Modify rule S2864: add benchmark results

rules/S2864/java/rule.adoc

@@ -35,6 +35,63 @@ public void doSomethingWithMap(Map<String,Object> map) {
 }
 ----
 
+=== Benchmarks
+
+[options="header"]
+|===
+| Method| size| Runtime| Average time| Error margin
+| usingEntrySet| 10| Temurin 21| 27.48 ns/op| ±6.22 ns/op
+| usingEntrySet| 1000| Temurin 21| 2480.26 ns/op| ±899.05 ns/op
+| usingEntrySet| 10000| Temurin 21| 22745.78 ns/op| ±10505.46 ns/op
+| usingKeySet| 10| Temurin 21| 49.70 ns/op| ±3.78 ns/op
+| usingKeySet| 1000| Temurin 21| 5061.54 ns/op| ±2056.60 ns/op
+| usingKeySet| 10000| Temurin 21| 46689.04 ns/op| ±14509.97 ns/op
+|===
+
+*Benchmarking code*
+
+The results were generated by running the following snippet with https://github.com/openjdk/jmh[JMH].
+
+[source,java]
+----
+@BenchmarkMode({Mode.AverageTime})
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+@State(Scope.Benchmark)
+public class S2864 {
+  @Param({"10", "1000", "10000"})
+  int size;
+
+  Map<Integer,Integer> map;
+
+  @Setup(Level.Trial)
+  public void setup() {
+    Random random = new Random();
+    map = new HashMap<>();
+    for (int i = 0; i < size; i++) {
+      map.put(i, random.nextInt());
+    }
+  }
+
+  @Benchmark
+  public int usingKeySet() {
+    int sumKeysValues = 0;
+    for (Integer key : map.keySet()) {
+      sumKeysValues += key + map.get(key);
+    }
+    return sumKeysValues;
+  }
+
+  @Benchmark
+  public int usingEntrySet() {
+    int sumKeysValues = 0;
+    for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
+      sumKeysValues += entry.getKey() + entry.getValue();
+    }
+    return sumKeysValues;
+  }
+}
+----
+
 == Resources
 
 === Documentation