diff --git a/app/src/androidTest/java/mock/MockFDroidResources.java b/app/src/androidTest/java/mock/MockFDroidResources.java
index b559bd83e..5cb33798c 100644
--- a/app/src/androidTest/java/mock/MockFDroidResources.java
+++ b/app/src/androidTest/java/mock/MockFDroidResources.java
@@ -7,7 +7,7 @@ import org.fdroid.fdroid.R;
 
 public class MockFDroidResources extends MockResources {
 
-    private Context getStringDelegatingContext;
+    private final Context getStringDelegatingContext;
 
     public MockFDroidResources(Context getStringDelegatingContext) {
         this.getStringDelegatingContext = getStringDelegatingContext;
diff --git a/app/src/androidTest/java/mock/MockInstallablePackageManager.java b/app/src/androidTest/java/mock/MockInstallablePackageManager.java
index 4ff3aef71..b7056711f 100644
--- a/app/src/androidTest/java/mock/MockInstallablePackageManager.java
+++ b/app/src/androidTest/java/mock/MockInstallablePackageManager.java
@@ -10,7 +10,7 @@ import java.util.List;
 
 public class MockInstallablePackageManager extends MockPackageManager {
 
-    private List<PackageInfo> info = new ArrayList<>();
+    private final List<PackageInfo> info = new ArrayList<>();
 
     @Override
     public List<PackageInfo> getInstalledPackages(int flags) {
diff --git a/app/src/main/java/com/google/zxing/encode/QRCodeEncoder.java b/app/src/main/java/com/google/zxing/encode/QRCodeEncoder.java
index 6200701f6..075209055 100755
--- a/app/src/main/java/com/google/zxing/encode/QRCodeEncoder.java
+++ b/app/src/main/java/com/google/zxing/encode/QRCodeEncoder.java
@@ -37,7 +37,7 @@ public final class QRCodeEncoder {
     private static final int WHITE = 0xFFFFFFFF;
     private static final int BLACK = 0xFF000000;
 
-    private int dimension = Integer.MIN_VALUE;
+    private final int dimension;
     private String contents;
     private String displayContents;
     private String title;
diff --git a/app/src/main/java/org/fdroid/fdroid/installer/PrivilegedInstaller.java b/app/src/main/java/org/fdroid/fdroid/installer/PrivilegedInstaller.java
index ed9419d4c..b24adb690 100644
--- a/app/src/main/java/org/fdroid/fdroid/installer/PrivilegedInstaller.java
+++ b/app/src/main/java/org/fdroid/fdroid/installer/PrivilegedInstaller.java
@@ -76,7 +76,7 @@ public class PrivilegedInstaller extends Installer {
     private static final String PRIVILEGED_EXTENSION_SERVICE_INTENT = "org.fdroid.fdroid.privileged.IPrivilegedService";
     public static final String PRIVILEGED_EXTENSION_PACKAGE_NAME = "org.fdroid.fdroid.privileged";
 
-    private Activity mActivity;
+    private final Activity mActivity;
 
     private static final int REQUEST_CONFIRM_PERMS = 0;
 
diff --git a/config/pmd/rules.xml b/config/pmd/rules.xml
index 34bdcfbb1..7af77cb40 100644
--- a/config/pmd/rules.xml
+++ b/config/pmd/rules.xml
@@ -25,4 +25,5 @@
     <rule ref="rulesets/java/design.xml/NonCaseLabelInSwitchStatement"/>
     <rule ref="rulesets/java/design.xml/EqualsNull"/>
     <rule ref="rulesets/java/design.xml/IdempotentOperations"/>
+    <rule ref="rulesets/java/design.xml/ImmutableField"/>
 </ruleset>