1 files changed, 84 insertions, 0 deletions
diff --git a/api/src/main/java/io/opencensus/tags/TagKey.java b/api/src/main/java/io/opencensus/tags/TagKey.java
new file mode 100644
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+++ b/api/src/main/java/io/opencensus/tags/TagKey.java
@@ -0,0 +1,84 @@
+/*
+ * Copyright 2017, OpenCensus 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.opencensus.tags;
+
+import com.google.auto.value.AutoValue;
+import io.opencensus.internal.StringUtils;
+import io.opencensus.internal.Utils;
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * A key to a value stored in a {@link TagContext}.
+ *
+ * <p>Each {@code TagKey} has a {@code String} name. Names have a maximum length of {@link
+ * #MAX_LENGTH} and contain only printable ASCII characters.
+ *
+ * <p>{@code TagKey}s are designed to be used as constants. Declaring each key as a constant
+ * prevents key names from being validated multiple times.
+ *
+ * @since 0.8
+ */
+@Immutable
+@AutoValue
+public abstract class TagKey {
+  /**
+   * The maximum length for a tag key name. The value is {@value #MAX_LENGTH}.
+   *
+   * @since 0.8
+   */
+  public static final int MAX_LENGTH = 255;
+
+  TagKey() {}
+
+  /**
+   * Constructs a {@code TagKey} with the given name.
+   *
+   * <p>The name must meet the following requirements:
+   *
+   * <ol>
+   *   <li>It cannot be longer than {@link #MAX_LENGTH}.
+   *   <li>It can only contain printable ASCII characters.
+   * </ol>
+   *
+   * @param name the name of the key.
+   * @return a {@code TagKey} with the given name.
+   * @throws IllegalArgumentException if the name is not valid.
+   * @since 0.8
+   */
+  public static TagKey create(String name) {
+    Utils.checkArgument(isValid(name), "Invalid TagKey name: %s", name);
+    return new AutoValue_TagKey(name);
+  }
+
+  /**
+   * Returns the name of the key.
+   *
+   * @return the name of the key.
+   * @since 0.8
+   */
+  public abstract String getName();
+
+  /**
+   * Determines whether the given {@code String} is a valid tag key.
+   *
+   * @param name the tag key name to be validated.
+   * @return whether the name is valid.
+   */
+  private static boolean isValid(String name) {
+    return !name.isEmpty() && name.length() <= MAX_LENGTH && StringUtils.isPrintableString(name);
+  }
+}