Add missing hashCode() implementation

Summary:
Whenever a Java class implements equals(), it has to implement hashCode(), otherwise
there might be weird behavior when inserting instances of the class in a hash map for
example. This adds two missing hashCode() implementations and extends tests to test
the hashCode() implementations.

Test Plan: make jtest

Reviewers: rven, igor, sdong, yhchiang

Reviewed By: yhchiang

Subscribers: anthony, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D43017

Makefile

@@ -1037,7 +1037,7 @@ rocksdbjava: $(java_libobjects)
 jclean:
 	cd java;$(MAKE) clean;
 
-jtest:
+jtest: rocksdbjava
 	cd java;$(MAKE) sample;$(MAKE) test;
 
 jdb_bench:

java/src/main/java/org/rocksdb/AbstractSlice.java

@@ -105,6 +105,11 @@ abstract class AbstractSlice<T> extends RocksObject {
     return compare0(nativeHandle_, other.nativeHandle_);
   }
 
+  @Override
+  public int hashCode() {
+    return toString().hashCode();
+  }
+
   /**
    * If other is a slice object, then
    * we defer to {@link #compare(AbstractSlice) compare}

java/src/main/java/org/rocksdb/WBWIRocksIterator.java

@@ -117,6 +117,18 @@ public class WBWIRocksIterator extends AbstractRocksIterator<WriteBatchWithIndex
       }
     }
 
+    /**
+     * Generates a hash code for the Write Entry. NOTE: The hash code is based
+     * on the string representation of the key, so it may not work correctly
+     * with exotic custom comparators.
+     *
+     * @return The hash code for the Write Entry
+     */
+    @Override
+    public int hashCode() {
+      return (key == null) ? 0 : key.hashCode();
+    }
+
     @Override
     public boolean equals(Object other) {
       if(other == null) {

java/src/test/java/org/rocksdb/SliceTest.java

@@ -52,6 +52,7 @@ public class SliceTest {
       slice = new Slice("abc");
       slice2 = new Slice("abc");
       assertThat(slice.equals(slice2)).isTrue();
+      assertThat(slice.hashCode() == slice2.hashCode()).isTrue();
     } finally {
       if (slice != null) {
         slice.dispose();

java/src/test/java/org/rocksdb/WriteBatchWithIndexTest.java

@@ -252,6 +252,7 @@ public class WriteBatchWithIndexTest {
       it = wbwi.newIterator();
       it.seekToFirst();
       assertThat(it.entry().equals(expected[0])).isTrue();
+      assertThat(it.entry().hashCode() == expected[0].hashCode()).isTrue();
     } finally {
       if(it != null) {
         it.dispose();