diff --git a/src/main/java/net/onrc/onos/api/flowmanager/Flow.java b/src/main/java/net/onrc/onos/api/flowmanager/Flow.java
index 0622be6..8fe9577 100644
--- a/src/main/java/net/onrc/onos/api/flowmanager/Flow.java
+++ b/src/main/java/net/onrc/onos/api/flowmanager/Flow.java
@@ -1,37 +1,83 @@
 package net.onrc.onos.api.flowmanager;
 
+import static com.google.common.base.Preconditions.checkNotNull;
 import net.onrc.onos.api.batchoperation.BatchOperationTarget;
 import net.onrc.onos.core.matchaction.MatchActionOperations;
 import net.onrc.onos.core.matchaction.match.Match;
 
 /**
- * An interface class to define flow object which is managed by
+ * An abstract class to define flow object which is managed by
  * FlowManagerModule.
- * <p>
- * The flow objects (eg. path, tree, disjoint-paths, etc.) must implement this
- * interface.
  */
-public interface Flow extends BatchOperationTarget {
+public abstract class Flow implements BatchOperationTarget {
+    private final FlowId id;
+
+    /**
+     * Creates Flow object using specified ID.
+     *
+     * @param id the ID to be assigned
+     */
+    public Flow(FlowId id) {
+        this.id = checkNotNull(id);
+    }
+
     /**
      * Gets ID for this flow object.
      *
-     * @return ID for this object.
+     * @return ID for this object
      */
-    public FlowId getId();
+    public FlowId getId() {
+        return id;
+    }
 
     /**
      * Gets traffic filter for this flow object.
      *
-     * @return a traffic filter for this flow object.
+     * @return a traffic filter for this flow object
      */
-    public Match getMatch();
+    public abstract Match getMatch();
 
     /**
-     * Compiles this object to MatchAction plan.
+     * Compiles this object to MatchAction operations.
      * <p>
      * This method is called by FlowManagerModule to create MatchAction operations.
      *
-     * @return a MatchAction plan of this flow object.
+     * @return a MatchActionOperations of this flow object
      */
-    public MatchActionOperations compile();
+    public abstract MatchActionOperations compile();
+
+    /**
+     * Generates a hash code using the FlowId.
+     *
+     * @return hashcode
+     */
+    @Override
+    public int hashCode() {
+        return (id == null) ? 0 : id.hashCode();
+    }
+
+    /**
+     * Compares two flow objects by type (class) and FlowId.
+     *
+     * @param obj other Flow object
+     * @return true if equal, false otherwise
+     */
+    @Override
+    public boolean equals(Object obj) {
+        if (this == obj) {
+            return true;
+        }
+        if ((obj == null) || !(obj instanceof Flow)) {
+            return false;
+        }
+        Flow other = (Flow) obj;
+        if (id == null) {
+            if (other.id != null) {
+                return false;
+            }
+        } else if (!id.equals(other.id)) {
+            return false;
+        }
+        return true;
+    }
 }
diff --git a/src/main/java/net/onrc/onos/api/flowmanager/OpticalPathFlow.java b/src/main/java/net/onrc/onos/api/flowmanager/OpticalPathFlow.java
index 64b7e2e..4018a7c 100644
--- a/src/main/java/net/onrc/onos/api/flowmanager/OpticalPathFlow.java
+++ b/src/main/java/net/onrc/onos/api/flowmanager/OpticalPathFlow.java
@@ -4,6 +4,8 @@
 
 import net.onrc.onos.core.matchaction.MatchActionOperations;
 import net.onrc.onos.core.matchaction.action.Action;
+import net.onrc.onos.core.matchaction.match.PacketMatch;
+import net.onrc.onos.core.matchaction.match.PacketMatchBuilder;
 import net.onrc.onos.core.util.PortNumber;
 
 /**
@@ -11,30 +13,42 @@
  * <p>
  * TODO: Think this: How do we deal the optical path flow going through the
  * regenerators? Can we express it with multiple OpticalPathFlow objects?
+ * <p>
+ * NOTE: This class is not fully supported for the August release.
  */
 public class OpticalPathFlow extends PathFlow {
-    protected int lambda;
+    private final int lambda;
 
