diff --git a/app/build.gradle b/app/build.gradle
index cee18bede..ce5e451e1 100644
--- a/app/build.gradle
+++ b/app/build.gradle
@@ -74,7 +74,7 @@ apply plugin: "net.ltgt.errorprone"
 
 tasks.withType(JavaCompile) {
     options.compilerArgs += [
-        '-Xep:ClassCanBeStatic:OFF',
+        '-Xep:ClassCanBeStatic:ERROR',
         '-Xep:DefaultCharset:OFF',
         '-Xep:EqualsHashCode:OFF',
         '-Xep:FragmentInjection:OFF',
diff --git a/app/src/main/java/com/keepassdroid/collections/VariantDictionary.java b/app/src/main/java/com/keepassdroid/collections/VariantDictionary.java
index 473dce2a3..c1f441af6 100644
--- a/app/src/main/java/com/keepassdroid/collections/VariantDictionary.java
+++ b/app/src/main/java/com/keepassdroid/collections/VariantDictionary.java
@@ -35,7 +35,7 @@ public class VariantDictionary {
 
     private Map<String, VdType> dict = new HashMap<String, VdType>();
 
-    private class VdType {
+    private static class VdType {
         public static final byte None = 0x00;
         public static final byte UInt32 = 0x04;
         public static final byte UInt64 =0x05;
diff --git a/app/src/main/java/com/keepassdroid/database/PwDatabaseV4.java b/app/src/main/java/com/keepassdroid/database/PwDatabaseV4.java
index 29e7b782d..fd4f554ac 100644
--- a/app/src/main/java/com/keepassdroid/database/PwDatabaseV4.java
+++ b/app/src/main/java/com/keepassdroid/database/PwDatabaseV4.java
@@ -104,7 +104,7 @@ public class PwDatabaseV4 extends PwDatabase {
 
     public String localizedAppName = "KeePassDroid";
     
-    public class MemoryProtectionConfig {
+    public static class MemoryProtectionConfig {
     	public boolean protectTitle = false;
     	public boolean protectUserName = false;
     	public boolean protectPassword = false;
@@ -470,7 +470,7 @@ private String dbNameFromPath(String dbPath) {
 		return filename.substring(0, lastExtDot);
 	}
 
-	private class GroupHasCustomData extends GroupHandler<PwGroup> {
+	private static class GroupHasCustomData extends GroupHandler<PwGroup> {
 
 		public boolean hasCustomData = false;
 
@@ -489,7 +489,7 @@ public boolean operate(PwGroup group) {
 		}
 	}
 
-	private class EntryHasCustomData extends EntryHandler<PwEntry> {
+	private static class EntryHasCustomData extends EntryHandler<PwEntry> {
 
         public boolean hasCustomData = false;
 
@@ -537,4 +537,4 @@ public void clearCache() {
 		binPool.clear();
 	}
 
-}
\ No newline at end of file
+}
diff --git a/app/src/main/java/com/keepassdroid/database/PwDbHeaderV4.java b/app/src/main/java/com/keepassdroid/database/PwDbHeaderV4.java
index d93fa9640..99333a1ca 100644
--- a/app/src/main/java/com/keepassdroid/database/PwDbHeaderV4.java
+++ b/app/src/main/java/com/keepassdroid/database/PwDbHeaderV4.java
@@ -51,7 +51,7 @@ public class PwDbHeaderV4 extends PwDbHeader {
 	public static final int FILE_VERSION_32_4 =           0x00040000;
 	public static final int FILE_VERSION_32 =             FILE_VERSION_32_4;
 
-    public class PwDbHeaderV4Fields {
+    public static class PwDbHeaderV4Fields {
         public static final byte EndOfHeader = 0;
 		public static final byte Comment = 1;
         public static final byte CipherID = 2;
@@ -68,19 +68,19 @@ public class PwDbHeaderV4Fields {
 
     }
 
-	public class PwDbInnerHeaderV4Fields {
+	public static class PwDbInnerHeaderV4Fields {
 		public static final byte EndOfHeader = 0;
 		public static final byte InnerRandomStreamID = 1;
 		public static final byte InnerRandomstreamKey = 2;
 		public static final byte Binary = 3;
 	}
 
-	public class KdbxBinaryFlags {
+	public static class KdbxBinaryFlags {
 		public static final byte None = 0;
 		public static final byte Protected = 1;
 	}
 
-	public class HeaderAndHash {
+	public static class HeaderAndHash {
 		public byte[] header;
 		public byte[] hash;
 
diff --git a/app/src/main/java/com/keepassdroid/database/PwEntryV4.java b/app/src/main/java/com/keepassdroid/database/PwEntryV4.java
index beb17d255..b08a5dd7b 100644
--- a/app/src/main/java/com/keepassdroid/database/PwEntryV4.java
+++ b/app/src/main/java/com/keepassdroid/database/PwEntryV4.java
@@ -62,7 +62,7 @@ public class PwEntryV4 extends PwEntry implements ITimeLogger {
 	public String tags = "";
 	public Map<String, String> customData = new HashMap<String, String>();
 
-	public class AutoType implements Cloneable {
+	public static class AutoType implements Cloneable {
 		private static final long OBF_OPT_NONE = 0;
 		
 		public boolean enabled = true;
diff --git a/app/src/main/java/com/keepassdroid/database/security/ProtectedBinary.java b/app/src/main/java/com/keepassdroid/database/security/ProtectedBinary.java
index e217eb800..00a57871e 100644
--- a/app/src/main/java/com/keepassdroid/database/security/ProtectedBinary.java
+++ b/app/src/main/java/com/keepassdroid/database/security/ProtectedBinary.java
@@ -56,7 +56,7 @@ public class ProtectedBinary {
 	private static final SecureRandom secureRandom = new SecureRandom();
 	private FileParams fileParams;
 
-	private class FileParams {
+	private static class FileParams {
 
 		private File dataFile;
 		public CipherOutputStream cos;
diff --git a/app/src/main/java/com/keepassdroid/utils/SprEngineV4.java b/app/src/main/java/com/keepassdroid/utils/SprEngineV4.java
index 67c9ad97a..4e057c156 100644
--- a/app/src/main/java/com/keepassdroid/utils/SprEngineV4.java
+++ b/app/src/main/java/com/keepassdroid/utils/SprEngineV4.java
@@ -35,7 +35,7 @@ public class SprEngineV4 extends SprEngine {
 	private final String STR_REF_START = "{REF:";
 	private final String STR_REF_END = "}";
 
-	public class TargetResult {
+	public static class TargetResult {
 		public PwEntryV4 entry;
 		public char wanted;