Add numPeers() and getPeers() accessors to PeerGroup. Resolves issue 106.

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

@@ -382,7 +382,7 @@ public class Peer {
         // TODO: Block locators should be abstracted out rather than special cased here.
         List<Sha256Hash> blockLocator = new LinkedList<Sha256Hash>();
         // For now we don't do the exponential thinning as suggested here: 
-        //  https://en.bitcoin.it/wiki/Protocol_specification#getblocks
+        //   https://en.bitcoin.it/wiki/Protocol_specification#getblocks
         // However, this should be taken seriously going forward. The old implementation only added the hash of the 
         // genesis block and the current chain head, which randomly led us to halt block fetching when ending on a
         // chain that turned out not to be the longest. This happened roughly once a week. 

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

@@ -28,9 +28,7 @@ import java.io.IOException;
 import java.net.ConnectException;
 import java.net.InetSocketAddress;
 import java.net.SocketTimeoutException;
-import java.util.Collections;
+import java.util.*;
-import java.util.HashSet;
-import java.util.Set;
 import java.util.concurrent.*;
 import java.util.concurrent.atomic.AtomicInteger;
 
@@ -157,6 +155,24 @@ public class PeerGroup {
         return peerPool.getMaximumPoolSize();
     }
     
+    /**
+     * Returns a newly allocated list containing the currently connected peers. If all you care about is the count,
+     * use numPeers().
+     */
+    public synchronized List<Peer> getPeers() {
+        ArrayList<Peer> result = new ArrayList<Peer>(peers.size());
+        result.addAll(peers);
+        return result;
+    }
+
+    /**
+     * Returns the number of currently connected peers. To be informed when this count changes, register a 
+     * {@link PeerEventListener} and use the onPeerConnected/onPeerDisconnected methods.
+     */
+    public synchronized int numPeers() {
+        return peers.size();
+    }
+
     /**
      * Add an address to the list of potential peers to connect to
      */

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

@@ -16,23 +16,21 @@
 
 package com.google.bitcoin.core;
 
-import static org.junit.Assert.assertEquals;
-import static org.junit.Assert.assertNull;
-import static org.junit.Assert.assertTrue;
-
 import com.google.bitcoin.discovery.PeerDiscovery;
 import com.google.bitcoin.discovery.PeerDiscoveryException;
 import com.google.bitcoin.store.MemoryBlockStore;
-
 import org.junit.Before;
 import org.junit.Test;
 
 import java.io.IOException;
 import java.net.InetSocketAddress;
+import java.util.List;
 import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.LinkedBlockingQueue;
 import java.util.concurrent.Semaphore;
 
+import static org.junit.Assert.*;
+
 public class PeerGroupTest extends TestWithNetworkConnections {
     static final NetworkParameters params = NetworkParameters.unitTests();
 
@@ -111,6 +109,12 @@ public class PeerGroupTest extends TestWithNetworkConnections {
         peerGroup.addPeer(p1);
         peerGroup.addPeer(p2);
         
+        // Check the peer accessors.
+        assertEquals(2, peerGroup.numPeers());
+        List<Peer> tmp = peerGroup.getPeers();
+        assertEquals(p1, tmp.get(0));
+        assertEquals(p2, tmp.get(1));
+
         // Set up a little block chain. We heard about b1 but not b2 (it is pending download). b3 is solved whilst we
         // are downloading the chain.
         Block b1 = TestUtils.createFakeBlock(params, blockStore).block;