diff --git a/pmd-core/src/main/java/net/sourceforge/pmd/RuleSetFactory.java b/pmd-core/src/main/java/net/sourceforge/pmd/RuleSetFactory.java
index 43e27de5ba..11bf9fe36d 100644
--- a/pmd-core/src/main/java/net/sourceforge/pmd/RuleSetFactory.java
+++ b/pmd-core/src/main/java/net/sourceforge/pmd/RuleSetFactory.java
@@ -190,7 +190,7 @@ public class RuleSetFactory {
         List<RuleSetReferenceId> references = RuleSetReferenceId.parse(referenceString);
         if (references.isEmpty()) {
             throw new RuleSetNotFoundException(
-                    "No RuleSetReferenceId can be parsed from the string: <" + referenceString + ">");
+                    "No RuleSetReferenceId can be parsed from the string: <" + referenceString + '>');
         }
         return createRuleSet(references.get(0));
     }
@@ -620,7 +620,7 @@ public class RuleSetFactory {
             if (referencedRule instanceof RuleReference) {
                 RuleReference ruleReference = (RuleReference) referencedRule;
                 if (LOG.isLoggable(Level.WARNING)) {
-                    LOG.warning("Use Rule name " + ruleReference.getRuleSetReference().getRuleSetFileName() + "/"
+                    LOG.warning("Use Rule name " + ruleReference.getRuleSetReference().getRuleSetFileName() + '/'
                             + ruleReference.getOriginalName() + " instead of the deprecated Rule name "
                             + otherRuleSetReferenceId
                             + ". PMD " + PMDVersion.getNextMajorRelease()
@@ -650,9 +650,9 @@ public class RuleSetFactory {
 
         if (warnDeprecated && ruleReference.isDeprecated()) {
             if (LOG.isLoggable(Level.WARNING)) {
-                LOG.warning("Use Rule name " + ruleReference.getRuleSetReference().getRuleSetFileName() + "/"
+                LOG.warning("Use Rule name " + ruleReference.getRuleSetReference().getRuleSetFileName() + '/'
                         + ruleReference.getOriginalName() + " instead of the deprecated Rule name "
-                        + ruleSetReferenceId.getRuleSetFileName() + "/" + ruleReference.getName()
+                        + ruleSetReferenceId.getRuleSetFileName() + '/' + ruleReference.getName()
                         + ". PMD " + PMDVersion.getNextMajorRelease()
                         + " will remove support for this deprecated Rule name usage.");
             }
diff --git a/pmd-core/src/main/java/net/sourceforge/pmd/RuleSetReferenceId.java b/pmd-core/src/main/java/net/sourceforge/pmd/RuleSetReferenceId.java
index 2f49c29b50..69458994a0 100644
--- a/pmd-core/src/main/java/net/sourceforge/pmd/RuleSetReferenceId.java
+++ b/pmd-core/src/main/java/net/sourceforge/pmd/RuleSetReferenceId.java
@@ -251,7 +251,7 @@ public class RuleSetReferenceId {
             int index = name.indexOf('-');
             if (index >= 0) {
                 // Standard short name
-                result = "rulesets/" + name.substring(0, index) + "/" + name.substring(index + 1) + ".xml";
+                result = "rulesets/" + name.substring(0, index) + '/' + name.substring(index + 1) + ".xml";
             } else {
                 // A release RuleSet?
                 if (name.matches("[0-9]+.*")) {
@@ -428,7 +428,7 @@ public class RuleSetReferenceId {
             if (allRules) {
                 return ruleSetFileName;
             } else {
-                return ruleSetFileName + "/" + ruleName;
+                return ruleSetFileName + '/' + ruleName;
             }
 
