Fix find bugs

src/main/java/com/yahoo/sketches/cpc/CompressedState.java

@@ -42,13 +42,12 @@
  */
 final class CompressedState {
   private static final String LS = System.getProperty("line.separator");
+  private boolean csvIsValid = false;
+  private boolean windowIsValid = false;
   final int lgK;
   final short seedHash;
   int fiCol = 0;
   boolean mergeFlag = false; //compliment of HIP Flag
-  boolean csvIsValid = false;
-  boolean windowIsValid = false;
-
   long numCoupons = 0;
 
   double kxp;
@@ -60,7 +59,7 @@ final class CompressedState {
   int[] cwStream = null; //may be longer than required
   int cwLengthInts = 0;
 
-  int cpcRequiredBytes = 0;
+  //int cpcRequiredBytes = 0;
 
   private CompressedState(final int lgK, final short seedHash) {
     this.lgK = lgK;

src/main/java/com/yahoo/sketches/theta/CompactSketch.java

@@ -39,6 +39,14 @@ public abstract class CompactSketch extends Sketch {
 
   //Sketch
 
+  @Override
+  public CompactSketch compact() { return this; }
+
+  @Override
+  public CompactSketch compact(final boolean dstOrdered, final WritableMemory dstMem) {
+    return this;
+  }
+
   @Override
   public Family getFamily() {
     return Family.COMPACT;
@@ -112,6 +120,7 @@ static final long[] compactCachePart(final long[] srcCache, final int lgArrLongs
     return cacheOut;
   }
 
+  //compactCache and dstMem must be valid
   static final Memory loadCompactMemory(final long[] compactCache, final short seedHash,
       final int curCount, final long thetaLong, final WritableMemory dstMem,
       final byte flags, final int preLongs) {
@@ -144,7 +153,7 @@ static final Memory loadCompactMemory(final long[] compactCache, final short see
     if (preLongs > 2) {
       insertThetaLong(dstMem, thetaLong);
     }
-    if ((compactCache != null) && (curCount > 0)) {
+    if (curCount > 0) {
       dstMem.putLongArray(preLongs << 3, compactCache, 0, curCount);
     }
     return dstMem;

src/main/java/com/yahoo/sketches/theta/ConcurrentDirectQuickSelectSketch.java

@@ -241,7 +241,8 @@ private void advanceEpoch() {
     ConcurrentPropagationService.resetExecutorService(Thread.currentThread().getId());
     //noinspection NonAtomicOperationOnVolatileField
     // this increment of a volatile field is done within the scope of the propagation
-    // synchronization and hence is done by a single thread
+    // synchronization and hence is done by a single thread.
+    // Ignore a FindBugs warning
     epoch_++;
     endPropagation(null, true);
     initBgPropagationService();

src/main/java/com/yahoo/sketches/theta/ConcurrentHeapQuickSelectSketch.java

@@ -239,6 +239,7 @@ private void advanceEpoch() {
     //noinspection NonAtomicOperationOnVolatileField
     // this increment of a volatile field is done within the scope of the propagation
     // synchronization and hence is done by a single thread
+    // Ignore a FindBugs warning
     epoch_++;
     endPropagation(null, true);
     initBgPropagationService();

src/main/java/com/yahoo/sketches/theta/Sketch.java

@@ -20,6 +20,7 @@
 import static com.yahoo.sketches.theta.PreambleUtil.SER_VER_BYTE;
 
 import com.yahoo.memory.Memory;
+import com.yahoo.memory.WritableMemory;
 import com.yahoo.sketches.BinomialBoundsN;
 import com.yahoo.sketches.Family;
 import com.yahoo.sketches.SketchesArgumentException;
@@ -142,6 +143,38 @@ else if (serVer == 2) {
 
   //Sketch interface, defined here with Javadocs
 
+  /**
+   * Converts this sketch to an ordered CompactSketch on the Java heap.
+   *
+   * <p>If this sketch is already in compact form this operation returns <i>this</i>.
+   *
+   * @return this sketch as an ordered CompactSketch on the Java heap.
+   */
+  public abstract CompactSketch compact();
+
+  /**
+   * Convert this sketch to a CompactSketch in the chosen form.
+   *
+   * <p>If this sketch is already in compact form this operation returns <i>this</i>.
+   *
+   * <p>Otherwise, this compacting process converts the hash table form of an UpdateSketch to
+   * a simple list of the valid hash values from the hash table.  Any hash values equal to or
+   * greater than theta will be discarded.  The number of valid values remaining in the
+   * Compact Sketch depends on a number of factors, but may be larger or smaller than
+   * <i>Nominal Entries</i> (or <i>k</i>). It will never exceed 2<i>k</i>.  If it is critical
+   * to always limit the size to no more than <i>k</i>, then <i>rebuild()</i> should be called
+   * on the UpdateSketch prior to this.
+   *
+   * @param dstOrdered
+   * <a href="{@docRoot}/resources/dictionary.html#dstOrdered">See Destination Ordered</a>
+   *
+   * @param dstMem
+   * <a href="{@docRoot}/resources/dictionary.html#dstMem">See Destination Memory</a>.
+   *
+   * @return this sketch as a CompactSketch in the chosen form
+   */
+  public abstract CompactSketch compact(final boolean dstOrdered, final WritableMemory dstMem);
+
   /**
    * Gets the number of hash values less than the given theta.
    * @param theta the given theta as a double between zero and one.