     /**
      * Constructor.
      *
-     * @param id ID for this new Flow object.
-     * @param inPort Ingress port number at the ingress edge node.
-     * @param path Path between ingress and egress edge node.
-     * @param actions The list of Action objects at the egress edge node.
-     * @param lambda The lambda to be used throughout the path.
+     * @param id the ID for this new Flow object
+     * @param ingressPort the Ingress port number at the ingress edge node
+     * @param path the Path between ingress and egress edge node
+     * @param egressActions the list of Action objects at the egress edge node
+     * @param lambda the lambda to be used throughout the path
      */
-    public OpticalPathFlow(String id,
-            PortNumber inPort, Path path, List<Action> actions, int lambda) {
-        super(id, null, inPort, path, actions);
+    public OpticalPathFlow(FlowId id,
+            PortNumber ingressPort, Path path, List<Action> egressActions, int lambda) {
+        super(id, ingressPort, path, egressActions);
         this.lambda = lambda;
-        // TODO Auto-generated constructor stub
+    }
+
+    /**
+     * Gets traffic filter for this flow.
+     * <p>
+     * This method only returns wildcard match, because the ingress transponder
+     * port does not have filtering functionality.
+     */
+    @Override
+    public PacketMatch getMatch() {
+        return (new PacketMatchBuilder()).build();
     }
 
     /**
      * Gets lambda which is used throughout the path.
      *
-     * @return lambda which is used throughout the path.
+     * @return lambda which is used throughout the path
      */
     public int getLambda() {
         return lambda;
@@ -43,6 +57,6 @@
     @Override
     public MatchActionOperations compile() {
         // TODO Auto-generated method stub
-        return super.compile();
+        return null;
     }
 }
diff --git a/src/main/java/net/onrc/onos/api/flowmanager/PacketPathFlow.java b/src/main/java/net/onrc/onos/api/flowmanager/PacketPathFlow.java
index 040b9a7..8a15f3f 100644
--- a/src/main/java/net/onrc/onos/api/flowmanager/PacketPathFlow.java
+++ b/src/main/java/net/onrc/onos/api/flowmanager/PacketPathFlow.java
@@ -1,5 +1,7 @@
 package net.onrc.onos.api.flowmanager;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 import java.util.List;
 
 import net.onrc.onos.core.matchaction.MatchActionOperations;
@@ -13,28 +15,36 @@
  * TODO: Think this: Do we need a bandwidth constraint?
  */
 public class PacketPathFlow extends PathFlow {
-    private int hardTimeout;
-    private int idleTimeout;
+    private final PacketMatch match;
+    private final int hardTimeout;
+    private final int idleTimeout;
 
     /**
      * Constructor.
      *
-     * @param id ID for this new Flow object.
-     * @param match Match object at the source node of the path.
-     * @param inPort Ingress port number at the ingress edge node.
-     * @param path Path between ingress and egress edge node.
-     * @param edgeActions The list of Action objects at the egress edge node.
-     * @param hardTimeout hard-timeout value.
-     * @param idleTimeout idle-timeout value.
+     * @param id ID for this new Flow object
+     * @param match the Match object at the source node of the path
+     * @param ingressPort the Ingress port number at the ingress edge node
+     * @param path the Path between ingress and egress edge node
+     * @param egressActions the list of Action objects at the egress edge node
+     * @param hardTimeout the hard-timeout value in seconds, or 0 for no timeout
+     * @param idleTimeout the idle-timeout value in seconds, or 0 for no timeout
      */
-    public PacketPathFlow(String id,
-            PacketMatch match, PortNumber inPort, Path path, List<Action> edgeActions,
+    public PacketPathFlow(FlowId id,
+            PacketMatch match, PortNumber ingressPort, Path path,
+            List<Action> egressActions,
             int hardTimeout, int idleTimeout) {
-        super(id, match, inPort, path, edgeActions);
+        super(id, ingressPort, path, egressActions);
+        this.match = checkNotNull(match);
         this.hardTimeout = hardTimeout;
         this.idleTimeout = idleTimeout;
     }
 
