In comments and one log output, use wording "best chain" instead of "main chain".

core/src/main/java/org/bitcoinj/core/AbstractBlockChain.java

@@ -581,7 +581,7 @@ public abstract class AbstractBlockChain {
                     // newStoredBlock is a part of the same chain, there's no fork. This happens when we receive a block
                     // that we already saw and linked into the chain previously, which isn't the chain head.
                     // Re-processing it is confusing for the wallet so just skip.
-                    log.warn("Saw duplicated block in main chain at height {}: {}",
+                    log.warn("Saw duplicated block in best chain at height {}: {}",
                             newBlock.getHeight(), newBlock.getHeader().getHash());
                     return;
                 }
@@ -757,7 +757,7 @@ public abstract class AbstractBlockChain {
         // Then build a list of all blocks in the old part of the chain and the new part.
         final LinkedList<StoredBlock> oldBlocks = getPartialChain(head, splitPoint, blockStore);
         final LinkedList<StoredBlock> newBlocks = getPartialChain(newChainHead, splitPoint, blockStore);
-        // Disconnect each transaction in the previous main chain that is no longer in the new main chain
+        // Disconnect each transaction in the previous best chain that is no longer in the new best chain
         StoredBlock storedNewHead = splitPoint;
         if (shouldVerifyTransactions()) {
             for (StoredBlock oldBlock : oldBlocks) {

core/src/main/java/org/bitcoinj/core/Transaction.java

@@ -1123,7 +1123,7 @@ public class Transaction extends ChildMessage {
             // the design we use today where scripts are executed independently but share a stack. This left the
             // OP_CODESEPARATOR instruction having no purpose as it was only meant to be used internally, not actually
             // ever put into scripts. Deleting OP_CODESEPARATOR is a step that should never be required but if we don't
-            // do it, we could split off the main chain.
+            // do it, we could split off the best chain.
             connectedScript = Script.removeAllInstancesOfOp(connectedScript, ScriptOpCodes.OP_CODESEPARATOR);
 
             // Set the input to the script of its output. Bitcoin Core does this but the step has no obvious purpose as

core/src/main/java/org/bitcoinj/core/TransactionConfidence.java

@@ -49,7 +49,7 @@ import static com.google.common.base.Preconditions.*;
  * <li>Receiving it from multiple peers on the network. If your network connection is not being intercepted,
  *     hearing about a transaction from multiple peers indicates the network has accepted the transaction and
  *     thus miners likely have too (miners have the final say in whether a transaction becomes valid or not).</li>
- * <li>Seeing the transaction appear appear in a block on the main chain. Your confidence increases as the transaction
+ * <li>Seeing the transaction appear appear in a block on the best chain. Your confidence increases as the transaction
  *     becomes further buried under work. Work can be measured either in blocks (roughly, units of time), or
  *     amount of work done.</li>
  * </ul>

core/src/main/java/org/bitcoinj/wallet/Wallet.java

@@ -2178,7 +2178,7 @@ public class Wallet extends BaseTaggableObject
 
         // This TX may spend our existing outputs even though it was not pending. This can happen in unit
         // tests, if keys are moved between wallets, if we're catching up to the chain given only a set of keys,
-        // or if a dead coinbase transaction has moved back onto the main chain.
+        // or if a dead coinbase transaction has moved back onto the best chain.
         boolean isDeadCoinbase = tx.isCoinBase() && dead.containsKey(tx.getHash());
         if (isDeadCoinbase) {
             // There is a dead coinbase tx being received on the best chain. A coinbase tx is made dead when it moves

core/src/test/java/org/bitcoinj/core/ChainSplitTest.java

@@ -276,7 +276,7 @@ public class ChainSplitTest {
 
     @Test
     public void testForking6() throws Exception {
-        // Test the case in which a side chain block contains a tx, and then it appears in the main chain too.
+        // Test the case in which a side chain block contains a tx, and then it appears in the best chain too.
         Block b1 = UNITTEST.getGenesisBlock().createNextBlock(someOtherGuy);
         chain.add(b1);
         // genesis -> b1

core/src/test/java/org/bitcoinj/wallet/WalletTest.java

@@ -509,7 +509,7 @@ public class WalletTest extends TestWithWallet {
 
     @Test
     public void sideChain() throws Exception {
-        // The wallet receives a coin on the main chain, then on a side chain. Balance is equal to both added together
+        // The wallet receives a coin on the best chain, then on a side chain. Balance is equal to both added together
         // as we assume the side chain tx is pending and will be included shortly.
         Coin v1 = COIN;
         sendMoneyToWallet(AbstractBlockChain.NewBlockType.BEST_CHAIN, v1);
@@ -1413,7 +1413,7 @@ public class WalletTest extends TestWithWallet {
 
     @Test
     public void pending3() throws Exception {
-        // Check that if we receive a pending tx, and it's overridden by a double spend from the main chain, we
+        // Check that if we receive a pending tx, and it's overridden by a double spend from the best chain, we
         // are notified that it's dead. This should work even if the pending tx inputs are NOT ours, ie, they don't
         // connect to anything.
         Coin nanos = COIN;
@@ -1457,7 +1457,7 @@ public class WalletTest extends TestWithWallet {
         Threading.waitForUserCode();
         assertEquals(t1, called[0]);
         assertEquals(nanos, wallet.getBalance(Wallet.BalanceType.ESTIMATED));
-        // Now receive a double spend on the main chain.
+        // Now receive a double spend on the best chain.
         called[0] = called[1] = null;
         sendMoneyToWallet(AbstractBlockChain.NewBlockType.BEST_CHAIN, t2);
         Threading.waitForUserCode();