singleton classes should have private constructors

h2/src/main/org/h2/engine/Engine.java

@@ -33,6 +33,8 @@ public class Engine implements SessionFactory {
             SysProperties.DELAY_WRONG_PASSWORD_MIN;
     private boolean jmx;
 
+    private Engine() {}
+    
     public static Engine getInstance() {
         return INSTANCE;
     }

h2/src/main/org/h2/tools/MultiDimension.java

@@ -24,7 +24,7 @@ public class MultiDimension implements Comparator<long[]> {
 
     private static final MultiDimension INSTANCE = new MultiDimension();
 
-    protected MultiDimension() {
+    private MultiDimension() {
         // don't allow construction by normal code
     }