/test/java/io/mincong/ocpjp/date/Java8DateTest.java
+++ b/ocp/src/test/java/io/mincong/ocpjp/date/Java8DateTest.java
@@ -49,6 +49,7 @@ public void localDate_constructWithMonth() throws Exception {
   }
 
   @Test
+  @SuppressWarnings("TemporalAccessorGetChronoField") // Fail on purpose
   public void localDate_temporalField() throws Exception {
     LocalDate d = LocalDate.of(2018, 1, 2);
     assertThat(d.get(ChronoField.YEAR)).isEqualTo(2018);
diff --git a/ocp/src/test/java/io/mincong/ocpjp/nio/PathTest.java b/ocp/src/test/java/io/mincong/ocpjp/nio/PathTest.java
index 79ec6e43..c74b7b0c 100644
--- a/ocp/src/test/java/io/mincong/ocpjp/nio/PathTest.java
+++ b/ocp/src/test/java/io/mincong/ocpjp/nio/PathTest.java
@@ -1,6 +1,6 @@
 package io.mincong.ocpjp.nio;
 
-import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.*;
 
 import java.io.FileInputStream;
 import java.io.IOException;
@@ -102,9 +102,10 @@ public void toAbsolutePath_nonexistent() throws Exception {
     assertThat(p.getFileName().toString()).isEqualTo("nonexistent");
   }
 
-  @Test(expected = NoSuchFileException.class)
+  @Test
   public void toRealPath_nonexistent() throws Exception {
-    r.resolve("nonexistent").toRealPath();
+    assertThatThrownBy(() -> r.resolve("nonexistent").toRealPath())
+        .isInstanceOf(NoSuchFileException.class);
   }
 
   @Test
diff --git a/ocp/src/test/java/io/mincong/ocpjp/regex/FormattingTest.java b/ocp/src/test/java/io/mincong/ocpjp/regex/FormattingTest.java
index 492ee946..dc95ed34 100644
--- a/ocp/src/test/java/io/mincong/ocpjp/regex/FormattingTest.java
+++ b/ocp/src/test/java/io/mincong/ocpjp/regex/FormattingTest.java
@@ -41,6 +41,7 @@
  *
  * @author Mincong Huang
  */
+@SuppressWarnings({"FormatString", "ArrayToString"})
 public class FormattingTest {
 
   @Rule public final TemporaryFolder tempFolder = new TemporaryFolder();
diff --git a/pom.xml b/pom.xml
index 034f6cb4..35f37a3b 100644
--- a/pom.xml
+++ b/pom.xml
@@ -19,9 +19,12 @@
 
     <!-- Dependency versions -->
     <assertjVersion>3.17.2</assertjVersion>
+    <immutablesVersion>2.8.8</immutablesVersion>
     <jacksonVersion>2.11.3</jacksonVersion>
+    <jmh.version>1.26</jmh.version>
     <junitVersion>4.13.1</junitVersion>
     <junit5Version>5.7.0</junit5Version>
+    <errorProneVersion>2.4.0</errorProneVersion>
   </properties>
 
   <modules>
@@ -99,6 +102,11 @@
         <artifactId>assertj-core</artifactId>
         <version>${assertjVersion}</version>
       </dependency>
+      <dependency>
+        <groupId>org.immutables</groupId>
+        <artifactId>value</artifactId>
+        <version>${immutablesVersion}</version>
+      </dependency>
     </dependencies>
   </dependencyManagement>
 
@@ -157,7 +165,36 @@
         </plugin>
       </plugins>
     </pluginManagement>
+
     <plugins>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-compiler-plugin</artifactId>
+        <configuration>
+          <compilerArgs>
+            <arg>-XDcompilePolicy=simple</arg>
+            <arg>-Xplugin:ErrorProne</arg>
+          </compilerArgs>
+          <annotationProcessorPaths>
+            <!-- https://errorprone.info/docs/installation -->
+            <path>
+              <groupId>com.google.errorprone</groupId>
+              <artifactId>error_prone_core</artifactId>
+              <version>${errorProneVersion}</version>
+            </path>
+            <path>
+              <groupId>org.immutables</groupId>
+              <artifactId>value</artifactId>
+              <version>${immutablesVersion}</version>
+            </path>
+            <path>
+              <groupId>org.openjdk.jmh</groupId>
+              <artifactId>jmh-generator-annprocess</artifactId>
+              <version>${jmh.version}</version>
+            </path>
+          </annotationProcessorPaths>
+        </configuration>
+      </plugin>
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-failsafe-plugin</artifactId>
diff --git a/vavr/src/test/java/io/mincongh/vavr/JavaListTest.java b/vavr/src/test/java/io/mincongh/vavr/JavaListTest.java
index 5da653f1..69431100 100644
--- a/vavr/src/test/java/io/mincongh/vavr/JavaListTest.java
+++ b/vavr/src/test/java/io/mincongh/vavr/JavaListTest.java
@@ -150,6 +150,7 @@ public void remove_ArraysArrayList() {
   }
 
   @Test
+  @SuppressWarnings("CollectionIncompatibleType")
   public void remove_ArrayList() {
     List<String> animals = new ArrayList<>();
     animals.add("🐱");
diff --git a/xml/src/main/java/io/mincongh/xml/xstream/model/PhoneNumber.java b/xml/src/main/java/io/mincongh/xml/xstream/model/PhoneNumber.java
index e3beaa2a..2f41d50f 100644
--- a/xml/src/main/java/io/mincongh/xml/xstream/model/PhoneNumber.java
+++ b/xml/src/main/java/io/mincongh/xml/xstream/model/PhoneNumber.java
@@ -40,6 +40,6 @@ public int hashCode() {
 
   @Override
   public String toString() {
-    return String.format(Locale.ENGLISH, "+%.2d %s", code, number);
+    return String.format(Locale.ENGLISH, "+%02d %s", code, number);
   }
 }
diff --git a/xml/src/test/java/io/mincongh/xml/xstream/model/PhoneNumberTest.java b/xml/src/test/java/io/mincongh/xml/xstream/model/PhoneNumberTest.java
new file mode 100644
index 00000000..b9afca17
--- /dev/null
+++ b/xml/src/test/java/io/mincongh/xml/xstream/model/PhoneNumberTest.java
@@ -0,0 +1,14 @@
+package io.mincongh.xml.xstream.model;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+class PhoneNumberTest {
+
+  @Test
+  void toStr() {
+    var phoneNumber = new PhoneNumber(1, "2345");
+    assertThat(String.valueOf(phoneNumber)).isEqualTo("+01 2345");
+  }
+}