+    @Override
+    public PacketMatch getMatch() {
+        return match;
+    }
+
     /**
      * Gets idle-timeout value.
      *
@@ -56,6 +66,6 @@
     @Override
     public MatchActionOperations compile() {
         // TODO Auto-generated method stub
-        return super.compile();
+        return null;
     }
 }
diff --git a/src/main/java/net/onrc/onos/api/flowmanager/Path.java b/src/main/java/net/onrc/onos/api/flowmanager/Path.java
index df61e51..2f733a4 100644
--- a/src/main/java/net/onrc/onos/api/flowmanager/Path.java
+++ b/src/main/java/net/onrc/onos/api/flowmanager/Path.java
@@ -1,5 +1,7 @@
 package net.onrc.onos.api.flowmanager;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 import java.util.ArrayList;
 import java.util.Iterator;
 import java.util.List;
@@ -18,6 +20,35 @@
     }
 
     /**
+     * Constructor to create the object from the list of FlowLinks.
+     *
+     * @param links the list of FlowLinks
+     * @throws IllegalArgumentException if the links does not form a single
+     *         connected directed path topology
+     */
+    public Path(List<FlowLink> links) {
+        super();
+        checkNotNull(links);
+        for (FlowLink link : links) {
+            if (!addLink(link)) {
+                throw new IllegalArgumentException();
+            }
+        }
+    }
+
+    /**
+     * Adds FlowLink to the Path object.
+     *
+     * @param link the FlowLink object to be added
+     * @return true if succeeded, false otherwise
+     */
+    public boolean addLink(FlowLink link) {
+        // TODO check connectivity
+        checkNotNull(link);
+        return add(link);
+    }
+
+    /**
      * Gets a list of switch DPIDs of the path.
      *
      * @return a list of Dpid objects
@@ -29,7 +60,7 @@
 
         List<Dpid> dpids = new ArrayList<Dpid>(size() + 1);
         dpids.add(getSrcDpid());
-        for (FlowLink link: this) {
+        for (FlowLink link : this) {
             dpids.add(link.getDstDpid());
         }
         return dpids;
diff --git a/src/main/java/net/onrc/onos/api/flowmanager/PathFlow.java b/src/main/java/net/onrc/onos/api/flowmanager/PathFlow.java
index d1b10f6..0cc35ce 100644
--- a/src/main/java/net/onrc/onos/api/flowmanager/PathFlow.java
+++ b/src/main/java/net/onrc/onos/api/flowmanager/PathFlow.java
@@ -1,64 +1,41 @@
 package net.onrc.onos.api.flowmanager;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 import java.util.List;
 
-import net.onrc.onos.core.matchaction.MatchActionOperations;
 import net.onrc.onos.core.matchaction.action.Action;
-import net.onrc.onos.core.matchaction.match.Match;
 import net.onrc.onos.core.util.PortNumber;
 
 /**
- * A path flow.
- * <p>
- * TODO: Think this: Should this class be an abstract class? Is it enough to
- * have only the PacketPathFlow and OpticalPathFlow classes?
+ * An abstract class expressing a path flow.
  */
-public class PathFlow implements Flow {
-    protected final FlowId id;
-    protected Match match;
-    protected PortNumber ingressPort;
-    protected Path path;
-    protected List<Action> egressActions;
+public abstract class PathFlow extends Flow {
+    private final PortNumber ingressPort;
+    private final Path path;
+    private final List<Action> egressActions;
 
     /**
-     * Constructor.
+     * Creates the new flow instance.
      *
-     * @param id ID for this new PathFlow object.
-     * @param match Match object at the ingress node of the path.
-     * @param ingressPort The ingress port number at the ingress node of the
-     *        path.
-     * @param path Path between ingress and egress edge node.
-     * @param egressActions The list of Action objects at the egress edge node.
+     * @param id ID for this new PathFlow object
+     * @param ingressPort the ingress port number at the ingress node of the
+     *        path
+     * @param path the Path between ingress and egress edge node
+     * @param egressActions the list of Action objects at the egress edge node
      */
-    public PathFlow(String id,
-            Match match, PortNumber ingressPort, Path path, List<Action> egressActions) {
-        this.id = new FlowId(id);
-        this.match = match;
-        this.ingressPort = ingressPort;
-        this.path = path;
-        this.egressActions = egressActions;
-    }
-
-    @Override
-    public FlowId getId() {
-        return id;
-    }
-
-    @Override
-    public Match getMatch() {
-        return match;
-    }
-
-    @Override
-    public MatchActionOperations compile() {
-        // TODO Auto-generated method stub
-        return null;
+    public PathFlow(FlowId id,
+            PortNumber ingressPort, Path path, List<Action> egressActions) {
+        super(id);
+        this.ingressPort = checkNotNull(ingressPort);
+        this.path = checkNotNull(path);
+        this.egressActions = checkNotNull(egressActions);
     }
 
