diff --git a/base/src/main/java/org/bitcoinj/base/utils/Fiat.java b/base/src/main/java/org/bitcoinj/base/utils/Fiat.java
index ea61ac985d8..eafcaa11f2f 100644
--- a/base/src/main/java/org/bitcoinj/base/utils/Fiat.java
+++ b/base/src/main/java/org/bitcoinj/base/utils/Fiat.java
@@ -46,7 +46,7 @@ public final class Fiat implements Monetary, Comparable<Fiat> {
 
     private Fiat(final String currencyCode, final long value) {
         this.value = value;
-        this.currencyCode = currencyCode;
+        this.currencyCode = Objects.requireNonNull(currencyCode);
     }
 
     public static Fiat valueOf(final String currencyCode, final long value) {
diff --git a/core/src/main/java/org/bitcoinj/store/BlockStore.java b/core/src/main/java/org/bitcoinj/store/BlockStore.java
index 5a815eaa13f..5581d9a26b4 100644
--- a/core/src/main/java/org/bitcoinj/store/BlockStore.java
+++ b/core/src/main/java/org/bitcoinj/store/BlockStore.java
@@ -19,6 +19,7 @@
 import org.bitcoinj.core.BlockChain;
 import org.bitcoinj.base.Sha256Hash;
 import org.bitcoinj.core.StoredBlock;
+import org.jspecify.annotations.Nullable;
 
 /**
  * An implementor of BlockStore saves StoredBlock objects to disk. Different implementations store them in
@@ -42,6 +43,7 @@ public interface BlockStore {
      * Returns the StoredBlock given a hash. The returned values block.getHash() method will be equal to the
      * parameter. If no such block is found, returns null.
      */
+    @Nullable
     StoredBlock get(Sha256Hash hash) throws BlockStoreException;
 
     /**
@@ -56,7 +58,7 @@ public interface BlockStore {
      * Sets the {@link StoredBlock} that represents the top of the chain of greatest total work.
      */
     void setChainHead(StoredBlock chainHead) throws BlockStoreException;
-    
+
     /** Closes the store. */
     void close() throws BlockStoreException;
 }
diff --git a/core/src/main/java/org/bitcoinj/store/FullPrunedBlockStore.java b/core/src/main/java/org/bitcoinj/store/FullPrunedBlockStore.java
index 65e7092702b..3188bbc40e9 100644
--- a/core/src/main/java/org/bitcoinj/store/FullPrunedBlockStore.java
+++ b/core/src/main/java/org/bitcoinj/store/FullPrunedBlockStore.java
@@ -21,6 +21,7 @@
 import org.bitcoinj.core.StoredUndoableBlock;
 import org.bitcoinj.core.UTXO;
 import org.bitcoinj.core.UTXOProvider;
+import org.jspecify.annotations.Nullable;
 
 
 /**
@@ -66,6 +67,7 @@ public interface FullPrunedBlockStore extends BlockStore, UTXOProvider {
      * Returns the StoredBlock that was added as a StoredUndoableBlock given a hash. The returned values block.getHash()
      * method will be equal to the parameter. If no such block is found, returns null.
      */
+    @Nullable
     StoredBlock getOnceUndoableStoredBlock(Sha256Hash hash) throws BlockStoreException;
 
     /**
@@ -73,11 +75,13 @@ public interface FullPrunedBlockStore extends BlockStore, UTXOProvider {
      * block is found, returns null. Note that this may return null more often than get(Sha256Hash hash) as not all
      * {@link StoredBlock}s have a {@link StoredUndoableBlock} copy stored as well.
      */
+    @Nullable
     StoredUndoableBlock getUndoBlock(Sha256Hash hash) throws BlockStoreException;
     
     /**
      * Gets a {@link UTXO} with the given hash and index, or null if none is found
      */
+    @Nullable
     UTXO getTransactionOutput(Sha256Hash hash, long index) throws BlockStoreException;
     
     /**
@@ -103,6 +107,7 @@ public interface FullPrunedBlockStore extends BlockStore, UTXOProvider {
      * been fully verified and the point in the chain at which the unspent transaction output set in this
      * store represents.
      */
+    @Nullable
     StoredBlock getVerifiedChainHead() throws BlockStoreException;
 
