PR 6524 with amendments (#6569)

Co-authored-by: junwense <970597653@qq.com>
Co-authored-by: SeanJ <44492651+junwense@users.noreply.github.com>

api/incubator/src/main/java/io/opentelemetry/api/incubator/propagation/CaseInsensitiveMap.java

@@ -14,18 +14,31 @@ class CaseInsensitiveMap extends HashMap<String, String> {
 
   private static final long serialVersionUID = -4202518750189126871L;
 
+  CaseInsensitiveMap() {}
+
   CaseInsensitiveMap(Map<String, String> carrier) {
-    super(carrier);
+    if (carrier != null) {
+      this.putAll(carrier);
+    }
   }
 
   @Override
   public String put(String key, String value) {
-    return super.put(key.toLowerCase(Locale.ROOT), value);
+    return super.put(getKeyLowerCase(key), value);
+  }
+
+  @Override
+  public void putAll(Map<? extends String, ? extends String> m) {
+    m.forEach(this::put);
+  }
+
+  private static String getKeyLowerCase(String key) {
+    return key.toLowerCase(Locale.ROOT);
   }
 
   @Override
   @Nullable
   public String get(Object key) {
-    return super.get(((String) key).toLowerCase(Locale.ROOT));
+    return super.get(getKeyLowerCase((String) key));
   }
 }

api/incubator/src/test/java/io/opentelemetry/api/incubator/propagation/CaseInsensitiveMapTest.java

@@ -0,0 +1,69 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.api.incubator.propagation;
+
+import static io.opentelemetry.sdk.testing.assertj.OpenTelemetryAssertions.assertThat;
+
+import java.util.HashMap;
+import java.util.Map;
+import org.junit.jupiter.api.Test;
+
+class CaseInsensitiveMapTest {
+
+  @Test
+  void createByConstructor() {
+    Map<String, String> map = new HashMap<>();
+    map.put("Key1", "test");
+    map.put("Key2", "test2");
+
+    CaseInsensitiveMap caseInsensitiveMap = new CaseInsensitiveMap(map);
+
+    Map<String, String> standardMap = new HashMap<>();
+    standardMap.put("key1", "test");
+    standardMap.put("key2", "test2");
+
+    assertThat(caseInsensitiveMap).isEqualTo(standardMap);
+  }
+
+  @Test
+  void putAll() {
+    CaseInsensitiveMap caseInsensitiveMap = new CaseInsensitiveMap();
+    Map<String, String> standardMap = new HashMap<>();
+    standardMap.put("key1", "test");
+    standardMap.put("key2", "test2");
+    caseInsensitiveMap.putAll(standardMap);
+    assertThat(caseInsensitiveMap).isEqualTo(standardMap);
+  }
+
+  @Test
+  void putIfAbsent() {
+    CaseInsensitiveMap caseInsensitiveMap = new CaseInsensitiveMap();
+    caseInsensitiveMap.putIfAbsent("key1", "test");
+    assertThat(caseInsensitiveMap.get("KEY1")).isEqualTo("test");
+    caseInsensitiveMap.putIfAbsent("key1", "nope");
+    assertThat(caseInsensitiveMap.get("KEY1")).isEqualTo("test");
+  }
+
+  @Test
+  void createByConstructorWithNullMap() {
+    CaseInsensitiveMap caseInsensitiveMap = new CaseInsensitiveMap(null);
+    assertThat(caseInsensitiveMap).isEmpty();
+  }
+
+  @Test
+  void caseInsensitivity() {
+    CaseInsensitiveMap caseInsensitiveMap = new CaseInsensitiveMap(null);
+
+    assertThat(caseInsensitiveMap).isEmpty();
+
+    caseInsensitiveMap.put("KEY1", "test1");
+    caseInsensitiveMap.put("KEY2", "test2");
+    assertThat(caseInsensitiveMap.get("key1")).isEqualTo("test1");
+    assertThat(caseInsensitiveMap.get("key2")).isEqualTo("test2");
+    assertThat(caseInsensitiveMap.get("kEy2")).isEqualTo("test2");
+    assertThat(caseInsensitiveMap.get("KEY2")).isEqualTo("test2");
+  }
+}