Reworked a bit the Extension interface

api/src/main/java/io/cloudevents/Extension.java

@@ -17,12 +17,39 @@
 
 package io.cloudevents;
 
-import java.util.Map;
+import io.cloudevents.lang.Nullable;
 
+import javax.annotation.ParametersAreNonnullByDefault;
+import java.util.Set;
+
+/**
+ * Materialized CloudEvent Extension interface to read/write the extension attributes key/values.
+ */
+@ParametersAreNonnullByDefault
 public interface Extension {
 
-    void readFromEvent(CloudEvent event);
+    /**
+     * Fill this materialized extension with values from a {@link CloudEventExtensions} implementation
+     *
+     * @param extensions
+     */
+    void readFrom(CloudEventExtensions extensions);
+
+    /**
+     * Get the attribute of extension named {@code key}
+     *
+     * @param key the name of the extension attribute
+     * @return the extension value
+     * @throws IllegalArgumentException if the key is unknown to this extension
+     */
+    @Nullable
+    Object getValue(String key) throws IllegalArgumentException;
 
-    Map<String, Object> asMap();
+    /**
+     * Get the set of possible extension attribute keys
+     *
+     * @return set of possible extension attribute keys
+     */
+    Set<String> getKeys();
 
 }

core/src/main/java/io/cloudevents/core/extensions/DistributedTracingExtension.java

@@ -17,17 +17,20 @@
 
 package io.cloudevents.core.extensions;
 
-import io.cloudevents.CloudEvent;
+import io.cloudevents.CloudEventExtensions;
 import io.cloudevents.Extension;
+import io.cloudevents.core.extensions.impl.ExtensionUtils;
 
+import java.util.Arrays;
 import java.util.Collections;
-import java.util.HashMap;
-import java.util.Map;
+import java.util.HashSet;
+import java.util.Set;
 
 public final class DistributedTracingExtension implements Extension {
 
     public static final String TRACEPARENT = "traceparent";
     public static final String TRACESTATE = "tracestate";
+    private static final Set<String> KEY_SET = Collections.unmodifiableSet(new HashSet<>(Arrays.asList(TRACEPARENT, TRACESTATE)));
 
     private String traceparent;
     private String tracestate;
@@ -49,23 +52,31 @@ public void setTracestate(String tracestate) {
     }
 
     @Override
-    public void readFromEvent(CloudEvent event) {
-        Object tp = event.getExtension(TRACEPARENT);
+    public void readFrom(CloudEventExtensions extensions) {
+        Object tp = extensions.getExtension(TRACEPARENT);
         if (tp != null) {
             this.traceparent = tp.toString();
         }
-        Object ts = event.getExtension(TRACESTATE);
+        Object ts = extensions.getExtension(TRACESTATE);
         if (ts != null) {
             this.tracestate = ts.toString();
         }
     }
 
     @Override
-    public Map<String, Object> asMap() {
-        HashMap<String, Object> map = new HashMap<>();
-        map.put(TRACEPARENT, this.traceparent);
-        map.put(TRACESTATE, this.tracestate);
-        return Collections.unmodifiableMap(map);
+    public Object getValue(String key) {
+        switch (key) {
+            case TRACEPARENT:
+                return this.traceparent;
+            case TRACESTATE:
+                return this.tracestate;
+        }
+        throw ExtensionUtils.generateInvalidKeyException(this.getClass().getSimpleName(), key);
+    }
+
+    @Override
+    public Set<String> getKeys() {
+        return KEY_SET;
     }
 
     @Override

core/src/main/java/io/cloudevents/core/extensions/ExtensionsParser.java

@@ -51,7 +51,7 @@ public <T extends Extension> T parseExtension(Class<T> extensionClass, CloudEven
         Supplier<Extension> factory = extensionFactories.get(extensionClass);
         if (factory != null) {
             Extension ext = factory.get();
-            ext.readFromEvent(event);
+            ext.readFrom(event);
             return (T) ext;
         }
         return null;

core/src/main/java/io/cloudevents/core/extensions/impl/ExtensionUtils.java

@@ -0,0 +1,29 @@
+/*
+ * Copyright 2018-Present The CloudEvents Authors
+ * <p>
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * <p>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package io.cloudevents.core.extensions.impl;
+
+public final class ExtensionUtils {
+
+    private ExtensionUtils() {
+    }
+
+    public static IllegalArgumentException generateInvalidKeyException(String extensionName, String key) {
+        return new IllegalArgumentException(extensionName + " doesn't expect the attribute key \"" + key + "\"");
+    }
+
+}

core/src/main/java/io/cloudevents/core/impl/BaseCloudEventBuilder.java

@@ -95,7 +95,12 @@ public SELF withExtension(String key, boolean value) {
     }
 
     public SELF withExtension(Extension extension) {
-        this.extensions.putAll(extension.asMap());
+        for (String key : extension.getKeys()) {
+            Object value = extension.getValue(key);
+            if (value != null) {
+                this.extensions.put(key, value);
+            }
+        }
         return self;
     }