diff --git a/src/seedu/addressbook/commands/AddCommand.java b/src/seedu/addressbook/commands/AddCommand.java
index b0f202d42..3857221be 100644
--- a/src/seedu/addressbook/commands/AddCommand.java
+++ b/src/seedu/addressbook/commands/AddCommand.java
@@ -67,4 +67,7 @@ public CommandResult execute() {
         }
     }
 
+    @Override
+    public boolean isMutating() {   return true; }
+
 }
diff --git a/src/seedu/addressbook/commands/ClearCommand.java b/src/seedu/addressbook/commands/ClearCommand.java
index 330146aaa..3aeb46e23 100644
--- a/src/seedu/addressbook/commands/ClearCommand.java
+++ b/src/seedu/addressbook/commands/ClearCommand.java
@@ -16,4 +16,7 @@ public CommandResult execute() {
         addressBook.clear();
         return new CommandResult(MESSAGE_SUCCESS);
     }
+
+    @Override
+    public boolean isMutating() { return true;  }
 }
diff --git a/src/seedu/addressbook/commands/Command.java b/src/seedu/addressbook/commands/Command.java
index ccd4fcbf4..e075aa674 100644
--- a/src/seedu/addressbook/commands/Command.java
+++ b/src/seedu/addressbook/commands/Command.java
@@ -65,4 +65,6 @@ public int getTargetIndex() {
     public void setTargetIndex(int targetIndex) {
         this.targetIndex = targetIndex;
     }
+
+    public boolean isMutating() { return false; }
 }
diff --git a/src/seedu/addressbook/commands/DeleteCommand.java b/src/seedu/addressbook/commands/DeleteCommand.java
index 1dd78f85e..70e8cab78 100644
--- a/src/seedu/addressbook/commands/DeleteCommand.java
+++ b/src/seedu/addressbook/commands/DeleteCommand.java
@@ -39,4 +39,7 @@ public CommandResult execute() {
         }
     }
 
+    @Override
+    public boolean isMutating() {   return true;    }
+
 }