     /**
      * Gets the ingress port number at the ingress node of the path.
      *
-     * @return The ingress port number at the ingress node of the path.
+     * @return the ingress port number at the ingress node of the path
      */
     public PortNumber getIngressPortNumber() {
         return ingressPort;
@@ -67,7 +44,7 @@
     /**
      * Gets the path from ingress to egress edge node.
      *
-     * @return The path object from ingress to egress edge node.
+     * @return the path object from ingress to egress edge node
      */
     public Path getPath() {
         return path;
@@ -76,7 +53,7 @@
     /**
      * Gets the list of Action objects at the egress edge node.
      *
-     * @return The list of Action objects at the egress edge node.
+     * @return the list of Action objects at the egress edge node
      */
     public List<Action> getEgressActions() {
         return egressActions;
diff --git a/src/main/java/net/onrc/onos/api/flowmanager/SingleDstTreeFlow.java b/src/main/java/net/onrc/onos/api/flowmanager/SingleDstTreeFlow.java
index af403ed..92533cb 100644
--- a/src/main/java/net/onrc/onos/api/flowmanager/SingleDstTreeFlow.java
+++ b/src/main/java/net/onrc/onos/api/flowmanager/SingleDstTreeFlow.java
@@ -1,15 +1,19 @@
 package net.onrc.onos.api.flowmanager;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 import java.util.Collection;
-import java.util.Collections;
-import java.util.HashSet;
+import java.util.List;
 import java.util.Set;
 
 import net.onrc.onos.core.matchaction.MatchActionOperations;
-import net.onrc.onos.core.matchaction.action.OutputAction;
+import net.onrc.onos.core.matchaction.action.Action;
 import net.onrc.onos.core.matchaction.match.PacketMatch;
 import net.onrc.onos.core.util.SwitchPort;
 
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableSet;
+
 /**
  * A Flow object expressing the multipoints-to-point tree flow for the packet
  * layer.
@@ -17,37 +21,36 @@
  * NOTE: This class might generate the MatchActionPlan which includes the MAC
  * address modifications or other the label-switching-like schemes.
  */
-public class SingleDstTreeFlow implements Flow {
-    protected final FlowId id;
-    protected PacketMatch match;
-    protected Set<SwitchPort> ingressPorts;
-    protected Tree tree;
-    protected OutputAction outputAction;
+public class SingleDstTreeFlow extends Flow {
+    private final PacketMatch match;
+    private final Set<SwitchPort> ingressPorts;
+    private final Tree tree;
+    private final List<Action> actions;
 
     /**
      * Creates new instance using Tree object.
+     * <p>
+     * For now, the actions parameter must be the list of a single
+     * ModifyDstMacAction object and a single OutputAction object. But in the
+     * future, the parameter should accept any type of the list of IAction
+     * objects.
      *
-     * @param id ID for this object.
-     * @param match Traffic filter for the tree.
-     * @param ingressPorts A set of ingress ports of the tree.
-     * @param tree Tree object specifying tree topology for this object.
-     * @param outputAction OutputAction object at the egress edge switch.
+     * @param id ID for this object
+     * @param match the traffic filter for the tree
+     * @param ingressPorts the set of ingress ports of the tree
+     * @param tree the Tree object specifying tree topology for this object
+     * @param actions the list of Action objects at the egress edge switch
      */
-    public SingleDstTreeFlow(String id, PacketMatch match,
-            Collection<SwitchPort> ingressPorts, Tree tree, OutputAction outputAction) {
-        this.id = new FlowId(id);
-        this.match = match;
-        this.ingressPorts = new HashSet<SwitchPort>(ingressPorts);
-        this.tree = tree;
-        this.outputAction = outputAction;
+    public SingleDstTreeFlow(FlowId id, PacketMatch match,
+            Collection<SwitchPort> ingressPorts, Tree tree, List<Action> actions) {
+        super(id);
+        this.match = checkNotNull(match);
+        this.ingressPorts = ImmutableSet.copyOf(checkNotNull(ingressPorts));
+        this.tree = checkNotNull(tree);
+        this.actions = ImmutableList.copyOf(checkNotNull(actions));
 
