Add API methods.

org.jhotdraw8.base/src/main/java/org.jhotdraw8.base/org/jhotdraw8/base/converter/IdResolver.java

@@ -17,7 +17,21 @@ public interface IdResolver {
      * @param id the id
      * @return the object
      */
-    @Nullable Object getObject(String id);
+    @Nullable Object getObject(@Nullable String id);
+
+    /**
+     * Gets the object of the specified class for the specified id.
+     * Returns null if the id has no object of this type.
+     *
+     * @param clazz the clazz
+     * @param id    the id
+     * @return the object
+     */
+    default <T> @Nullable T getObject(@NonNull Class<T> clazz, @Nullable String id) {
+        Object object = getObject(id);
+        if (clazz.isInstance(object)) return clazz.cast(object);
+        return null;
+    }
 
     /**
      * Absolutize the given external URI, so that it can be used inside

org.jhotdraw8.collection/src/main/java/org.jhotdraw8.collection/org/jhotdraw8/collection/pair/UnorderedPair.java

@@ -14,6 +14,17 @@
  * @author Werner Randelshofer
  */
 public interface UnorderedPair<V> {
+    /**
+     * Returns other() if someone is equal to either(),
+     * returns either() otherwise.
+     *
+     * @param someone either or other
+     * @return
+     */
+    default V getOther(V someone) {
+        V either = either();
+        return Objects.equals(either, someone) ? other() : either;
+    }
 
     V either();
 

org.jhotdraw8.fxbase/src/main/java/org.jhotdraw8.fxbase/org/jhotdraw8/fxbase/tree/TreeNode.java

@@ -214,6 +214,19 @@ default T getChild(int index) {
         return path;
     }
 
+    /**
+     * Returns the depth of this node.
+     *
+     * @return depth (0 if the node is the root)
+     */
+    default @NonNull int getDepth() {
+        int depth = 0;
+        for (T node = getParent(); node != null; node = node.getParent()) {
+            depth++;
+        }
+        return depth;
+    }
+
     /**
      * Returns an iterable which can iterate through this figure and all its
      * descendants in postorder sequence.