Do not use AbstractTag<T> for Span.setTag(), but the actual impls.

opentracing-api/src/main/java/io/opentracing/Span.java

@@ -13,7 +13,7 @@
  */
 package io.opentracing;
 
-import io.opentracing.tag.AbstractTag;
+import io.opentracing.tag.Tag;
 import java.util.Map;
 
 /**
@@ -45,8 +45,8 @@ public interface Span {
     /** Same as {@link #setTag(String, String)}, but for numeric values. */
     Span setTag(String key, Number value);
 
-    /** Same as {@link #setTag(String, String)}, but with typed value. */
-    <T> Span setTag(AbstractTag<T> key, T value);
+    /** Same as {@link #setTag(String, String)}, but with using Tag<T>. */
+    <T> Span setTag(Tag<T> tag, T value);
 
     /**
      * Log key:value pairs to the Span with the current walltime timestamp.

opentracing-api/src/main/java/io/opentracing/tag/AbstractTag.java

@@ -15,13 +15,14 @@
 
 import io.opentracing.Span;
 
-public abstract class AbstractTag<T> {
+public abstract class AbstractTag<T> implements Tag<T> {
     protected final String key;
 
     public AbstractTag(String tagKey) {
         this.key = tagKey;
     }
 
+    @Override
     public String getKey() {
         return key;
     }

opentracing-api/src/main/java/io/opentracing/tag/Tag.java

@@ -0,0 +1,20 @@
+/*
+ * Copyright 2016-2018 The OpenTracing Authors
+ *
+ * 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
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * 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.opentracing.tag;
+
+import io.opentracing.Span;
+
+public interface Tag<T> {
+    String getKey();
+}

opentracing-mock/src/main/java/io/opentracing/mock/MockSpan.java

@@ -16,7 +16,7 @@
 import io.opentracing.References;
 import io.opentracing.Span;
 import io.opentracing.SpanContext;
-import io.opentracing.tag.AbstractTag;
+import io.opentracing.tag.Tag;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
@@ -137,8 +137,8 @@ public MockSpan setTag(String key, Number value) {
     }
 
     @Override
-    public <T> Span setTag(AbstractTag<T> key, T value) {
-        return setObjectTag(key.getKey(), value);
+    public <T> MockSpan setTag(Tag<T> tag, T value) {
+        return setObjectTag(tag.getKey(), value);
     }
 
     private synchronized MockSpan setObjectTag(String key, Object value) {

opentracing-noop/src/main/java/io/opentracing/noop/NoopSpan.java

@@ -15,7 +15,7 @@
 
 import io.opentracing.Span;
 import io.opentracing.SpanContext;
-import io.opentracing.tag.AbstractTag;
+import io.opentracing.tag.Tag;
 
 import java.util.Map;
 
@@ -44,9 +44,7 @@ public void finish(long finishMicros) {}
     public NoopSpan setTag(String key, Number value) { return this; }
 
     @Override
-    public <T> Span setTag(AbstractTag<T> key, T value) {
-        return this;
-    }
+    public <T> NoopSpan setTag(Tag<T> tag, T value) { return this; }
 
     @Override
     public NoopSpan log(Map<String, ?> fields) { return this; }