diff --git a/core/src/test/java/hudson/PluginManagerTest.java b/core/src/test/java/hudson/PluginManagerTest.java
index 62d1967ac9..527fed7ad3 100644
--- a/core/src/test/java/hudson/PluginManagerTest.java
+++ b/core/src/test/java/hudson/PluginManagerTest.java
@@ -25,6 +25,7 @@
 package hudson;
 
 import java.io.File;
+import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import org.apache.tools.ant.filters.StringInputStream;
 import org.junit.Test;
@@ -145,7 +146,7 @@ public class PluginManagerTest {
         File newFolder = tmp.newFolder("myJar");
         String manifestPath = "META-INF/MANIFEST.MF";
         new File("META-INF").mkdir();
-        FileUtils.write(new File(newFolder, manifestPath), SAMPLE_MANIFEST_FILE);
+        FileUtils.write(new File(newFolder, manifestPath), SAMPLE_MANIFEST_FILE, StandardCharsets.UTF_8);
         
         final File f = new File(tmp.getRoot(), "my.hpi");
         try(ZipOutputStream out = new ZipOutputStream(Files.newOutputStream(f.toPath()))) {
diff --git a/core/src/test/java/hudson/util/ConsistentHashTest.java b/core/src/test/java/hudson/util/ConsistentHashTest.java
index 42ad8e2903..8fefa64b2a 100644
--- a/core/src/test/java/hudson/util/ConsistentHashTest.java
+++ b/core/src/test/java/hudson/util/ConsistentHashTest.java
@@ -66,7 +66,7 @@ public class ConsistentHashTest {
         assertEquals(z,x);
         all.add(itr.next());
         all.add(itr.next());
-        assertTrue(!itr.hasNext());
+        assertFalse(itr.hasNext());
         assertEquals(3,all.size());
     }
 
diff --git a/core/src/test/java/jenkins/model/lazy/AbstractLazyLoadRunMapTest.java b/core/src/test/java/jenkins/model/lazy/AbstractLazyLoadRunMapTest.java
index 044ebcfd3b..cd275b5597 100644
--- a/core/src/test/java/jenkins/model/lazy/AbstractLazyLoadRunMapTest.java
+++ b/core/src/test/java/jenkins/model/lazy/AbstractLazyLoadRunMapTest.java
@@ -39,7 +39,6 @@ import java.util.Map.Entry;
 import java.util.NoSuchElementException;
 import java.util.Set;
 import java.util.SortedMap;
-import java.util.concurrent.Callable;
 import java.util.concurrent.Future;
 import java.util.concurrent.Semaphore;
 import java.util.concurrent.atomic.AtomicInteger;
@@ -201,7 +200,7 @@ public class AbstractLazyLoadRunMapTest {
         final FakeMap m = localExpiredBuilder.add(1).add(2).make();
 
         // force index creation
-        m.entrySet();
+        assertEquals(2, m.entrySet().size());
 
         m.search(1, Direction.EXACT).asserts(1);
         assertNull(m.search(3, Direction.EXACT));
@@ -259,10 +258,9 @@ public class AbstractLazyLoadRunMapTest {
 
     @Test
     public void identity() {
-        assertTrue(a.equals(a));
-        assertTrue(!a.equals(b));
-        a.hashCode();
-        b.hashCode();
+        assertEquals(a, a);
+        assertNotEquals(a, b);
+        assertNotEquals(a.hashCode(), b.hashCode());
     }
 
     @Issue("JENKINS-15439")
@@ -285,7 +283,7 @@ public class AbstractLazyLoadRunMapTest {
     }
 
     @Issue("JENKINS-18065")
-    @Test public void all() throws Exception {
+    @Test public void all() {
         assertEquals("[]", a.getLoadedBuilds().keySet().toString());
         Set<Map.Entry<Integer,Build>> entries = a.entrySet();
         assertEquals("[]", a.getLoadedBuilds().keySet().toString());
@@ -399,18 +397,8 @@ public class AbstractLazyLoadRunMapTest {
             slowBuilderLoadCount.put(i, new AtomicInteger());
         }
         final FakeMap m = slowBuilder.make();
-        Future<Build> firstLoad = Timer.get().submit(new Callable<Build>() {
-            @Override
-            public Build call() throws Exception {
-                return m.getByNumber(2);
-            }
-        });
-        Future<Build> secondLoad = Timer.get().submit(new Callable<Build>() {
-            @Override
-            public Build call() throws Exception {
-                return m.getByNumber(2);
-            }
-        });
+        Future<Build> firstLoad = Timer.get().submit(() -> m.getByNumber(2));
+        Future<Build> secondLoad = Timer.get().submit(() -> m.getByNumber(2));
         slowBuilderStartSemaphores.get(2).acquire(1);
         // now one of them is inside retrieve(…); the other is waiting for the lock
         slowBuilderEndSemaphores.get(2).release(2); // allow both to proceed
diff --git a/core/src/test/java/jenkins/plugins/DetachedPluginsUtilTest.java b/core/src/test/java/jenkins/plugins/DetachedPluginsUtilTest.java
index 611a9b34d7..a07a756507 100644
--- a/core/src/test/java/jenkins/plugins/DetachedPluginsUtilTest.java
+++ b/core/src/test/java/jenkins/plugins/DetachedPluginsUtilTest.java
@@ -6,6 +6,7 @@ import org.apache.commons.io.IOUtils;
 import org.junit.Test;
 
 import java.io.IOException;
+import java.nio.charset.StandardCharsets;
 import java.util.List;
 import java.util.stream.Collectors;
 
@@ -29,12 +30,10 @@ public class DetachedPluginsUtilTest {
         final List<DetachedPluginsUtil.DetachedPlugin> detachedPlugins = DetachedPluginsUtil.getDetachedPlugins();
         if (JavaUtils.isRunningWithJava8OrBelow()) {
             assertEquals(0, detachedPlugins.stream()
-                    .filter(plugin -> plugin.getShortName().equals("jaxb"))
-                    .collect(Collectors.toList()).size());
+                    .filter(plugin -> plugin.getShortName().equals("jaxb")).count());
         } else if (JavaUtils.getCurrentJavaRuntimeVersionNumber().isNewerThanOrEqualTo(new VersionNumber("11.0.2"))) {
             assertEquals(1, detachedPlugins.stream()
-                    .filter(plugin -> plugin.getShortName().equals("jaxb"))
-                    .collect(Collectors.toList()).size());
+                    .filter(plugin -> plugin.getShortName().equals("jaxb")).count());
 
             final List<DetachedPluginsUtil.DetachedPlugin> detachedPluginsSince2_161 =
                     DetachedPluginsUtil.getDetachedPlugins(new VersionNumber("2.161"));
@@ -49,8 +48,8 @@ public class DetachedPluginsUtilTest {
      */
     @Test
     public void checkSplitPluginsFileFormat() throws IOException {
-        final List<String> splitPluginsLines = IOUtils.readLines(getClass().getResourceAsStream("/jenkins/split-plugins.txt"));
-        assertTrue(!splitPluginsLines.isEmpty());
+        final List<String> splitPluginsLines = IOUtils.readLines(getClass().getResourceAsStream("/jenkins/split-plugins.txt"), StandardCharsets.UTF_8);
+        assertFalse(splitPluginsLines.isEmpty());
 
         // File is not only comments
         final List<String> linesWithoutComments = splitPluginsLines.stream()