Have upperY and lowerY on GroundFindingFunction be set via mutators.

src/main/java/com/sk89q/worldedit/EditSession.java

@@ -2757,7 +2757,9 @@ public class EditSession {
                           double density, TreeGenerator treeGenerator)
             throws WorldEditException {
 
-        ForestGenerator generator = new ForestGenerator(this, treeGenerator, lowerY, upperY);
+        ForestGenerator generator = new ForestGenerator(this, treeGenerator);
+        generator.setLowerY(lowerY);
+        generator.setUpperY(upperY);
         generator.setDensity(density);
         int affected = 0;
 

src/main/java/com/sk89q/worldedit/commands/RegionCommands.java

@@ -576,10 +576,9 @@ public class RegionCommands {
             flatRegion = CuboidRegion.makeCuboid(region);
         }
 
-        int upperY = flatRegion.getMaximumY();
-        int lowerY = flatRegion.getMinimumY();
-
-        FloraPlacer generator = new FloraPlacer(editSession, lowerY, upperY);
+        FloraPlacer generator = new FloraPlacer(editSession);
+        generator.setLowerY(flatRegion.getMinimumY());
+        generator.setUpperY(flatRegion.getMaximumY());
         generator.setDensity(density);
         int affected = 0;
 

src/main/java/com/sk89q/worldedit/generator/FloraPlacer.java

@@ -44,11 +44,9 @@ public class FloraPlacer extends GroundGenerator {
      * Create a new instance.
      *
      * @param editSession the edit session
-     * @param lowerY the lower Y
-     * @param upperY the upper Y (lowerY <= upperY)
      */
-    public FloraPlacer(EditSession editSession, int lowerY, int upperY) {
-        super(editSession, lowerY, upperY);
+    public FloraPlacer(EditSession editSession) {
+        super(editSession);
         this.editSession = editSession;
     }
 

src/main/java/com/sk89q/worldedit/generator/ForestGenerator.java

@@ -40,11 +40,9 @@ public class ForestGenerator extends GroundGenerator {
      *
      * @param editSession the edit session
      * @param treeGenerator a tree generator
-     * @param lowerY the lower Y boundary to end the ground search from
-     * @param upperY the upper Y boundary to start the ground search at
      */
-    public ForestGenerator(EditSession editSession, TreeGenerator treeGenerator, int lowerY, int upperY) {
-        super(editSession, lowerY, upperY);
+    public ForestGenerator(EditSession editSession, TreeGenerator treeGenerator) {
+        super(editSession);
         this.editSession = editSession;
         this.treeGenerator = treeGenerator;
     }

src/main/java/com/sk89q/worldedit/generator/GroundGenerator.java

@@ -34,11 +34,9 @@ public abstract class GroundGenerator extends GroundFindingFunction {
      * Create a new instance.
      *
      * @param editSession the edit session
-     * @param lowerY the lower Y
-     * @param upperY the upper Y (lowerY <= upperY)
      */
-    protected GroundGenerator(EditSession editSession, int lowerY, int upperY) {
-        super(editSession, lowerY, upperY);
+    protected GroundGenerator(EditSession editSession) {
+        super(editSession);
     }
 
     /**

src/main/java/com/sk89q/worldedit/operation/GroundFindingFunction.java

@@ -26,6 +26,13 @@ import com.sk89q.worldedit.blocks.BlockID;
 /**
  * An abstract implementation of {@link com.sk89q.worldedit.operation.FlatRegionFunction}
  * that searches for the ground, which is considered to be any non-air block.
+ * <p>
+ * It functions by starting from the upperY in each column and traversing
+ * down the column until it finds the first non-air block, at which point
+ * {@link #apply(com.sk89q.worldedit.Vector, com.sk89q.worldedit.blocks.BaseBlock)}
+ * is called on that non-air block. {@link #shouldContinue(com.sk89q.worldedit.Vector2D)}
+ * is called before each column is traversed, which allows implementations
+ * to "skip" a column and avoid the ground search.
  */
 public abstract class GroundFindingFunction implements FlatRegionFunction {
 
@@ -37,13 +44,9 @@ public abstract class GroundFindingFunction implements FlatRegionFunction {
      * Create a new instance.
      *
      * @param editSession the edit session
-     * @param lowerY the lower Y
-     * @param upperY the upper Y (lowerY <= upperY)
      */
-    protected GroundFindingFunction(EditSession editSession, int lowerY, int upperY) {
+    protected GroundFindingFunction(EditSession editSession) {
         this.editSession = editSession;
-        this.lowerY = lowerY;
-        this.upperY = upperY;
         checkYBounds();
     }
 
@@ -118,7 +121,8 @@ public abstract class GroundFindingFunction implements FlatRegionFunction {
     }
 
     /**
-     * Returns whether a search for the ground should be performed for the given point.
+     * Returns whether a search for the ground should be performed for the given
+     * column. Return false if the column is to be skipped.
      *
      * @param pt the point
      * @return true if the search should begin
@@ -129,6 +133,12 @@ public abstract class GroundFindingFunction implements FlatRegionFunction {
 
     /**
      * Apply the function to the given ground block.
+     * <p>
+     * The block above the given ground block may or may not be air, but it is
+     * a block that can be replaced. For example, this block may be a tall
+     * grass block or a flower. However, the ground block also could be
+     * the flower or grass block itself, depending on the configuration of the
+     * GroundFindingFunction.
      *
      * @param pt the position
      * @param block the block