diff --git a/AlgebraicDataflowArchitectureModel/src/simulator/Resource.java b/AlgebraicDataflowArchitectureModel/src/simulator/Resource.java
index 5f7e67d..665d4a9 100644
--- a/AlgebraicDataflowArchitectureModel/src/simulator/Resource.java
+++ b/AlgebraicDataflowArchitectureModel/src/simulator/Resource.java
@@ -89,6 +89,10 @@
 	public ResourceState getState() {
 		return state;
 	}
+	
+	public void changeState(ResourceState state) {
+		this.state = state;
+	}
 
 	public Constant getParameter() {
 		return parameter;
@@ -99,22 +103,26 @@
 		if (resourceHierarchy.getChildren().size() > 0) {
 			children = new ArrayList<>();
 			ResourceHierarchy childRes = resourceHierarchy.getChildren().iterator().next();
-			Map<String, ResourceState> childStates = ((CompositeResourceState) state).getChildStates();
 			if (childRes.getNumParameters() > 0) {
 				// List or Map type.
-				for (Map.Entry<String, ResourceState> childEnt: childStates.entrySet()) {
-					String childParam = childEnt.getKey();
-					ResourceState childState = childEnt.getValue();
-					Type thisType = resourceHierarchy.getResourceStateType();
-					if (DataConstraintModel.typeList.isAncestorOf(thisType)) {
-						children.add(new Resource(childRes, this, new Constant(childParam, DataConstraintModel.typeInt), childState));
-					} else if (DataConstraintModel.typeMap.isAncestorOf(thisType)) {
-						children.add(new Resource(childRes, this, new Constant(childParam, DataConstraintModel.typeString), childState));						
+				if (state instanceof CompositeResourceState) {
+					// If the list or map is not nil.
+					Map<String, ResourceState> childStates = ((CompositeResourceState) state).getChildStates();
+					for (Map.Entry<String, ResourceState> childEnt: childStates.entrySet()) {
+						String childParam = childEnt.getKey();
+						ResourceState childState = childEnt.getValue();
+						Type thisType = resourceHierarchy.getResourceStateType();
+						if (DataConstraintModel.typeList.isAncestorOf(thisType)) {
+							children.add(new Resource(childRes, this, new Constant(childParam, DataConstraintModel.typeInt), childState));
+						} else if (DataConstraintModel.typeMap.isAncestorOf(thisType)) {
+							children.add(new Resource(childRes, this, new Constant(childParam, DataConstraintModel.typeString), childState));						
+						}
 					}
 				}
 			} else {
 				// Json type.
 				if (this.children == null) {
+					Map<String, ResourceState> childStates = ((CompositeResourceState) state).getChildStates();
 					for (Map.Entry<String, ResourceState> childEnt: childStates.entrySet()) {
 						String childParam = childEnt.getKey();
 						ResourceState childState = childEnt.getValue();
diff --git a/AlgebraicDataflowArchitectureModel/src/simulator/SystemState.java b/AlgebraicDataflowArchitectureModel/src/simulator/SystemState.java
index 9e331e5..4adf78d 100644
--- a/AlgebraicDataflowArchitectureModel/src/simulator/SystemState.java
+++ b/AlgebraicDataflowArchitectureModel/src/simulator/SystemState.java
@@ -13,7 +13,9 @@
 import models.algebra.Term;
 import models.algebra.Type;
 import models.dataConstraintModel.DataConstraintModel;
+import models.dataConstraintModel.ResourceHierarchy;
 import models.dataFlowModel.DataTransferChannel;
+import simulator.states.CompositeResourceState;
 import simulator.states.JsonResourceState;
 import simulator.states.ListResourceState;
 import simulator.states.MapResourceState;
@@ -66,10 +68,27 @@
 				if (listValue.getSymbol().equals(DataConstraintModel.append)) {
 					Resource res = getResource(resourceIdentifier);
 					ResourceState state = res.getState();
-					Expression childExp = new Constant(Integer.toString(((ListResourceState) state).getChildStates().size()));
-					ResourceIdentifier childResourceIdentifier = new ResourceIdentifier(resourceIdentifier, 
-																						childExp, 
-																						resourceIdentifier.getResourceHierarchy().getChildren().iterator().next());
+					Expression childExp = null;
+					if (state instanceof ListResourceState) {
+						childExp = new Constant(Integer.toString(((ListResourceState) state).getChildStates().size()));
+					} else if (state instanceof PrimitiveResourceState && ((PrimitiveResourceState) state).getValue().getSymbol().equals(DataConstraintModel.nil)) {
+						// If the value of state is nil.
+						childExp = new Constant("0");
+						ResourceState parentState = res.getParent().getState();
+						ResourceState newState = new ListResourceState();
+						if (parentState instanceof CompositeResourceState) {
+							((CompositeResourceState) parentState).replaceChildState(state, newState);
+						}
+						state = newState;
+						res.changeState(state);
+					}
+					ResourceHierarchy childResourceHierarchy = null;
+					if (resourceIdentifier.getResourceHierarchy().getChildren() != null && resourceIdentifier.getResourceHierarchy().getChildren().size() > 0) {
+						childResourceHierarchy = resourceIdentifier.getResourceHierarchy().getChildren().iterator().next();
+					} else {
+						childResourceHierarchy = new ResourceHierarchy(resourceIdentifier.getResourceHierarchy(), 1);
+					}
+					ResourceIdentifier childResourceIdentifier = new ResourceIdentifier(resourceIdentifier, childExp, childResourceHierarchy);
 					Map.Entry<ResourceState, ResourceIdentifier> childInfo = createResourceState(childResourceIdentifier, listValue.getChild(1));
 					((ListResourceState) state).addChildState(childInfo.getKey());
 					return childInfo.getValue();
@@ -84,9 +103,23 @@
 					if (childExp instanceof Constant) {
 						Resource res = getResource(resourceIdentifier);
 						ResourceState state = res.getState();
-						ResourceIdentifier childResourceIdentifier = new ResourceIdentifier(resourceIdentifier, 
-																							childExp, 
-																							resourceIdentifier.getResourceHierarchy().getChildren().iterator().next());
+						if (state instanceof PrimitiveResourceState && ((PrimitiveResourceState) state).getValue().getSymbol().equals(DataConstraintModel.nil)) {
+							// If the value of state is nil.
+							ResourceState parentState = res.getParent().getState();
+							ResourceState newState = new MapResourceState();
+							if (parentState instanceof CompositeResourceState) {
+								((CompositeResourceState) parentState).replaceChildState(state, newState);
+							}
+							state = newState;
+							res.changeState(state);
+						}
+						ResourceHierarchy childResourceHierarchy = null;
+						if (resourceIdentifier.getResourceHierarchy().getChildren() != null && resourceIdentifier.getResourceHierarchy().getChildren().size() > 0) {
+							childResourceHierarchy = resourceIdentifier.getResourceHierarchy().getChildren().iterator().next();
+						} else {
+							childResourceHierarchy = new ResourceHierarchy(resourceIdentifier.getResourceHierarchy(), 1);
+						}
+						ResourceIdentifier childResourceIdentifier = new ResourceIdentifier(resourceIdentifier, childExp, childResourceHierarchy);
 						String childId = ((Constant) childExp).toString();
 						Map.Entry<ResourceState, ResourceIdentifier> childInfo = createResourceState(childResourceIdentifier, mapValue.getChild(2));
 						((MapResourceState) state).addChildState(childId, childInfo.getKey());
diff --git a/AlgebraicDataflowArchitectureModel/src/simulator/states/CompositeResourceState.java b/AlgebraicDataflowArchitectureModel/src/simulator/states/CompositeResourceState.java
index 3cc29ca..5e6311f 100644
--- a/AlgebraicDataflowArchitectureModel/src/simulator/states/CompositeResourceState.java
+++ b/AlgebraicDataflowArchitectureModel/src/simulator/states/CompositeResourceState.java
@@ -2,11 +2,13 @@
 