         // TODO: check if the tree is a MP2P tree.
-        // TODO: check consistency among inPorts, tree, and action.
-    }
-
-    @Override
-    public FlowId getId() {
-        return id;
+        // TODO: check consistency among ingressPorts, tree, and actions.
     }
 
     @Override
@@ -64,27 +67,27 @@
     /**
      * Gets the ingress ports of the tree.
      *
-     * @return The ingress ports of the tree.
+     * @return the ingress ports of the tree
      */
     public Collection<SwitchPort> getIngressPorts() {
-        return Collections.unmodifiableCollection(ingressPorts);
+        return ingressPorts;
     }
 
     /**
      * Gets the tree.
      *
-     * @return The tree object.
+     * @return the tree object
      */
     public Tree getTree() {
         return tree;
     }
 
     /**
-     * Gets the output action for the tree.
+     * Gets the list of actions at the egress edge switch.
      *
-     * @return The OutputAction object.
+     * @return the list of actions at the egress edge switch
      */
-    public OutputAction getOutputAction() {
-        return outputAction;
+    public List<Action> getEgressActions() {
+        return actions;
     }
 }
diff --git a/src/main/java/net/onrc/onos/api/flowmanager/SingleSrcTreeFlow.java b/src/main/java/net/onrc/onos/api/flowmanager/SingleSrcTreeFlow.java
index 7430b7f..bb896c8 100644
--- a/src/main/java/net/onrc/onos/api/flowmanager/SingleSrcTreeFlow.java
+++ b/src/main/java/net/onrc/onos/api/flowmanager/SingleSrcTreeFlow.java
@@ -1,5 +1,7 @@
 package net.onrc.onos.api.flowmanager;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 import java.util.Set;
 
 import net.onrc.onos.core.matchaction.MatchActionOperations;
@@ -12,9 +14,10 @@
 /**
  * A Flow object expressing the point-to-multipoints tree flow for the packet
  * layer.
+ * <p>
+ * NOTE: This class is not fully supported for the August release.
  */
-public class SingleSrcTreeFlow implements Flow {
-    protected final FlowId id;
+public class SingleSrcTreeFlow extends Flow {
     protected PacketMatch match;
     protected SwitchPort ingressPort;
     protected Tree tree;
@@ -23,31 +26,26 @@
     /**
      * Creates new instance using Tree object.
      *
-     * @param id ID for this object.
-     * @param match Traffic filter for the tree.
-     * @param ingressPort A ingress port of the tree.
-     * @param tree Tree object specifying tree topology for this object.
-     * @param outputActions The set of the pairs of the switch DPID and
-     *        OutputAction object at the egress edge switchs.
+     * @param id ID for this object
+     * @param match the traffic filter for the tree
+     * @param ingressPort an ingress port of the tree
+     * @param tree the tree object specifying tree topology for this object
+     * @param outputActions the set of the pairs of the switch DPID and
+     *        OutputAction object at the egress edge switchs
      */
-    public SingleSrcTreeFlow(String id, PacketMatch match,
+    public SingleSrcTreeFlow(FlowId id, PacketMatch match,
             SwitchPort ingressPort, Tree tree, Set<Pair<Dpid, OutputAction>> outputActions) {
-        this.id = new FlowId(id);
-        this.match = match;
-        this.ingressPort = ingressPort;
-        this.tree = tree;
-        this.outputActions = outputActions;
+        super(id);
+        this.match = checkNotNull(match);
+        this.ingressPort = checkNotNull(ingressPort);
+        this.tree = checkNotNull(tree);
+        this.outputActions = checkNotNull(outputActions);
 
         // TODO: check if the tree is a P2MP tree.
         // TODO: check consistency among rootPort, tree, and actions.
     }
 