         } else {
diff --git a/pmd-core/src/main/java/net/sourceforge/pmd/RuleSetWriter.java b/pmd-core/src/main/java/net/sourceforge/pmd/RuleSetWriter.java
index 0081100627..7f7fa97406 100644
--- a/pmd-core/src/main/java/net/sourceforge/pmd/RuleSetWriter.java
+++ b/pmd-core/src/main/java/net/sourceforge/pmd/RuleSetWriter.java
@@ -177,7 +177,7 @@ public class RuleSetWriter {
                 LanguageVersion maximumLanguageVersion = ruleReference.getOverriddenMaximumLanguageVersion();
                 Boolean deprecated = ruleReference.isOverriddenDeprecated();
                 String name = ruleReference.getOverriddenName();
-                String ref = ruleReference.getRuleSetReference().getRuleSetFileName() + "/"
+                String ref = ruleReference.getRuleSetReference().getRuleSetFileName() + '/'
                         + ruleReference.getRule().getName();
                 String message = ruleReference.getOverriddenMessage();
                 String externalInfoUrl = ruleReference.getOverriddenExternalInfoUrl();
diff --git a/pmd-core/src/main/java/net/sourceforge/pmd/RulesetsFactoryUtils.java b/pmd-core/src/main/java/net/sourceforge/pmd/RulesetsFactoryUtils.java
index c17fd98215..840b50fbc4 100644
--- a/pmd-core/src/main/java/net/sourceforge/pmd/RulesetsFactoryUtils.java
+++ b/pmd-core/src/main/java/net/sourceforge/pmd/RulesetsFactoryUtils.java
@@ -37,7 +37,7 @@ public final class RulesetsFactoryUtils {
             ruleSets = factory.createRuleSets(rulesets);
             printRuleNamesInDebug(ruleSets);
             if (ruleSets.ruleCount() == 0) {
-                String msg = "No rules found. Maybe you mispelled a rule name? (" + rulesets + ")";
+                String msg = "No rules found. Maybe you mispelled a rule name? (" + rulesets + ')';
                 LOG.log(Level.SEVERE, msg);
                 throw new IllegalArgumentException(msg);
             }
diff --git a/pmd-core/src/main/java/net/sourceforge/pmd/ant/internal/PMDTaskImpl.java b/pmd-core/src/main/java/net/sourceforge/pmd/ant/internal/PMDTaskImpl.java
index 734fbf2d51..e8928d5a7f 100644
--- a/pmd-core/src/main/java/net/sourceforge/pmd/ant/internal/PMDTaskImpl.java
+++ b/pmd-core/src/main/java/net/sourceforge/pmd/ant/internal/PMDTaskImpl.java
@@ -85,9 +85,9 @@ public class PMDTaskImpl {
         SourceLanguage version = task.getSourceLanguage();
         if (version != null) {
             LanguageVersion languageVersion = LanguageRegistry
-                    .findLanguageVersionByTerseName(version.getName() + " " + version.getVersion());
+                    .findLanguageVersionByTerseName(version.getName() + ' ' + version.getVersion());
             if (languageVersion == null) {
-                throw new BuildException("The following language is not supported:" + version + ".");
+                throw new BuildException("The following language is not supported:" + version + '.');
             }
             configuration.setDefaultLanguageVersion(languageVersion);
         }
diff --git a/pmd-core/src/main/java/net/sourceforge/pmd/cli/PMDParameters.java b/pmd-core/src/main/java/net/sourceforge/pmd/cli/PMDParameters.java
index acf68c391c..0ff46ddb50 100644
--- a/pmd-core/src/main/java/net/sourceforge/pmd/cli/PMDParameters.java
+++ b/pmd-core/src/main/java/net/sourceforge/pmd/cli/PMDParameters.java
@@ -167,7 +167,7 @@ public class PMDParameters {
         configuration.setAnalysisCacheLocation(params.cacheLocation);
 
         LanguageVersion languageVersion = LanguageRegistry
-                .findLanguageVersionByTerseName(params.getLanguage() + " " + params.getVersion());
+                .findLanguageVersionByTerseName(params.getLanguage() + ' ' + params.getVersion());
         if (languageVersion != null) {
             configuration.getLanguageVersionDiscoverer().setDefaultLanguageVersion(languageVersion);
         }
diff --git a/pmd-core/src/main/java/net/sourceforge/pmd/cpd/GUI.java b/pmd-core/src/main/java/net/sourceforge/pmd/cpd/GUI.java
index c706d8037c..322e0366e4 100644
--- a/pmd-core/src/main/java/net/sourceforge/pmd/cpd/GUI.java
+++ b/pmd-core/src/main/java/net/sourceforge/pmd/cpd/GUI.java
@@ -636,7 +636,7 @@ public class GUI implements CPDListener {
             int separatorPos = sourceId.lastIndexOf(File.separatorChar);
             label = "..." + sourceId.substring(separatorPos);
         } else {
-            label = "(" + sourceIDs.size() + " separate files)";
+            label = '(' + sourceIDs.size() + " separate files)";
         }
 
         match.setLabel(label);
diff --git a/pmd-core/src/main/java/net/sourceforge/pmd/cpd/SimpleRenderer.java b/pmd-core/src/main/java/net/sourceforge/pmd/cpd/SimpleRenderer.java
index dc2d4ef38b..b755b52bb2 100644
--- a/pmd-core/src/main/java/net/sourceforge/pmd/cpd/SimpleRenderer.java
+++ b/pmd-core/src/main/java/net/sourceforge/pmd/cpd/SimpleRenderer.java
@@ -46,7 +46,7 @@ public class SimpleRenderer implements Renderer {
         String source = match.getSourceCodeSlice();
 
         if (trimLeadingWhitespace) {
-            String[] lines = source.split("[" + PMD.EOL + "]");
+            String[] lines = source.split('[' + PMD.EOL + ']');
             int trimDepth = StringUtil.maxCommonLeadingWhitespaceForAll(lines);
             if (trimDepth > 0) {
                 lines = StringUtil.trimStartOn(lines, trimDepth);
diff --git a/pmd-core/src/main/java/net/sourceforge/pmd/dcd/graph/MemberNode.java b/pmd-core/src/main/java/net/sourceforge/pmd/dcd/graph/MemberNode.java
index 8f85abe991..01996dc4ea 100644
--- a/pmd-core/src/main/java/net/sourceforge/pmd/dcd/graph/MemberNode.java
+++ b/pmd-core/src/main/java/net/sourceforge/pmd/dcd/graph/MemberNode.java
@@ -81,7 +81,7 @@ public abstract class MemberNode<S extends MemberNode<S, T>, T extends Member>
 
