Fix more code issues

pgpainless-core/src/main/java/org/pgpainless/algorithm/AlgorithmSuite.java

@@ -16,7 +16,7 @@ import java.util.Set;
  */
 public class AlgorithmSuite {
 
-    private static AlgorithmSuite defaultAlgorithmSuite = new AlgorithmSuite(
+    private static final AlgorithmSuite defaultAlgorithmSuite = new AlgorithmSuite(
             Arrays.asList(
                     SymmetricKeyAlgorithm.AES_256,
                     SymmetricKeyAlgorithm.AES_192,

pgpainless-core/src/main/java/org/pgpainless/decryption_verification/DecryptionBuilder.java

@@ -24,7 +24,7 @@ public class DecryptionBuilder implements DecryptionBuilderInterface {
 
     class DecryptWithImpl implements DecryptWith {
 
-        private BufferedInputStream inputStream;
+        private final BufferedInputStream inputStream;
 
         DecryptWithImpl(InputStream inputStream) {
             this.inputStream = new BufferedInputStream(inputStream, BUFFER_SIZE);

pgpainless-core/src/main/java/org/pgpainless/encryption_signing/EncryptionResult.java

@@ -145,7 +145,7 @@ public final class EncryptionResult {
         private CompressionAlgorithm compressionAlgorithm;
 
         private final MultiMap<SubkeyIdentifier, PGPSignature> detachedSignatures = new MultiMap<>();
-        private Set<SubkeyIdentifier> recipients = new HashSet<>();
+        private final Set<SubkeyIdentifier> recipients = new HashSet<>();
         private String fileName = "";
         private Date modificationDate = new Date(0L); // NOW
         private StreamEncoding encoding = StreamEncoding.BINARY;

pgpainless-core/src/main/java/org/pgpainless/key/info/KeyRingInfo.java

@@ -53,7 +53,7 @@ public class KeyRingInfo {
     private static final Pattern PATTERN_EMAIL = Pattern.compile("[A-Z0-9a-z._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,6}");
 
     private final PGPKeyRing keys;
-    private Signatures signatures;
+    private final Signatures signatures;
     private final Date evaluationDate;
     private final String primaryUserId;
 

pgpainless-core/src/test/java/org/pgpainless/decryption_verification/MissingPassphraseForDecryptionTest.java

@@ -18,7 +18,6 @@ import java.nio.charset.StandardCharsets;
 import java.security.InvalidAlgorithmParameterException;
 import java.security.NoSuchAlgorithmException;
 import java.util.List;
-import javax.annotation.Nullable;
 
 import org.bouncycastle.openpgp.PGPException;
 import org.bouncycastle.openpgp.PGPPublicKey;
@@ -41,7 +40,7 @@ import org.pgpainless.util.Passphrase;
 
 public class MissingPassphraseForDecryptionTest {
 
-    private String passphrase = "dragon123";
+    private final String passphrase = "dragon123";
     private PGPSecretKeyRing secretKeys;
     private byte[] message;
 
@@ -63,7 +62,6 @@ public class MissingPassphraseForDecryptionTest {
     @Test
     public void invalidPostponedKeysStrategyTest() {
         SecretKeyPassphraseProvider callback = new SecretKeyPassphraseProvider() {
-            @Nullable
             @Override
             public Passphrase getPassphraseFor(Long keyId) {
                 fail("MUST NOT get called in if postponed key strategy is invalid.");
@@ -88,7 +86,6 @@ public class MissingPassphraseForDecryptionTest {
     public void interactiveStrategy() throws PGPException, IOException {
         // interactive callback
         SecretKeyPassphraseProvider callback = new SecretKeyPassphraseProvider() {
-            @Nullable
             @Override
             public Passphrase getPassphraseFor(Long keyId) {
                 // is called in interactive mode
@@ -121,7 +118,6 @@ public class MissingPassphraseForDecryptionTest {
         List<PGPPublicKey> encryptionKeys = info.getEncryptionSubkeys(EncryptionPurpose.ANY);
 
         SecretKeyPassphraseProvider callback = new SecretKeyPassphraseProvider() {
-            @Nullable
             @Override
             public Passphrase getPassphraseFor(Long keyId) {
                 fail("MUST NOT get called in non-interactive mode.");

pgpainless-core/src/test/java/org/pgpainless/provider/ProviderFactoryTest.java

@@ -8,27 +8,15 @@ import static org.junit.jupiter.api.Assertions.assertEquals;
 
 import java.security.Provider;
 
+import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.Test;
 
 public class ProviderFactoryTest {
 
-    @Test
-    public void providerFactoryDefaultIsBouncyCastleTest() {
-        assertEquals("BC", ProviderFactory.getProviderName());
-    }
-
-    @Test
-    public void setCustomProviderTest() {
-        ProviderFactory.setFactory(customProviderFactory);
-        assertEquals("PL", ProviderFactory.getProviderName());
-        // Reset back to BouncyCastle
-        ProviderFactory.setFactory(new BouncyCastleProviderFactory());
-    }
-
-    private ProviderFactory customProviderFactory = new ProviderFactory() {
+    private final ProviderFactory customProviderFactory = new ProviderFactory() {
 
         @SuppressWarnings("deprecation")
-        Provider provider = new Provider("PL", 1L, "PGPainlessTestProvider") {
+        final Provider provider = new Provider("PL", 1L, "PGPainlessTestProvider") {
 
         };
 
@@ -42,4 +30,21 @@ public class ProviderFactoryTest {
             return provider.getName();
         }
     };
+
+    @Test
+    public void providerFactoryDefaultIsBouncyCastleTest() {
+        assertEquals("BC", ProviderFactory.getProviderName());
+    }
+
+    @Test
+    public void setCustomProviderTest() {
+        ProviderFactory.setFactory(customProviderFactory);
+        assertEquals("PL", ProviderFactory.getProviderName());
+    }
+
+    @AfterEach
+    public void resetToDefault() {
+        // Reset back to BouncyCastle
+        ProviderFactory.setFactory(new BouncyCastleProviderFactory());
+    }
 }

pgpainless-sop/src/main/java/org/pgpainless/sop/DetachInbandSignatureAndMessageImpl.java

@@ -37,11 +37,13 @@ public class DetachInbandSignatureAndMessageImpl implements DetachInbandSignatur
 
         return new ReadyWithResult<Signatures>() {
 
-            private ByteArrayOutputStream sigOut = new ByteArrayOutputStream();
-            @Override
-            public Signatures writeTo(OutputStream messageOutputStream) throws SOPGPException.NoSignature, IOException {
+            private final ByteArrayOutputStream sigOut = new ByteArrayOutputStream();
 
-                PGPSignatureList signatures = null;
+            @Override
+            public Signatures writeTo(OutputStream messageOutputStream)
+                    throws SOPGPException.NoSignature, IOException {
+
+                PGPSignatureList signatures;
                 try {
                     signatures = ClearsignedMessageUtil.detachSignaturesFromInbandClearsignedMessage(messageInputStream, messageOutputStream);
                 } catch (WrongConsumingMethodException e) {