     @Override
-    public FlowId getId() {
-        return id;
-    }
-
-    @Override
     public PacketMatch getMatch() {
         return match;
     }
diff --git a/src/main/java/net/onrc/onos/api/flowmanager/Tree.java b/src/main/java/net/onrc/onos/api/flowmanager/Tree.java
index 49b05dc..1e161a2 100644
--- a/src/main/java/net/onrc/onos/api/flowmanager/Tree.java
+++ b/src/main/java/net/onrc/onos/api/flowmanager/Tree.java
@@ -1,5 +1,7 @@
 package net.onrc.onos.api.flowmanager;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
@@ -25,14 +27,43 @@
     }
 
     /**
-     * Creates new instance using Path object.
+     * Creates new instance from the specified Path object.
+     *
+     * @param path the Path object
+     * @throws IllegalArgumentException if the path object does not form a
+     *         single connected directed path topology
      */
     public Tree(Path path) {
         super();
-        // TODO implement
+        checkNotNull(path);
+
+        for (FlowLink link : path) {
+            if (!addLink(link)) {
+                throw new IllegalArgumentException();
+            }
+        }
+    }
+
+    /**
+     * Creates new instance using FlowLinks object.
+     *
+     * @param links the FlowLinks object
+     * @throws IllegalArgumentException if the links object does not form a
+     *         single connected directed tree topology
+     */
+    public Tree(FlowLinks links) {
+        super();
+        checkNotNull(links);
+
+        for (FlowLink link : links) {
+            if (!addLink(link)) {
+                throw new IllegalArgumentException();
+            }
+        }
     }
 
     private void addPort(SwitchPort port) {
+        checkNotNull(port);
         if (!ports.containsKey(port.dpid())) {
             ports.put(port.dpid(), new HashSet<PortNumber>());
         }
@@ -49,6 +80,7 @@
      * @return true if succeeded, false otherwise.
      */
     public boolean addLink(FlowLink link) {
+        checkNotNull(link);
         if (links.size() > 0) {
             if (!hasDpid(link.getSrcDpid()) && !hasDpid(link.getDstDpid())) {
                 // no attaching point
diff --git a/src/main/java/net/onrc/onos/core/newintent/PathFlowIntent.java b/src/main/java/net/onrc/onos/core/newintent/PathFlowIntent.java
index 988a8b3..39a1898 100644
--- a/src/main/java/net/onrc/onos/core/newintent/PathFlowIntent.java
+++ b/src/main/java/net/onrc/onos/core/newintent/PathFlowIntent.java
@@ -1,33 +1,33 @@
 package net.onrc.onos.core.newintent;
 
-import com.google.common.base.Objects;
-import net.onrc.onos.api.flowmanager.PathFlow;
+import static com.google.common.base.Preconditions.checkNotNull;
+import net.onrc.onos.api.flowmanager.PacketPathFlow;
 import net.onrc.onos.api.newintent.AbstractIntent;
 import net.onrc.onos.api.newintent.InstallableIntent;
 import net.onrc.onos.api.newintent.IntentId;
 
-import static com.google.common.base.Preconditions.checkNotNull;
+import com.google.common.base.Objects;
 
 /**
- * Intent containing {@link PathFlow} object, which defines an explicit path.
+ * Intent containing {@link PacketPathFlow} object, which defines an explicit path.
  *
  * It is intended to establish a path by using Flow Manager's API.
  */
 public class PathFlowIntent extends AbstractIntent implements InstallableIntent {
 
-    private final PathFlow flow;
+    private final PacketPathFlow flow;
 
-    public PathFlowIntent(IntentId id, PathFlow flow) {
+    public PathFlowIntent(IntentId id, PacketPathFlow flow) {
         super(id);
         this.flow = checkNotNull(flow);
     }
 
     /**
-     * Returns {@link PathFlow} object, which defines an explicit path.
+     * Returns {@link PacketPathFlow} object, which defines an explicit path.
      *
-     * @return {@link PathFlow path}
+     * @return {@link PacketPathFlow path}
      */
-    public PathFlow getFlow() {
+    public PacketPathFlow getFlow() {
         return flow;
 
     }