 import java.util.Map;
 
-abstract public class CompositeResourceState extends ResourceState {
+public abstract class CompositeResourceState extends ResourceState {
 	@Override
 	boolean hasChildren() {
 		return true;
 	}
 	
-	abstract public Map<String, ResourceState> getChildStates();
+	public abstract Map<String, ResourceState> getChildStates();
+
+	public abstract void replaceChildState(ResourceState state, ResourceState newState);
 }
diff --git a/AlgebraicDataflowArchitectureModel/src/simulator/states/JsonResourceState.java b/AlgebraicDataflowArchitectureModel/src/simulator/states/JsonResourceState.java
index 327da4f..afd1c20 100644
--- a/AlgebraicDataflowArchitectureModel/src/simulator/states/JsonResourceState.java
+++ b/AlgebraicDataflowArchitectureModel/src/simulator/states/JsonResourceState.java
@@ -33,4 +33,13 @@
 	public void addChildState(String param, ResourceState childState) {
 		children.put(param, childState);
 	}
+
+	@Override
+	public void replaceChildState(ResourceState state, ResourceState newState) {
+		for (Map.Entry<String, ResourceState> childEnt: children.entrySet()) {
+			if (childEnt.getValue().equals(state)) {
+				children.put(childEnt.getKey(), newState);
+			}
+		}
+	}
 }
diff --git a/AlgebraicDataflowArchitectureModel/src/simulator/states/ListResourceState.java b/AlgebraicDataflowArchitectureModel/src/simulator/states/ListResourceState.java
index e49bd72..83367e0 100644
--- a/AlgebraicDataflowArchitectureModel/src/simulator/states/ListResourceState.java
+++ b/AlgebraicDataflowArchitectureModel/src/simulator/states/ListResourceState.java
@@ -29,13 +29,23 @@
 	@Override
 	public Map<String, ResourceState> getChildStates() {
 		Map<String, ResourceState> childParams = new HashMap<>();
-		for (int i = 1; i <= children.size(); i++) {
+		for (int i = 0; i < children.size(); i++) {
 			childParams.put(Integer.toString(i), children.get(i));
 		}
-		return null;
+		return childParams;
 	}
 
 	public void addChildState(ResourceState childState) {
 		children.add(childState);
 	}
+
+
+	@Override
+	public void replaceChildState(ResourceState state, ResourceState newState) {
+		for (int i = 0; i < children.size(); i++) {
+			if (children.get(i).equals(state)) {
+				children.set(i, newState);
+			}
+		}
+	}
 }
diff --git a/AlgebraicDataflowArchitectureModel/src/simulator/states/MapResourceState.java b/AlgebraicDataflowArchitectureModel/src/simulator/states/MapResourceState.java
index 1f36406..5bd2fc7 100644
--- a/AlgebraicDataflowArchitectureModel/src/simulator/states/MapResourceState.java
+++ b/AlgebraicDataflowArchitectureModel/src/simulator/states/MapResourceState.java
@@ -33,4 +33,13 @@
 	public void addChildState(String param, ResourceState childState) {
 		children.put(param, childState);
 	}
+
+	@Override
+	public void replaceChildState(ResourceState state, ResourceState newState) {
+		for (Map.Entry<String, ResourceState> childEnt: children.entrySet()) {
+			if (childEnt.getValue().equals(state)) {
+				children.put(childEnt.getKey(), newState);
+			}
+		}
+	}
 }