     /**
diff --git a/core/src/main/java/org/bitcoinj/store/MemoryBlockStore.java b/core/src/main/java/org/bitcoinj/store/MemoryBlockStore.java
index 26eb7b7e2ee..516859f64e2 100644
--- a/core/src/main/java/org/bitcoinj/store/MemoryBlockStore.java
+++ b/core/src/main/java/org/bitcoinj/store/MemoryBlockStore.java
@@ -20,6 +20,7 @@
 import org.bitcoinj.base.Sha256Hash;
 import org.bitcoinj.core.StoredBlock;
 import org.bitcoinj.core.VerificationException;
+import org.jspecify.annotations.Nullable;
 
 import java.util.LinkedHashMap;
 import java.util.Map;
@@ -28,10 +29,10 @@
  * Keeps {@link StoredBlock}s in memory. Used primarily for unit testing.
  */
 public class MemoryBlockStore implements BlockStore {
-    private LinkedHashMap<Sha256Hash, StoredBlock> blockMap = new LinkedHashMap<Sha256Hash, StoredBlock>() {
+    private @Nullable LinkedHashMap<Sha256Hash, StoredBlock> blockMap = new LinkedHashMap<Sha256Hash, StoredBlock>() {
         @Override
         protected boolean removeEldestEntry(Map.Entry<Sha256Hash, StoredBlock> eldest) {
-            return blockMap.size() > 5000;
+            return this.size() > 5000;
         }
     };
     private StoredBlock chainHead;
@@ -41,7 +42,7 @@ public MemoryBlockStore(Block genesisBlock) {
             Block genesisHeader = genesisBlock.asHeader();
             StoredBlock storedGenesis = new StoredBlock(genesisHeader, genesisHeader.getWork(), 0);
             put(storedGenesis);
-            setChainHead(storedGenesis);
+            chainHead = storedGenesis;
         } catch (BlockStoreException | VerificationException e) {
             throw new RuntimeException(e);  // Cannot happen.
         }
@@ -55,6 +56,7 @@ public synchronized final void put(StoredBlock block) throws BlockStoreException
     }
 
     @Override
+    @Nullable
     public synchronized StoredBlock get(Sha256Hash hash) throws BlockStoreException {
         if (blockMap == null) throw new BlockStoreException("MemoryBlockStore is closed");
         return blockMap.get(hash);
diff --git a/core/src/main/java/org/bitcoinj/store/MemoryFullPrunedBlockStore.java b/core/src/main/java/org/bitcoinj/store/MemoryFullPrunedBlockStore.java
index b4c2f807b9e..4fdcf268d9f 100644
--- a/core/src/main/java/org/bitcoinj/store/MemoryFullPrunedBlockStore.java
+++ b/core/src/main/java/org/bitcoinj/store/MemoryFullPrunedBlockStore.java
@@ -34,6 +34,7 @@
 
