core: avoid allocating Iterators in EquivalentAddressGroup, which is called for each new RPC

Benchmarked with 3 runs of 4 forks
Before:
Benchmark                         (direct)  (transport)    Mode     Cnt       Score     Error  Units
TransportBenchmark.unaryCall1024      true        NETTY  sample  255593  156248.670 ± 563.514  ns/op
Benchmark                         (direct)  (transport)    Mode     Cnt       Score     Error  Units
TransportBenchmark.unaryCall1024      true        NETTY  sample  261443  152753.415 ± 500.957  ns/op
Benchmark                         (direct)  (transport)    Mode     Cnt       Score     Error  Units
TransportBenchmark.unaryCall1024      true        NETTY  sample  258978  154205.374 ± 453.808  ns/op

After:
Benchmark                         (direct)  (transport)    Mode     Cnt       Score     Error  Units
TransportBenchmark.unaryCall1024      true        NETTY  sample  262194  152313.101 ± 502.563  ns/op
Benchmark                         (direct)  (transport)    Mode     Cnt       Score     Error  Units
TransportBenchmark.unaryCall1024      true        NETTY  sample  264811  150809.474 ± 477.459  ns/op
Benchmark                         (direct)  (transport)    Mode     Cnt       Score     Error  Units
TransportBenchmark.unaryCall1024      true        NETTY  sample  263606  151501.729 ± 593.149  ns/op

Author: carl-mastrangelo

core/src/main/java/io/grpc/EquivalentAddressGroup.java

@@ -50,13 +50,26 @@ public final class EquivalentAddressGroup {
 
   private final List<SocketAddress> addrs;
 
+  /**
+   * {@link SocketAddress} docs say that the addresses are immutable, so we cache the hashCode.
+   */
+  private final int hashCode;
+
+  /**
+   * List constructor.
+   */
   public EquivalentAddressGroup(List<SocketAddress> addrs) {
     Preconditions.checkArgument(!addrs.isEmpty(), "addrs is empty");
     this.addrs = Collections.unmodifiableList(new ArrayList<SocketAddress>(addrs));
+    hashCode = this.addrs.hashCode();
   }
 
+  /**
+   * Singleton constructor.
+   */
   public EquivalentAddressGroup(SocketAddress addr) {
     this.addrs = Collections.singletonList(addr);
+    hashCode = addrs.hashCode();
   }
 
   /**
@@ -73,14 +86,25 @@ public String toString() {
 
   @Override
   public int hashCode() {
-    return addrs.hashCode();
+    // Avoids creating an iterator on the underlying array list.
+    return hashCode;
   }
 
   @Override
   public boolean equals(Object other) {
     if (!(other instanceof EquivalentAddressGroup)) {
       return false;
     }
-    return addrs.equals(((EquivalentAddressGroup) other).addrs);
+    EquivalentAddressGroup that = (EquivalentAddressGroup) other;
+    if (addrs.size() != that.addrs.size()) {
+      return false;
+    }
+    // Avoids creating an iterator on the underlying array list.
+    for (int i = 0; i < addrs.size(); i++) {
+      if (!addrs.get(i).equals(that.addrs.get(i))) {
+        return false;
+      }
+    }
+    return true;
   }
 }