HBASE-18251 Remove unnecessary traversing to the first and last keys in

the CellSet (Toshihoro Suzuki)

hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/CellFlatMap.java

@@ -282,37 +282,85 @@ public abstract class CellFlatMap implements NavigableMap<Cell,Cell> {
   }
 
   // -------------------------------- Entry's getters --------------------------------
-  // all interfaces returning Entries are unsupported because we are dealing only with the keys
+
+  private static class CellFlatMapEntry implements Entry<Cell, Cell> {
+    private final Cell cell;
+
+    public CellFlatMapEntry (Cell cell) {
+      this.cell = cell;
+    }
+
+    @Override
+    public Cell getKey() {
+      return cell;
+    }
+
+    @Override
+    public Cell getValue() {
+      return cell;
+    }
+
+    @Override
+    public Cell setValue(Cell value) {
+      throw new UnsupportedOperationException();
+    }
+  }
+
   @Override
   public Entry<Cell, Cell> lowerEntry(Cell k) {
-    throw new UnsupportedOperationException();
+    Cell cell = lowerKey(k);
+    if (cell == null) {
+      return null;
+    }
+    return new CellFlatMapEntry(cell);
   }
 
   @Override
   public Entry<Cell, Cell> higherEntry(Cell k) {
-    throw new UnsupportedOperationException();
+    Cell cell = higherKey(k);
+    if (cell == null) {
+      return null;
+    }
+    return new CellFlatMapEntry(cell);
   }
 
   @Override
   public Entry<Cell, Cell> ceilingEntry(Cell k) {
-    throw new UnsupportedOperationException();
+    Cell cell = ceilingKey(k);
+    if (cell == null) {
+      return null;
+    }
+    return new CellFlatMapEntry(cell);
   }
 
   @Override
   public Entry<Cell, Cell> floorEntry(Cell k) {
-    throw new UnsupportedOperationException();
+    Cell cell = floorKey(k);
+    if (cell == null) {
+      return null;
+    }
+    return new CellFlatMapEntry(cell);
   }
 
   @Override
   public Entry<Cell, Cell> firstEntry() {
-    throw new UnsupportedOperationException();
+    Cell cell = firstKey();
+    if (cell == null) {
+      return null;
+    }
+    return new CellFlatMapEntry(cell);
   }
 
   @Override
   public Entry<Cell, Cell> lastEntry() {
-    throw new UnsupportedOperationException();
+    Cell cell = lastKey();
+    if (cell == null) {
+      return null;
+    }
+    return new CellFlatMapEntry(cell);
   }
 
+  // The following 2 methods (pollFirstEntry, pollLastEntry) are unsupported because these are updating methods.
   @Override
   public Entry<Cell, Cell> pollFirstEntry() {
     throw new UnsupportedOperationException();
@@ -323,7 +371,6 @@ public abstract class CellFlatMap implements NavigableMap<Cell,Cell> {
     throw new UnsupportedOperationException();
   }
 
-
   // -------------------------------- Updates --------------------------------
   // All updating methods below are unsupported.
   // Assuming an array of Cells will be allocated externally,

hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/CellSet.java

@@ -126,15 +126,12 @@ public class CellSet implements NavigableSet<Cell>  {
     throw new UnsupportedOperationException("Not implemented");
   }
 
-  // TODO: why do we have a double traversing through map? Recall we have Cell to Cell mapping...
-  // First for first/last key, which actually returns Cell and then get for the same Cell?
-  // TODO: Consider just return the first/lastKey(), should be twice more effective...
   public Cell first() {
-    return this.delegatee.get(this.delegatee.firstKey());
+    return this.delegatee.firstEntry().getValue();
   }
 
   public Cell last() {
-    return this.delegatee.get(this.delegatee.lastKey());
+    return this.delegatee.lastEntry().getValue();
   }
 
   public boolean add(Cell e) {