 import org.jspecify.annotations.Nullable;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.LinkedList;
@@ -194,10 +195,10 @@ protected static class StoredBlockAndWasUndoableFlag {
         public boolean wasUndoable;
         public StoredBlockAndWasUndoableFlag(StoredBlock block, boolean wasUndoable) { this.block = block; this.wasUndoable = wasUndoable; }
     }
-    private TransactionalHashMap<Sha256Hash, StoredBlockAndWasUndoableFlag> blockMap;
-    private TransactionalFullBlockMap fullBlockMap;
+    private @Nullable TransactionalHashMap<Sha256Hash, StoredBlockAndWasUndoableFlag> blockMap;
+    private @Nullable TransactionalFullBlockMap fullBlockMap;
     //TODO: Use something more suited to remove-heavy use?
-    private TransactionalHashMap<TransactionOutPoint, UTXO> transactionOutputMap;
+    private @Nullable TransactionalHashMap<TransactionOutPoint, UTXO> transactionOutputMap;
     private StoredBlock chainHead;
     private StoredBlock verifiedChainHead;
     private final int fullStoreDepth;
@@ -209,20 +210,19 @@ protected static class StoredBlockAndWasUndoableFlag {
      * @param fullStoreDepth The depth of blocks to keep FullStoredBlocks instead of StoredBlocks
      */
     public MemoryFullPrunedBlockStore(NetworkParameters params, int fullStoreDepth) {
+        network = params.network();
         blockMap = new TransactionalHashMap<>();
         fullBlockMap = new TransactionalFullBlockMap();
         transactionOutputMap = new TransactionalHashMap<>();
         this.fullStoreDepth = fullStoreDepth > 0 ? fullStoreDepth : 1;
         // Insert the genesis block.
+        StoredBlock storedGenesisHeader = new StoredBlock(params.getGenesisBlock().asHeader(), params.getGenesisBlock().getWork(), 0);
+        // The coinbase in the genesis block is not spendable
+        StoredUndoableBlock storedGenesis = new StoredUndoableBlock(params.getGenesisBlock().getHash(), Collections.emptyList());
         try {
-            StoredBlock storedGenesisHeader = new StoredBlock(params.getGenesisBlock().asHeader(), params.getGenesisBlock().getWork(), 0);
-            // The coinbase in the genesis block is not spendable
-            List<Transaction> genesisTransactions = new LinkedList<>();
-            StoredUndoableBlock storedGenesis = new StoredUndoableBlock(params.getGenesisBlock().getHash(), genesisTransactions);
             put(storedGenesisHeader, storedGenesis);
-            setChainHead(storedGenesisHeader);
-            setVerifiedChainHead(storedGenesisHeader);
-            network = params.network();
+            chainHead = storedGenesisHeader;
+            verifiedChainHead = storedGenesisHeader;
         } catch (BlockStoreException | VerificationException e) {
             throw new RuntimeException(e);  // Cannot happen.
         }
@@ -238,6 +238,8 @@ public synchronized void put(StoredBlock block) throws BlockStoreException {
     @Override
     public synchronized final void put(StoredBlock storedBlock, StoredUndoableBlock undoableBlock) throws BlockStoreException {
         Objects.requireNonNull(blockMap, "MemoryFullPrunedBlockStore is closed");
+        Objects.requireNonNull(fullBlockMap);
+        Objects.requireNonNull(blockMap);
         Sha256Hash hash = storedBlock.getHeader().getHash();
         fullBlockMap.put(hash, storedBlock.getHeight(), undoableBlock);
         blockMap.put(hash, new StoredBlockAndWasUndoableFlag(storedBlock, true));
@@ -292,6 +294,7 @@ public synchronized final void setVerifiedChainHead(StoredBlock chainHead) throw
             setChainHead(chainHead);
         // Potential leak here if not all blocks get setChainHead'd
         // Though the FullPrunedBlockStore allows for this, the current AbstractBlockChain will not do it.
+        Objects.requireNonNull(fullBlockMap);
         fullBlockMap.removeByHeight(chainHead.getHeight() - fullStoreDepth);
     }
     
@@ -324,6 +327,9 @@ public synchronized void removeUnspentTransactionOutput(UTXO out) throws BlockSt
 
     @Override
     public synchronized void beginDatabaseBatchWrite() throws BlockStoreException {
+        Objects.requireNonNull(blockMap);
+        Objects.requireNonNull(fullBlockMap);
+        Objects.requireNonNull(transactionOutputMap);
         blockMap.beginDatabaseBatchWrite();
         fullBlockMap.BeginTransaction();
         transactionOutputMap.beginDatabaseBatchWrite();
@@ -331,6 +337,9 @@ public synchronized void beginDatabaseBatchWrite() throws BlockStoreException {
 
     @Override
     public synchronized void commitDatabaseBatchWrite() throws BlockStoreException {
+        Objects.requireNonNull(blockMap);
+        Objects.requireNonNull(fullBlockMap);
+        Objects.requireNonNull(transactionOutputMap);
         blockMap.commitDatabaseBatchWrite();
         fullBlockMap.CommitTransaction();
         transactionOutputMap.commitDatabaseBatchWrite();
@@ -338,6 +347,9 @@ public synchronized void commitDatabaseBatchWrite() throws BlockStoreException {
 
     @Override
     public synchronized void abortDatabaseBatchWrite() throws BlockStoreException {
+        Objects.requireNonNull(blockMap);
+        Objects.requireNonNull(fullBlockMap);
+        Objects.requireNonNull(transactionOutputMap);
         blockMap.abortDatabaseBatchWrite();
         fullBlockMap.AbortTransaction();
         transactionOutputMap.abortDatabaseBatchWrite();
@@ -359,7 +371,7 @@ public Network network() {
     @Override
     public int getChainHeadHeight() throws UTXOProviderException {
         try {
-            return getVerifiedChainHead().getHeight();
+            return Objects.requireNonNull(getVerifiedChainHead()).getHeight();
         } catch (BlockStoreException e) {
             throw new UTXOProviderException(e);
         }
@@ -367,6 +379,7 @@ public int getChainHeadHeight() throws UTXOProviderException {
 