src/main/java/com/yahoo/sketches/theta/UnionImpl.java

@@ -37,6 +37,15 @@
  * @author Kevin Lang
  */
 final class UnionImpl extends Union {
+  /**
+   * Although the gadget object is initially an UpdateSketch, in the context of a Union it is used
+   * as a specialized buffer that happens to leverage much of the machinery of an UpdateSketch.
+   * However, in this context some of the key invariants of the sketch algorithm are intentionally
+   * violated as an optimization. As a result this object can not be considered as an UpdateSketch
+   * and should never be exported as an UpdateSketch. It's internal state is not necessarily
+   * finalized and may contain garbage. Also its internal concept of "nominal entries" or "k" can
+   * be meaningless. It is private for very good reasons.
+   */
   private final UpdateSketch gadget_;
   private final short seedHash_; //eliminates having to compute the seedHash on every update.
   private long unionThetaLong_; //when on-heap, this is the only copy

src/main/java/com/yahoo/sketches/theta/UpdateSketch.java

@@ -115,50 +115,11 @@ public static UpdateSketch heapify(final Memory srcMem, final long seed) {
   //Sketch interface
 
   @Override
-  public boolean isCompact() {
-    return false;
+  public CompactSketch compact() {
+    return compact(true, null);
   }
 
   @Override
-  public boolean isOrdered() {
-    return false;
-  }
-
-  //UpdateSketch interface
-
-  /**
-   * Returns a new builder
-   *
-   * @return a new builder
-   */
-  public static final UpdateSketchBuilder builder() {
-    return new UpdateSketchBuilder();
-  }
-
-  /**
-   * Resets this sketch back to a virgin empty state.
-   */
-  public abstract void reset();
-
-  /**
-   * Convert this UpdateSketch to a CompactSketch in the chosen form.
-   *
-   * <p>This compacting process converts the hash table form of an UpdateSketch to
-   * a simple list of the valid hash values from the hash table.  Any hash values equal to or
-   * greater than theta will be discarded.  The number of valid values remaining in the
-   * Compact Sketch depends on a number of factors, but may be larger or smaller than
-   * <i>Nominal Entries</i> (or <i>k</i>). It will never exceed 2<i>k</i>.  If it is critical
-   * to always limit the size to no more than <i>k</i>, then <i>rebuild()</i> should be called
-   * on the UpdateSketch prior to this.
-   *
-   * @param dstOrdered
-   * <a href="{@docRoot}/resources/dictionary.html#dstOrdered">See Destination Ordered</a>
-   *
-   * @param dstMem
-   * <a href="{@docRoot}/resources/dictionary.html#dstMem">See Destination Memory</a>.
-   *
-   * @return this sketch as a CompactSketch in the chosen form
-   */
   public CompactSketch compact(final boolean dstOrdered, final WritableMemory dstMem) {
     CompactSketch sketchOut = null;
     final int sw = (dstOrdered ? 2 : 0) | ((dstMem != null) ? 1 : 0);
@@ -184,14 +145,32 @@ public CompactSketch compact(final boolean dstOrdered, final WritableMemory dstM
     return sketchOut;
   }
 
+  @Override
+  public boolean isCompact() {
+    return false;
+  }
+
+  @Override
+  public boolean isOrdered() {
+    return false;
+  }
+
+  //UpdateSketch interface
+
   /**
-   * Converts this UpdateSketch to an ordered CompactSketch on the Java heap.
-   * @return this sketch as an ordered CompactSketch on the Java heap.
+   * Returns a new builder
+   *
+   * @return a new builder
    */
-  public CompactSketch compact() {
-    return compact(true, null);
+  public static final UpdateSketchBuilder builder() {
+    return new UpdateSketchBuilder();
   }
 
+  /**
+   * Resets this sketch back to a virgin empty state.
+   */
+  public abstract void reset();
+
   /**
    * Rebuilds the hash table to remove dirty values or to reduce the size
    * to nominal entries.

src/test/java/com/yahoo/sketches/theta/CompactSketchTest.java

@@ -241,6 +241,8 @@ public void checkDirectCompactSingleItemSketch() {
     sk.update(1);
     csk = sk.compact(true, WritableMemory.allocate(16));
     assertEquals(csk.getCurrentBytes(true), 16);
+    assertTrue(csk == csk.compact());
+    assertTrue(csk == csk.compact(true, null));
   }
 
   @Test