use Attributes.empty singleton when reading XML elements with no attributes

Author: clausnagel

src/main/java/org/xmlobjects/stream/XMLReader.java

@@ -328,8 +328,13 @@ public Attributes getAttributes() throws XMLReadException {
             throw new XMLReadException("Illegal to call getAttributes when event is not START_ELEMENT.");
         }
 
+        int count = reader.getAttributeCount();
+        if (count == 0) {
+            return Attributes.empty();
+        }
+
         Attributes attributes = new Attributes();
-        for (int i = 0; i < reader.getAttributeCount(); i++) {
+        for (int i = 0; i < count; i++) {
             attributes.add(reader.getAttributeName(i), reader.getAttributeValue(i));
         }
 
@@ -389,7 +394,7 @@ public String getMixedContent() throws XMLReadException {
 
     public <T> ObjectBuilder<T> getOrCreateBuilder(Class<? extends ObjectBuilder<T>> type) throws ObjectBuildException {
         ObjectBuilder<?> cachedBuilder = builderCache.get(type);
-        if (cachedBuilder != null && type.isAssignableFrom(cachedBuilder.getClass())) {
+        if (cachedBuilder != null) {
             return type.cast(cachedBuilder);
         } else {
             try {

src/main/java/org/xmlobjects/xml/Attributes.java

@@ -12,7 +12,36 @@
 import java.util.Map;
 
 public class Attributes {
-    private final Map<String, Map<String, TextContent>> attributes = new HashMap<>();
+    private static final Attributes EMPTY = new Attributes(Map.of()) {
+        @Override
+        public void add(String namespaceURI, String localName, TextContent value) {
+            throw new UnsupportedOperationException("Attributes.empty() is immutable.");
+        }
+
+        @Override
+        public void addAll(String namespaceURI, Map<String, TextContent> attributes) {
+            throw new UnsupportedOperationException("Attributes.empty() is immutable.");
+        }
+
+        @Override
+        public boolean isImmutable() {
+            return true;
+        }
+    };
+
+    private final Map<String, Map<String, TextContent>> attributes;
+
+    public static Attributes empty() {
+        return EMPTY;
+    }
+
+    public Attributes() {
+        this(new HashMap<>());
+    }
+
+    private Attributes(Map<String, Map<String, TextContent>> attributes) {
+        this.attributes = attributes;
+    }
 
     public void add(String namespaceURI, String localName, TextContent value) {
         attributes.computeIfAbsent(namespaceURI, v -> new HashMap<>()).put(localName, value);
@@ -78,9 +107,7 @@ public boolean isEmpty() {
         return attributes.isEmpty();
     }
 
-    public Attributes copy() {
-        Attributes copy = new Attributes();
-        copy.attributes.putAll(attributes);
-        return copy;
+    public boolean isImmutable() {
+        return false;
     }
 }