     @Override
     public List<UTXO> getOpenTransactionOutputs(List<ECKey> keys) throws UTXOProviderException {
+        Objects.requireNonNull(transactionOutputMap, "MemoryFullPrunedBlockStore is closed");
         // This is *NOT* optimal: We go through all the outputs and select the ones we are looking for.
         // If someone uses this store for production then they have a lot more to worry about than an inefficient impl :)
         List<UTXO> foundOutputs = new ArrayList<>();
diff --git a/core/src/main/java/org/bitcoinj/store/SPVBlockStore.java b/core/src/main/java/org/bitcoinj/store/SPVBlockStore.java
index e0ac1e52a8f..5359fa897aa 100644
--- a/core/src/main/java/org/bitcoinj/store/SPVBlockStore.java
+++ b/core/src/main/java/org/bitcoinj/store/SPVBlockStore.java
@@ -28,6 +28,7 @@
 
 import org.jspecify.annotations.Nullable;
 import java.io.File;
+import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.io.RandomAccessFile;
 import java.nio.Buffer;
@@ -65,7 +66,7 @@ public class SPVBlockStore implements BlockStore {
     // Magic header for the V2 format.
     static final byte[] HEADER_MAGIC_V2 = "SPV2".getBytes(StandardCharsets.US_ASCII);
 
-    protected volatile MappedByteBuffer buffer;
+    protected volatile @Nullable MappedByteBuffer buffer;
     protected final NetworkParameters params;
 
     // The entire ring-buffer is mmapped and accessing it should be as fast as accessing regular memory once it's
@@ -95,8 +96,8 @@ protected boolean removeEldestEntry(Map.Entry<Sha256Hash, Object> entry) {
         }
     };
     // Used to stop other applications/processes from opening the store.
-    protected FileLock fileLock = null;
-    protected RandomAccessFile randomAccessFile = null;
+    protected @Nullable FileLock fileLock;
+    protected final RandomAccessFile randomAccessFile;
     private final FileChannel channel;
     private int fileLength;
 
@@ -123,11 +124,16 @@ public SPVBlockStore(NetworkParameters params, File file, int capacity, boolean
         this.params = Objects.requireNonNull(params);
         checkArgument(capacity > 0);
 
-        try {
-            boolean exists = file.exists();
+        boolean exists = file.exists();
 
+        try {
             // Set up the backing file, empty if it doesn't exist.
             randomAccessFile = new RandomAccessFile(file, "rw");
+        } catch (FileNotFoundException e) {
+            throw new BlockStoreException(e);
+        }
+
+        try {
             channel = randomAccessFile.getChannel();
 
             // Lock the file.
@@ -192,7 +198,7 @@ public SPVBlockStore(NetworkParameters params, File file, int capacity, boolean
                         StandardCharsets.US_ASCII));
         } catch (Exception e) {
             try {
-                if (randomAccessFile != null) randomAccessFile.close();
+                randomAccessFile.close();
             } catch (IOException e2) {
                 throw new BlockStoreException(e2);
             }
@@ -201,6 +207,7 @@ public SPVBlockStore(NetworkParameters params, File file, int capacity, boolean
     }
 
     private void initNewStore(Block genesisBlock) throws Exception {
+        Objects.requireNonNull(buffer);
         ((Buffer) buffer).rewind();
         buffer.put(HEADER_MAGIC_V2);
         // Insert the genesis block.
@@ -225,6 +232,7 @@ private void migrateV1toV2() throws BlockStoreException, IOException {
 
         randomAccessFile.setLength(fileLength);
         // Map it into memory again because of the length change.
+        Objects.requireNonNull(buffer);
         buffer.force();
         buffer = channel.map(FileChannel.MapMode.READ_WRITE, 0, fileLength);
 
@@ -327,7 +335,7 @@ public StoredBlock get(Sha256Hash hash) throws BlockStoreException {
         } finally { lock.unlock(); }
     }
 
-    protected StoredBlock lastChainHead = null;
+    protected @Nullable StoredBlock lastChainHead;
 
     @Override
     public StoredBlock getChainHead() throws BlockStoreException {
@@ -366,14 +374,19 @@ public void setChainHead(StoredBlock chainHead) throws BlockStoreException {
 
