Make WalletEventListener an interface with a no-op implementation. Add an onChange() method to the default implementation that is called by the others, for cases where you don't care about what specifically changed, just that a change happened.

src/com/google/bitcoin/core/AbstractWalletEventListener.java

@@ -0,0 +1,80 @@
+/**
+ * Copyright 2011 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.bitcoin.core;
+
+import java.math.BigInteger;
+
+/**
+ * Implementing a subclass WalletEventListener allows you to learn when the contents of the wallet changes due to
+ * receiving money or a block chain re-organize. Methods are called with the event listener object locked so your
+ * implementation does not have to be thread safe. The default method implementations simply call onChange().
+ */
+public abstract class AbstractWalletEventListener implements WalletEventListener {
+    /**
+     * This is called on a Peer thread when a block is received that sends some coins to you. Note that this will
+     * also be called when downloading the block chain as the wallet balance catches up so if you don't want that
+     * register the event listener after the chain is downloaded. It's safe to use methods of wallet during the
+     * execution of this callback.
+     *
+     * @param wallet The wallet object that received the coins/
+     * @param tx The transaction which sent us the coins.
+     * @param prevBalance Balance before the coins were received.
+     * @param newBalance Current balance of the wallet.
+     */
+    public void onCoinsReceived(Wallet wallet, Transaction tx, BigInteger prevBalance, BigInteger newBalance) {
+        onChange();
+    }
+
+    /**
+     * This is called on a Peer thread when a block is received that triggers a block chain re-organization.<p>
+     *
+     * A re-organize means that the consensus (chain) of the network has diverged and now changed from what we
+     * believed it was previously. Usually this won't matter because the new consensus will include all our old
+     * transactions assuming we are playing by the rules. However it's theoretically possible for our balance to
+     * change in arbitrary ways, most likely, we could lose some money we thought we had.<p>
+     *
+     * It is safe to use methods of wallet whilst inside this callback.
+     *
+     * TODO: Finish this interface.
+     */
+    public void onReorganize() {
+        onChange();
+    }
+
+
+    /**
+     * This is called on a Peer thread when a transaction becomes <i>dead</i>. A dead transaction is one that has
+     * been overridden by a double spend from the network and so will never confirm no matter how long you wait.<p>
+     *
+     * A dead transaction can occur if somebody is attacking the network, or by accident if keys are being shared.
+     * You can use this event handler to inform the user of the situation. A dead spend will show up in the BitCoin
+     * C++ client of the recipient as 0/unconfirmed forever, so if it was used to purchase something,
+     * the user needs to know their goods will never arrive.
+     *
+     * @param deadTx The transaction that is newly dead.
+     * @param replacementTx The transaction that killed it.
+     */
+    public void onDeadTransaction(Transaction deadTx, Transaction replacementTx) {
+        onChange();
+    }
+
+    /**
+     * Called by the other default method implementations when something (anything) changes in the wallet.
+     */
+    public void onChange() {
+    }
+}

src/com/google/bitcoin/core/WalletEventListener.java

@@ -18,14 +18,13 @@ package com.google.bitcoin.core;
 
 import java.math.BigInteger;
 
-// TODO: Make this be an interface with a convenience abstract impl.
-
 /**
- * Implementing a subclass WalletEventListener allows you to learn when the contents of the wallet changes due to
+ * Implementing WalletEventListener allows you to learn when the contents of the wallet changes due to
  * receiving money or a block chain re-organize. Methods are called with the event listener object locked so your
- * implementation does not have to be thread safe. The default method implementations do nothing.
+ * implementation does not have to be thread safe. It may be convenient to derive from
+ * {@link AbstractWalletEventListener} instead.
  */
-public abstract class WalletEventListener {
+public interface WalletEventListener {
     /**
      * This is called on a Peer thread when a block is received that sends some coins to you. Note that this will
      * also be called when downloading the block chain as the wallet balance catches up so if you don't want that
@@ -37,8 +36,7 @@ public abstract class WalletEventListener {
      * @param prevBalance Balance before the coins were received.
      * @param newBalance Current balance of the wallet.
      */
-    public void onCoinsReceived(Wallet wallet, Transaction tx, BigInteger prevBalance, BigInteger newBalance) {
+    void onCoinsReceived(Wallet wallet, Transaction tx, BigInteger prevBalance, BigInteger newBalance);
-    }
 
     /**
      * This is called on a Peer thread when a block is received that triggers a block chain re-organization.<p>
@@ -52,9 +50,7 @@ public abstract class WalletEventListener {
      *
      * TODO: Finish this interface.
      */
-    public void onReorganize() {
+    void onReorganize();
-    }
-
 
     /**
      * This is called on a Peer thread when a transaction becomes <i>dead</i>. A dead transaction is one that has
@@ -68,6 +64,5 @@ public abstract class WalletEventListener {
      * @param deadTx The transaction that is newly dead.
      * @param replacementTx The transaction that killed it.
      */
-    public void onDeadTransaction(Transaction deadTx, Transaction replacementTx) {
+    void onDeadTransaction(Transaction deadTx, Transaction replacementTx);
-    }
 }

src/com/google/bitcoin/examples/PingService.java

@@ -16,19 +16,7 @@
 
 package com.google.bitcoin.examples;
 
