implementing hashCode() as well for Operation and added test

java/com/google/devtools/moe/client/parser/Operation.java

@@ -39,9 +39,11 @@ public String toString() {
 
   @Override
   public boolean equals(Object o) {
-    if (!(o instanceof Operation)) {
-      return false;
-    }
-    return toString().equals(o.toString());
+    return o instanceof Operation && toString().equals(o.toString());
+  }
+
+  @Override
+  public int hashCode() {
+    return toString().hashCode();
   }
 }

javatests/com/google/devtools/moe/client/parser/OperationTest.java

@@ -0,0 +1,22 @@
+package com.google.devtools.moe.client.parser;
+
+import junit.framework.TestCase;
+
+import java.util.HashMap;
+
+import static org.hamcrest.CoreMatchers.is;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+public class OperationTest extends TestCase {
+
+  public void testHashCode() throws Exception {
+    HashMap<String, String> map1 = new HashMap<>();
+    map1.put("key", "value");
+
+    Operation operation1 = new Operation(Operator.EDIT, new Term("term1", map1));
+    Operation operation2 = new Operation(Operator.EDIT, new Term("term1", map1));
+
+    assertThat(operation1.hashCode(), is(operation2.hashCode()));
+    assertTrue(operation1.equals(operation2));
+  }
+}