     @Override
     public void close() throws BlockStoreException {
-        try {
+        if (buffer != null) {
             buffer.force();
             buffer = null;  // Allow it to be GCd and the underlying file mapping to go away.
-            fileLock.release();
+        }
+        try {
+            if (fileLock != null) {
+                fileLock.release();
+            }
             randomAccessFile.close();
-            blockCache.clear();
         } catch (IOException e) {
             throw new BlockStoreException(e);
+        } finally {
+            blockCache.clear();
         }
     }
 
@@ -395,6 +408,7 @@ public void close() throws BlockStoreException {
 
     /** Returns the offset from the file start where the latest block should be written (end of prev block). */
     int getRingCursor() {
+        Objects.requireNonNull(buffer);
         int c = buffer.getInt(4);
         checkState(c >= FILE_PROLOGUE_BYTES, () ->
                 "integer overflow");
@@ -403,10 +417,12 @@ int getRingCursor() {
 
     private void setRingCursor(int newCursor) {
         checkArgument(newCursor >= 0);
+        Objects.requireNonNull(buffer);
         buffer.putInt(4, newCursor);
     }
 
     public void clear() throws Exception {
+        Objects.requireNonNull(buffer);
         lock.lock();
         try {
             // Clear caches
diff --git a/core/src/main/java/org/bitcoinj/store/package-info.java b/core/src/main/java/org/bitcoinj/store/package-info.java
index 5d4a8c335da..2d5bafd49f1 100644
--- a/core/src/main/java/org/bitcoinj/store/package-info.java
+++ b/core/src/main/java/org/bitcoinj/store/package-info.java
@@ -19,4 +19,7 @@
  * buffer of headers on disk and is suitable for lightweight user wallets, a store that can calculate a full indexed
  * UTXO set (i.e. it can query address balances), and a memory only store useful for unit tests.
  */
+@NullMarked
 package org.bitcoinj.store;
+
+import org.jspecify.annotations.NullMarked;
diff --git a/core/src/main/java/org/bitcoinj/utils/BlockFileLoader.java b/core/src/main/java/org/bitcoinj/utils/BlockFileLoader.java
index 062e719ce59..b0120fed6a7 100644
--- a/core/src/main/java/org/bitcoinj/utils/BlockFileLoader.java
+++ b/core/src/main/java/org/bitcoinj/utils/BlockFileLoader.java
@@ -22,6 +22,7 @@
 import org.bitcoinj.core.MessageSerializer;
 import org.bitcoinj.core.NetworkParameters;
 import org.bitcoinj.core.ProtocolException;
+import org.jspecify.annotations.Nullable;
 
 import java.io.BufferedInputStream;
 import java.io.File;
@@ -109,7 +110,7 @@ public BlockFileLoader(Network network, List<File> files) {
     public class BlockFileIterator implements Iterator<ByteBuffer> {
         private final File file;
         private final BufferedInputStream currentFileStream;
-        private ByteBuffer nextBlock = null;
+        private @Nullable ByteBuffer nextBlock = null;
 
         public BlockFileIterator(File blockFile) throws FileNotFoundException {
             this.file = blockFile;
@@ -124,6 +125,7 @@ public boolean hasNext() {
         }
 
         @Override
+        @Nullable
         public ByteBuffer next() throws NoSuchElementException {
             if (!hasNext())
                 throw new NoSuchElementException();
diff --git a/core/src/main/java/org/bitcoinj/utils/BtcFormat.java b/core/src/main/java/org/bitcoinj/utils/BtcFormat.java
index 2ae6df33977..d473d2bb6b0 100644
--- a/core/src/main/java/org/bitcoinj/utils/BtcFormat.java
+++ b/core/src/main/java/org/bitcoinj/utils/BtcFormat.java
@@ -18,6 +18,7 @@
 
 import org.bitcoinj.base.Coin;
 import org.bitcoinj.utils.BtcAutoFormat.Style;
+import org.jspecify.annotations.Nullable;
 
 import java.math.BigDecimal;
 import java.math.BigInteger;
@@ -1309,6 +1310,7 @@ else if (qty instanceof Coin)
       * {@link Coin} object that represents the parsed value.
       * @see NumberFormat */
     @Override
+    @Nullable
     public final Object parseObject(String source, ParsePosition pos) { return parse(source, pos); }
 