     @Override
     public String toString() {
-        return name + " " + desc;
+        return name + ' ' + desc;
     }
 
     public String toStringLong() {
diff --git a/pmd-core/src/main/java/net/sourceforge/pmd/lang/BaseLanguageModule.java b/pmd-core/src/main/java/net/sourceforge/pmd/lang/BaseLanguageModule.java
index 8b7827c247..d02b956dda 100644
--- a/pmd-core/src/main/java/net/sourceforge/pmd/lang/BaseLanguageModule.java
+++ b/pmd-core/src/main/java/net/sourceforge/pmd/lang/BaseLanguageModule.java
@@ -99,7 +99,7 @@ public abstract class BaseLanguageModule implements Language {
 
     @Override
     public String toString() {
-        return "LanguageModule:" + name + "(" + this.getClass().getSimpleName() + ")";
+        return "LanguageModule:" + name + '(' + this.getClass().getSimpleName() + ')';
     }
 
     @Override
diff --git a/pmd-core/src/main/java/net/sourceforge/pmd/lang/ast/xpath/Attribute.java b/pmd-core/src/main/java/net/sourceforge/pmd/lang/ast/xpath/Attribute.java
index 07a0eaa3cc..9e2de28f7a 100644
--- a/pmd-core/src/main/java/net/sourceforge/pmd/lang/ast/xpath/Attribute.java
+++ b/pmd-core/src/main/java/net/sourceforge/pmd/lang/ast/xpath/Attribute.java
@@ -75,6 +75,6 @@ public class Attribute {
 
     @Override
     public String toString() {
-        return name + ":" + getValue() + ":" + parent;
+        return name + ':' + getValue() + ':' + parent;
     }
 }
diff --git a/pmd-core/src/main/java/net/sourceforge/pmd/lang/dfa/VariableAccess.java b/pmd-core/src/main/java/net/sourceforge/pmd/lang/dfa/VariableAccess.java
index dca5da4676..2676138530 100644
--- a/pmd-core/src/main/java/net/sourceforge/pmd/lang/dfa/VariableAccess.java
+++ b/pmd-core/src/main/java/net/sourceforge/pmd/lang/dfa/VariableAccess.java
@@ -51,13 +51,13 @@ public class VariableAccess {
     @Override
     public String toString() {
         if (isDefinition()) {
-            return "Definition(" + variableName + ")";
+            return "Definition(" + variableName + ')';
         }
         if (isReference()) {
-            return "Reference(" + variableName + ")";
+            return "Reference(" + variableName + ')';
         }
         if (isUndefinition()) {
-            return "Undefinition(" + variableName + ")";
+            return "Undefinition(" + variableName + ')';
         }
         throw new RuntimeException("Access type was never set");
     }
diff --git a/pmd-core/src/main/java/net/sourceforge/pmd/lang/rule/ImportWrapper.java b/pmd-core/src/main/java/net/sourceforge/pmd/lang/rule/ImportWrapper.java
index adee83c793..b094c383a8 100644
--- a/pmd-core/src/main/java/net/sourceforge/pmd/lang/rule/ImportWrapper.java
+++ b/pmd-core/src/main/java/net/sourceforge/pmd/lang/rule/ImportWrapper.java
@@ -101,6 +101,6 @@ public class ImportWrapper {
 
     @Override
     public String toString() {
-        return "Import[name=" + name + ",fullname=" + fullname + ",static*=" + isStaticDemand + "]";
+        return "Import[name=" + name + ",fullname=" + fullname + ",static*=" + isStaticDemand + ']';
     }
 }
diff --git a/pmd-core/src/main/java/net/sourceforge/pmd/properties/AbstractProperty.java b/pmd-core/src/main/java/net/sourceforge/pmd/properties/AbstractProperty.java
index 935d8e4b66..ab1815ddef 100644
--- a/pmd-core/src/main/java/net/sourceforge/pmd/properties/AbstractProperty.java
+++ b/pmd-core/src/main/java/net/sourceforge/pmd/properties/AbstractProperty.java
@@ -100,9 +100,9 @@ import org.apache.commons.lang3.StringUtils;
 
     @Override
     public String toString() {
-        return "[PropertyDescriptor: name=" + name() + ","
-               + " type=" + (isMultiValue() ? "List<" + type() + ">" : type()) + ","
-               + " value=" + defaultValue() + "]";
+        return "[PropertyDescriptor: name=" + name() + ','
+               + " type=" + (isMultiValue() ? "List<" + type() + '>' : type()) + ','
+               + " value=" + defaultValue() + ']';
     }