ttools: make certain ShapeForm implementations effectively @Equality

SizeForm and PairLinkForm are now singletons.  MarkForm overrides
equals and hashCode.  This is so that certain distinct but equivalent
ShapePlotters can be identified as the same, though neither MarkForm
nor Plotter is currently declared with the annotation @Equality.
This is used by MatchPlotter so that adding layers can be done by
inserting layer attributes into existing layers, so it makes it
tidier rather than being essential.  Maybe declare MarkForm as
@Equality at some point, it could be done.

ttools/src/main/uk/ac/starlink/ttools/plot2/geom/CubePlotType.java

@@ -64,15 +64,15 @@ public Plotter[] getPlotters() {
         List<Plotter> list = new ArrayList<Plotter>();
         ShapeForm[] forms = new ShapeForm[] {
             MarkForm.SINGLE,
-            new SizeForm(),
+            SizeForm.getInstance(),
             MultiPointForm
            .createVectorForm( new CartesianVectorCoordSet( axisNames_ ),
                               true ),
             MultiPointForm
            .createErrorForm( CartesianErrorCoordSet
                             .createAllAxesErrorCoordSet( axisNames_ ),
                              StyleKeys.ERROR_SHAPE_3D ),
-            new PairLinkForm(),
+            PairLinkForm.getInstance(),
             MarkForm.PAIR,
         };
         Plotter[] shapePlotters =

ttools/src/main/uk/ac/starlink/ttools/plot2/geom/PlanePlotType.java

@@ -63,7 +63,7 @@ public Plotter[] getPlotters() {
         List<Plotter> list = new ArrayList<Plotter>();
         ShapeForm[] forms = new ShapeForm[] {
             MarkForm.SINGLE,
-            new SizeForm(),
+            SizeForm.getInstance(),
             MultiPointForm
            .createVectorForm( new CartesianVectorCoordSet( axisNames_ ), true ),
             MultiPointForm
@@ -72,7 +72,7 @@ public Plotter[] getPlotters() {
                              StyleKeys.ERROR_SHAPE_2D ),
             MultiPointForm
            .createEllipseForm( new PlaneEllipseCoordSet(), true ),
-            new PairLinkForm(),
+            PairLinkForm.getInstance(),
             MarkForm.PAIR,
         };
         Plotter[] shapePlotters =

ttools/src/main/uk/ac/starlink/ttools/plot2/geom/SkyPlotType.java

@@ -56,11 +56,11 @@ public Plotter[] getPlotters() {
         List<Plotter> list = new ArrayList<Plotter>();
         ShapeForm[] forms = new ShapeForm[] {
             MarkForm.SINGLE,
-            new SizeForm(),
+            SizeForm.getInstance(),
             MultiPointForm.createVectorForm( new SkyVectorCoordSet( true ),
                                              true ),
             MultiPointForm.createEllipseForm( new SkyEllipseCoordSet(), true ),
-            new PairLinkForm(),
+            PairLinkForm.getInstance(),
             MarkForm.PAIR,
         };
         Plotter[] shapePlotters =

ttools/src/main/uk/ac/starlink/ttools/plot2/geom/SpherePlotType.java

@@ -52,8 +52,8 @@ public Plotter[] getPlotters() {
         List<Plotter> list = new ArrayList<Plotter>();
         ShapeForm[] forms = new ShapeForm[] {
             MarkForm.SINGLE,
-            new SizeForm(),
-            new PairLinkForm(),
+            SizeForm.getInstance(),
+            PairLinkForm.getInstance(),
             MarkForm.PAIR,
         };
         Plotter[] shapePlotters =

ttools/src/main/uk/ac/starlink/ttools/plot2/layer/MarkForm.java

@@ -82,6 +82,22 @@ public Coord[] getExtraCoords() {
         return new Coord[ 0 ];
     }
 
+    @Override
+    public int hashCode() {
+        return npos_;
+    }
+
+    @Override
+    public boolean equals( Object o ) {
+        if ( o instanceof MarkForm ) {
+            MarkForm other = (MarkForm) o;
+            return this.npos_ == other.npos_;
+        }
+        else {
+            return false;
+        }
+    }
+
     /**
      * Factory method to create an instance of this class.
      *

ttools/src/main/uk/ac/starlink/ttools/plot2/layer/PairLinkForm.java

@@ -25,12 +25,21 @@
 
 /**
  * Draws a line between two related positions.
+ * Singleton class.
  *
  * @author   Mark Taylor
  * @since    28 Nov 2013
  */
 public class PairLinkForm implements ShapeForm {
 
+    private static final PairLinkForm instance_ = new PairLinkForm();
+
+    /**
+     * Private constructor prevents instantiation.
+     */
+    private PairLinkForm() {
+    }
+
     public String getFormName() {
         return "Link";
     }
@@ -56,6 +65,15 @@ public Outliner createOutliner( ConfigMap config ) {
         return new LinkOutliner();
     }
 
+    /**
+     * Returns the singleton instance of this class.
+     *
+     * @return sole instance
+     */
+    public static PairLinkForm getInstance() {
+        return instance_;
+    }
+
     /**
      * Returns an uncoloured icon suitable for use in a legend.
      *

ttools/src/main/uk/ac/starlink/ttools/plot2/layer/SizeForm.java

@@ -32,6 +32,7 @@
  * ShapeForm implementation that draws shaped markers of a size
  * given by an additional data coordinate.
  * Auto-scaling is provided.
+ * Singleton class.
  *
  * @author   Mark Taylor
  * @since    18 Feb 2013
@@ -50,6 +51,14 @@ public class SizeForm implements ShapeForm {
                                          16, 2, 64, false );
     private static final AuxScale SIZE_SCALE = new AuxScale( "globalsize" );
 
+    private static final SizeForm instance_ = new SizeForm();
+
+    /**
+     * Private constructor prevents instantiation.
+     */
+    private SizeForm() {
+    }
+
     public int getPositionCount() {
         return 1;
     }
@@ -90,6 +99,15 @@ public Outliner createOutliner( ConfigMap config ) {
         return new SizeOutliner( shape, scale, autoscale );
     }
 
+    /**
+     * Returns the sole instance of this class.
+     *
+     * @return  singleton instance
+     */
+    public static SizeForm getInstance() {
+        return instance_;
+    }
+
     /**
      * Returns the column index in a tuple sequenc at which the size
      * coordinate will be found.