     private static class ScaleMatcher {
@@ -1320,8 +1322,8 @@ private static class ScaleMatcher {
     /* Lazy initialization;  No reason to create all these objects unless needed for parsing */
     // coin indicator regex String; TODO: does this need to be volatile?
     private volatile String ci = "(" + COIN_SYMBOL + "|" + COIN_SYMBOL_ALT + "|B⃦|" + COIN_CODE + "|XBT)";
-    private Pattern coinPattern;
-    private volatile ScaleMatcher[] denoms;
+    private @Nullable Pattern coinPattern;
+    private volatile ScaleMatcher @Nullable [] denoms;
     ScaleMatcher[] denomMatchers() {
         ScaleMatcher[] result = denoms;
         if (result == null) { synchronized(this) {
@@ -1412,6 +1414,7 @@ protected static void prefixUnitsIndicator(DecimalFormat numberFormat, int scale
      * @return a Coin object representing the parsed value
      * @see java.text.ParsePosition
      */
+    @Nullable
     public Coin parse(String source, ParsePosition pos) {
         DecimalFormatSymbols anteSigns = null;
         int parseScale = COIN_SCALE; // default
@@ -1427,6 +1430,7 @@ public Coin parse(String source, ParsePosition pos) {
                     }
                 }
                 if (parseScale == COIN_SCALE) {
+                    Objects.requireNonNull(coinPattern);
                     Matcher matcher = coinPattern.matcher(source);
                     matcher.find();
                     anteSigns = setSymbolAndCode(numberFormat, matcher.group());
diff --git a/core/src/main/java/org/bitcoinj/utils/VersionTally.java b/core/src/main/java/org/bitcoinj/utils/VersionTally.java
index 066b99b6f86..8575a4c9ca0 100644
--- a/core/src/main/java/org/bitcoinj/utils/VersionTally.java
+++ b/core/src/main/java/org/bitcoinj/utils/VersionTally.java
@@ -20,6 +20,7 @@
 import org.bitcoinj.core.StoredBlock;
 import org.bitcoinj.store.BlockStore;
 import org.bitcoinj.store.BlockStoreException;
+import org.jspecify.annotations.Nullable;
 
 import java.util.Stack;
 
@@ -75,6 +76,7 @@ public void add(final long version) {
      * @return the count for the block version, or null if the window is not yet
      * full.
      */
+    @Nullable
     public Integer getCountAtOrAbove(final long version) {
         if (versionsStored < versionWindow.length) {
             return null;
diff --git a/core/src/main/java/org/bitcoinj/utils/package-info.java b/core/src/main/java/org/bitcoinj/utils/package-info.java
index fe2cc0017d9..7663e129d99 100644
--- a/core/src/main/java/org/bitcoinj/utils/package-info.java
+++ b/core/src/main/java/org/bitcoinj/utils/package-info.java
@@ -16,4 +16,7 @@
  * Formatting monetary amounts, representing exchange rates, a program for loading Bitcoin Core saved block files,
  * a class to control how bitcoinj uses threads and misc other utility classes that don't fit anywhere else.
  */
-package org.bitcoinj.utils;
\ No newline at end of file
+@NullMarked
+package org.bitcoinj.utils;
+
+import org.jspecify.annotations.NullMarked;
diff --git a/core/src/test/java/org/bitcoinj/store/WalletProtobufSerializerTest.java b/core/src/test/java/org/bitcoinj/store/WalletProtobufSerializerTest.java
index e542f83e600..cc08ab38ca5 100644
--- a/core/src/test/java/org/bitcoinj/store/WalletProtobufSerializerTest.java
+++ b/core/src/test/java/org/bitcoinj/store/WalletProtobufSerializerTest.java
@@ -192,7 +192,9 @@ public void doubleSpend() throws Exception {
         myWallet.receiveFromBlock(doubleSpends.t2, null, BlockChain.NewBlockType.BEST_CHAIN, 0);
         Wallet wallet1 = roundTrip(myWallet);
         assertEquals(1, wallet1.getTransactions(true).size());
+        assertNotNull(doubleSpends.t1);
         Transaction t1 = wallet1.getTransaction(doubleSpends.t1.getTxId());
+        assertNotNull(t1);
         assertEquals(ConfidenceType.DEAD, t1.getConfidence().getConfidenceType());
         assertEquals(Coin.ZERO, wallet1.getBalance());
 
