Minor changes to PI runtime classes

- New class for action runtime parameter and its identifier
- PiAction builder
- Various indentifier builders

Change-Id: I265f71c868c21dbbbe633622b0c4330712f5a5ad
diff --git a/core/api/src/main/java/org/onosproject/net/pi/runtime/PiActionParamId.java b/core/api/src/main/java/org/onosproject/net/pi/runtime/PiActionParamId.java
new file mode 100644
index 0000000..e941e10
--- /dev/null
+++ b/core/api/src/main/java/org/onosproject/net/pi/runtime/PiActionParamId.java
@@ -0,0 +1,61 @@
+/*
+ * Copyright 2017-present Open Networking Laboratory
+ *
+ * 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 org.onosproject.net.pi.runtime;
+
+import com.google.common.annotations.Beta;
+import org.onlab.util.Identifier;
+
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Preconditions.checkNotNull;
+
+/**
+ * Identifier of an action's runtime parameter in a match+action table of a protocol-independent pipeline.
+ */
+@Beta
+public final class PiActionParamId extends Identifier<String> {
+
+    private final String name;
+
+    // TODO: if needed, we might add here support for positional parameters.
+    // E.g. add a second constructor that takes the "position" of a parameter, vs. its name.
+
+    private PiActionParamId(String name) {
+        super(name);
+        this.name = name;
+    }
+
+    /**
+     * Returns the name of this parameter.
+     *
+     * @return parameter name
+     */
+    public String name() {
+        return name;
+    }
+
+    /**
+     * Returns a parameter identifier with the given name.
+     *
+     * @param name parameter name
+     * @return parameter identifier
+     */
+    public static PiActionParamId of(String name) {
+        checkNotNull(name);
+        checkArgument(!name.isEmpty(), "Name can't be empty");
+        return new PiActionParamId(name);
+    }
+}