Remove memory usage optimization that was complicating things.

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

@@ -65,7 +65,7 @@ public class TransactionConfidence implements Serializable {
     private Set<PeerAddress> broadcastBy;
     /** The Transaction that this confidence object is associated with. */
     private Transaction transaction;
-    private transient ArrayList<Listener> listeners;
+    private transient ArrayList<Listener> listeners = new ArrayList<Listener>(1);
 
     // TODO: The advice below is a mess. There should be block chain listeners, see issue 94.
     /**
@@ -81,8 +81,6 @@ public class TransactionConfidence implements Serializable {
     public synchronized void addEventListener(Listener listener) {
         if (listener == null)
             throw new IllegalArgumentException("listener is null");
-        if (listeners == null)
-            listeners = new ArrayList<Listener>(1);
         listeners.add(listener);
     }
 
@@ -90,8 +88,6 @@ public class TransactionConfidence implements Serializable {
         if (listener == null)
             throw new IllegalArgumentException("listener is null");
         listeners.remove(listener);
-        if (listeners.size() == 0)
-            listeners = null;
     }
 
     /** Describes the state of the transaction in general terms. Properties can be read to learn specifics. */
@@ -130,7 +126,8 @@ public class TransactionConfidence implements Serializable {
     /**
      * A confidence listener is informed when the level of confidence in a transaction is updated by something, like
      * for example a {@link Wallet}. You can add listeners to update your user interface or manage your order tracking
-     * system when confidence levels pass a certain threshold. <b>Note that confidence can go down as well as up.</b>
+     * system when confidence levels pass a certain threshold. <b>Note that confidence can go down as well as up.</b>.
+     * During listener execution, it's safe to remove the current listener but not others.
      */
     public interface Listener {
         public void onConfidenceChanged(Transaction tx);
@@ -331,11 +328,15 @@ public class TransactionConfidence implements Serializable {
     }
 
     private void runListeners() {
-        if (listeners == null) return;
-        for (Listener l : listeners) {
+        for (int i = 0; i < listeners.size(); i++) {
+            Listener l = listeners.get(i);
             synchronized (l) {
                 l.onConfidenceChanged(transaction);
             }
+            if (listeners.get(i) != l) {
+                // Listener removed itself.
+                i--;
+            }
         }
     }
 }

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

@@ -270,10 +270,19 @@ public class Wallet implements Serializable {
         // Event listeners may re-enter so we cannot make assumptions about wallet state after this loop completes.
         BigInteger balance = getBalance();
         BigInteger newBalance = balance.add(value);
-        for (WalletEventListener l : eventListeners) {
+        invokeOnCoinsReceived(tx, balance, newBalance);
+    }
+
+    private void invokeOnCoinsReceived(Transaction tx, BigInteger balance, BigInteger newBalance) {
+        for (int i = 0; i < eventListeners.size(); i++) {
+            WalletEventListener l = eventListeners.get(i);
             synchronized (l) {
                 l.onCoinsReceived(this, tx, balance, newBalance);
             }
+            if (eventListeners.get(i) != l) {
+                // Listener removed itself.
+                i--;
+            }
         }
     }
 
@@ -409,11 +418,7 @@ public class Wallet implements Serializable {
         //
         // TODO: Decide whether to run the event listeners, if a tx confidence listener already modified the wallet.
         if (!reorg && bestChain && valueDifference.compareTo(BigInteger.ZERO) > 0 && wtx == null) {
-            for (WalletEventListener l : eventListeners) {
-                synchronized (l) {
-                    l.onCoinsReceived(this, tx, prevBalance, getBalance());
-                }
-            }
+            invokeOnCoinsReceived(tx, prevBalance, getBalance());
         }
     }
 
@@ -452,11 +457,7 @@ public class Wallet implements Serializable {
             dead.put(doubleSpend.getHash(), doubleSpend);
             // Inform the event listeners of the newly dead tx.
             doubleSpend.getConfidence().setOverridingTransaction(tx);
-            for (WalletEventListener listener : eventListeners) {
-                synchronized (listener) {
-                    listener.onDeadTransaction(this, doubleSpend, tx);
-                }
-            }
+            invokeOnDeadTransaction(doubleSpend, tx);
         }
     }
 
@@ -1181,12 +1182,7 @@ public class Wallet implements Serializable {
                 Transaction replacement = doubleSpent.getSpentBy().getParentTransaction();
                 dead.put(tx.getHash(), tx);
                 pending.remove(tx.getHash());
-                // Inform the event listeners of the newly dead tx.
-                for (WalletEventListener listener : eventListeners) {
-                    synchronized (listener) {
-                        listener.onDeadTransaction(this, tx, replacement);
-                    }
-                }
+                invokeOnDeadTransaction(tx, replacement);
                 break;
             }
         }
@@ -1203,6 +1199,19 @@ public class Wallet implements Serializable {
         }
     }
 
+    private void invokeOnDeadTransaction(Transaction tx, Transaction replacement) {
+        for (int i = 0; i < eventListeners.size(); i++) {
+            WalletEventListener listener = eventListeners.get(i);
+            synchronized (listener) {
+                listener.onDeadTransaction(this, tx, replacement);
+            }
+            if (eventListeners.get(i) != listener) {
+                // Listener removed itself.
+                i--;
+            }
+        }
+    }
+
     /**
      * Returns an immutable view of the transactions currently waiting for network confirmations.
      */

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

@@ -22,7 +22,10 @@ import java.math.BigInteger;
  * 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. It may be convenient to derive from
- * {@link AbstractWalletEventListener} instead.
+ * {@link AbstractWalletEventListener} instead.<p>
+ *
+ * It is safe to call methods of the wallet during event listener execution, and also for a listener to remove itself.
+ * Other types of modifications generally aren't safe.
  */
 public interface WalletEventListener {
     /**

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

@@ -115,6 +115,10 @@ public class PingService {
             public void onBlocksDownloaded(Peer peer, Block block, int blocksLeft) {
                 super.onBlocksDownloaded(peer, block, blocksLeft);
 
+                // Don't bother printing during block chain downloads.
+                if (blocksLeft > 0)
+                    return;
+
                 Set<Transaction> transactions = wallet.getTransactions(false, false);
                 if (transactions.size() == 0) return;
                 System.out.println("Confidences of wallet transactions:");
@@ -149,9 +153,7 @@ public class PingService {
                             if (tx2.getConfidence().getConfidenceType() == TransactionConfidence.ConfidenceType.BUILDING) {
                                 // Coins were confirmed.
                                 bounceCoins(tx2);
-
-                                // TODO: Make this work.
-                                // tx2.getConfidence().removeEventListener(this);
+                                tx2.getConfidence().removeEventListener(this);
                             } else {
                                 System.out.println(String.format("Confidence of %s changed, is now: %s",
                                         tx2.getHashAsString(), tx2.getConfidence().toString()));