@@ -408,7 +410,7 @@ public void extensions() throws Exception {
             new WalletProtobufSerializer().readWallet(BitcoinNetwork.TESTNET, null, proto);
             fail();
         } catch (UnreadableWalletException e) {
-            assertTrue(e.getMessage().contains("mandatory"));
+            assertTrue(Objects.requireNonNull(e.getMessage()).contains("mandatory"));
         }
         Wallet wallet = new WalletProtobufSerializer().readWallet(BitcoinNetwork.TESTNET,
                 new WalletExtension[]{ new FooWalletExtension("com.whatever.required", true) },
diff --git a/core/src/test/java/org/bitcoinj/utils/ExchangeRateTest.java b/core/src/test/java/org/bitcoinj/utils/ExchangeRateTest.java
index 5740c20cae2..e4303daccab 100644
--- a/core/src/test/java/org/bitcoinj/utils/ExchangeRateTest.java
+++ b/core/src/test/java/org/bitcoinj/utils/ExchangeRateTest.java
@@ -51,11 +51,6 @@ public void currencyCodeMismatch() {
         rate.fiatToCoin(Fiat.parseFiat("USD", "1"));
     }
 
-    @Test(expected = NullPointerException.class)
-    public void constructMissingCurrencyCode() {
-        new ExchangeRate(Fiat.valueOf(null, 1));
-    }
-
     @Test(expected = IllegalArgumentException.class)
     public void constructNegativeCoin() {
         new ExchangeRate(Coin.valueOf(-1), Fiat.valueOf("EUR", 1));
diff --git a/core/src/test/java/org/bitcoinj/utils/VersionTallyTest.java b/core/src/test/java/org/bitcoinj/utils/VersionTallyTest.java
index d3f4c68fba8..8c71283ccc5 100644
--- a/core/src/test/java/org/bitcoinj/utils/VersionTallyTest.java
+++ b/core/src/test/java/org/bitcoinj/utils/VersionTallyTest.java
@@ -34,8 +34,10 @@
 
 import java.time.Instant;
 import java.time.temporal.ChronoUnit;
+import java.util.Objects;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertNull;
 
 public class VersionTallyTest {
@@ -64,7 +66,7 @@ public void testNullWhileEmpty() {
             assertNull(instance.getCountAtOrAbove(1));
             instance.add(1);
         }
-        assertEquals(TESTNET.getMajorityWindow(), instance.getCountAtOrAbove(1).intValue());
+        assertEquals(TESTNET.getMajorityWindow(), Objects.requireNonNull(instance.getCountAtOrAbove(1)).intValue());
     }
 
     /**
@@ -88,21 +90,21 @@ public void testVersionCounts() {
             assertNull(instance.getCountAtOrAbove(1));
             instance.add(1);
         }
-        assertEquals(TESTNET.getMajorityWindow(), instance.getCountAtOrAbove(1).intValue());
+        assertEquals(TESTNET.getMajorityWindow(), Objects.requireNonNull(instance.getCountAtOrAbove(1)).intValue());
 
         // Check the count updates as we replace with 2s
         for (int i = 0; i < TESTNET.getMajorityWindow(); i++) {
-            assertEquals(i, instance.getCountAtOrAbove(2).intValue());
+            assertEquals(i, Objects.requireNonNull(instance.getCountAtOrAbove(2)).intValue());
             instance.add(2);
         }
  
         // Inject a rogue 1
         instance.add(1);
-        assertEquals(TESTNET.getMajorityWindow() - 1, instance.getCountAtOrAbove(2).intValue());
+        assertEquals(TESTNET.getMajorityWindow() - 1, Objects.requireNonNull(instance.getCountAtOrAbove(2)).intValue());
 
         // Check we accept high values as well
         instance.add(10);
-        assertEquals(TESTNET.getMajorityWindow() - 1, instance.getCountAtOrAbove(2).intValue());
+        assertEquals(TESTNET.getMajorityWindow() - 1, Objects.requireNonNull(instance.getCountAtOrAbove(2)).intValue());
     }
 
     @Test
@@ -119,9 +121,10 @@ public void testInitialize() throws BlockStoreException {
             assertEquals(2, chainHead.getHeader().version());
             time = time.plus(1, ChronoUnit.MINUTES);
         }
+        assertNotNull(chainHead);
 
         VersionTally instance = new VersionTally(TESTNET);
         instance.initialize(blockStore, chainHead);
-        assertEquals(TESTNET.getMajorityWindow(), instance.getCountAtOrAbove(2).intValue());
+        assertEquals(TESTNET.getMajorityWindow(), Objects.requireNonNull(instance.getCountAtOrAbove(2)).intValue());
     }
 }