-import com.google.bitcoin.core.Address;
+import com.google.bitcoin.core.*;
-import com.google.bitcoin.core.BlockChain;
-import com.google.bitcoin.core.DownloadListener;
-import com.google.bitcoin.core.ECKey;
-import com.google.bitcoin.core.NetworkParameters;
-import com.google.bitcoin.core.PeerAddress;
-import com.google.bitcoin.core.PeerGroup;
-import com.google.bitcoin.core.ScriptException;
-import com.google.bitcoin.core.Transaction;
-import com.google.bitcoin.core.TransactionInput;
-import com.google.bitcoin.core.Utils;
-import com.google.bitcoin.core.Wallet;
-import com.google.bitcoin.core.WalletEventListener;
 import com.google.bitcoin.store.BlockStore;
 import com.google.bitcoin.store.BoundedOverheadBlockStore;
 
@@ -94,7 +82,7 @@ public class PingService {
         peerGroup.start();
 
         // We want to know when the balance changes.
-        wallet.addEventListener(new WalletEventListener() {
+        wallet.addEventListener(new AbstractWalletEventListener() {
             @Override
             public void onCoinsReceived(Wallet w, Transaction tx, BigInteger prevBalance, BigInteger newBalance) {
                 // Running on a peer thread.

src/com/google/bitcoin/examples/RefreshWallet.java

@@ -16,14 +16,7 @@
 
 package com.google.bitcoin.examples;
 
-import com.google.bitcoin.core.BlockChain;
+import com.google.bitcoin.core.*;
-import com.google.bitcoin.core.DownloadListener;
-import com.google.bitcoin.core.NetworkParameters;
-import com.google.bitcoin.core.PeerAddress;
-import com.google.bitcoin.core.PeerGroup;
-import com.google.bitcoin.core.Transaction;
-import com.google.bitcoin.core.Wallet;
-import com.google.bitcoin.core.WalletEventListener;
 import com.google.bitcoin.store.BlockStore;
 import com.google.bitcoin.store.MemoryBlockStore;
 
@@ -49,7 +42,7 @@ public class RefreshWallet {
         peerGroup.addAddress(new PeerAddress(InetAddress.getLocalHost()));
         peerGroup.start();
 
-        wallet.addEventListener(new WalletEventListener() {
+        wallet.addEventListener(new AbstractWalletEventListener() {
             @Override
             public void onCoinsReceived(Wallet w, Transaction tx, BigInteger prevBalance, BigInteger newBalance) {
                 System.out.println("\nReceived tx " + tx.getHashAsString());

tests/com/google/bitcoin/core/ChainSplitTests.java

@@ -51,7 +51,7 @@ public class ChainSplitTests {
         // TODO: Change this test to not use coinbase transactions as they are special (maturity rules).
         final boolean[] reorgHappened = new boolean[1];
         reorgHappened[0] = false;
-        wallet.addEventListener(new WalletEventListener() {
+        wallet.addEventListener(new AbstractWalletEventListener() {
             @Override
             public void onReorganize() {
                 reorgHappened[0] = true;
@@ -185,7 +185,7 @@ public class ChainSplitTests {
         // double spend on the new best chain.
 
         final boolean[] eventCalled = new boolean[1];
-        wallet.addEventListener(new WalletEventListener() {
+        wallet.addEventListener(new AbstractWalletEventListener() {
             @Override
             public void onDeadTransaction(Transaction deadTx, Transaction replacementTx) {
                 eventCalled[0] = true;
@@ -225,7 +225,7 @@ public class ChainSplitTests {
 
         final Transaction[] eventDead = new Transaction[1];
         final Transaction[] eventReplacement = new Transaction[1];
-        wallet.addEventListener(new WalletEventListener() {
+        wallet.addEventListener(new AbstractWalletEventListener() {
             @Override
             public void onDeadTransaction(Transaction deadTx, Transaction replacementTx) {
                 eventDead[0] = deadTx;

tests/com/google/bitcoin/core/WalletTest.java

@@ -101,7 +101,7 @@ public class WalletTest {
     public void listeners() throws Exception {
         final Transaction fakeTx = createFakeTx(params, Utils.toNanoCoins(1, 0), myAddress);
         final boolean[] didRun = new boolean[1];
-        WalletEventListener listener = new WalletEventListener() {
+        WalletEventListener listener = new AbstractWalletEventListener() {
             public void onCoinsReceived(Wallet w, Transaction tx, BigInteger prevBalance, BigInteger newBalance) {
                 assertTrue(prevBalance.equals(BigInteger.ZERO));
                 assertTrue(newBalance.equals(Utils.toNanoCoins(1, 0)));
@@ -249,7 +249,7 @@ public class WalletTest {
         // isn't tested because today BitCoinJ only learns about such transactions when they appear in the chain.
         final Transaction[] eventDead = new Transaction[1];
         final Transaction[] eventReplacement = new Transaction[1];
-        wallet.addEventListener(new WalletEventListener() {
+        wallet.addEventListener(new AbstractWalletEventListener() {
             @Override
             public void onDeadTransaction(Transaction deadTx, Transaction replacementTx) {
                 eventDead[0] = deadTx;