Using the Command Pattern to Undo and Redo in ArrayLists

Question

So, I have a program in which you can log in and add / remove friends to and from friends arraylist. I may also like a certain thing, and this thing will be stored in likes arraylist. I will be asked to make undo and redo options for whatever I do.

So, I want to add apple as a friend. After that, when I choose the undo option, I can undo this action, so apple will not be my friend. How can I approach this with the Command Pattern when the input is any name or word entered for storage in friends arraylist?

I did some research and found that using the team template might be my best bet, as it should be done within the Facebook class that I already have. I guess I will have to use two different stacks, but I'm a little lost in the subject.

I decided to add parts of what I have in order to get a little more information about what I need to do and what my program does.

In the driver program

 Facebook facebook1 = new Facebook(); if (userInput == 6) { System.out.println("Login"); String operand1 = getOperand("What is the Username? "); String operand2 = getOperand("What is the Password? "); System.out.println("Enter a friend to be added. "); String operand3 = getOperand("What is the Username? "); facebook1.friend(operand3); } if (userInput == 7) { System.out.println("Login"); String operand1 = getOperand("What is the Username? "); String operand2 = getOperand("What is the Password? "); System.out.println("Enter a friend to be removed. "); String operand3 = getOperand("What is the Username? "); facebook1.defriend(operand3); } if (userInput == 12) { System.out.println("Login"); String operand1 = getOperand("What is the Password? "); facebook1.undo(); } if (userInput == 13) { System.out.println("Login"); String operand1 = getOperand("What is the Password? "); facebook1.redo(); } 

In Facebook class

 ArrayList<FacebookUser> recommendedFriends = new ArrayList<FacebookUser>(); void friend(String newFriend) { boolean positiveChecker = false; for (int i = 0; i < recommendedFriends.size(); i++) { if (recommendedFriends.get(i).toString().equalsIgnoreCase(newFriend)) { System.out.println("Error: This friend already exists."); positiveChecker = true; } } if (positiveChecker == false) { FacebookUser friend = new FacebookUser(newFriend, newFriend ); recommendedFriends.add(friend); System.out.println(friend + " is now your friend."); } positiveChecker = false; } void defriend(String formerFriend) { boolean positiveChecker = false; for (int i = 0; i < recommendedFriends.size(); i++) { if (recommendedFriends.get(i).toString().equalsIgnoreCase(formerFriend)) { recommendedFriends.remove(i); System.out.println(formerFriend + " has been removed from your friends list."); positiveChecker = true; } if (recommendedFriends.size() == (i + 1) && recommendedFriends.get(i).toString() != formerFriend && positiveChecker == false) { System.out.println("Error: There is no friend with this username."); } } positiveChecker = false; } public interface Command { public void undo(); public void redo(); } 

Answer 1

When you cancel 2 things, you take a completely new action, you need to “forget” the history of the replay and replace it with a new command, right?

For instance...

• Add as friend Jim
• Add Friend Bill
• Add Friend Jill
• Remove Jim
• Cancel
• Cancel

The state should be "Jim" and "Bill."

So, you really need only one list and a pointer to the current "command", for example ...

 // Note: NOT thread safe! public class CommandStack { private List<Command> commands = Collections.emptyList(); private int nextPointer = 0; public void doCommand(Command command) { List<Command> newList = new ArrayList<>(nextPointer + 1) for(int k = 0; k < nextPointer; k++) { newList.add(commands.get(k)); } newList.add(command); commands = newList; nextPointer++; // Do the command here, or return it to whatever called this to be done, or maybe it has already been done by now or something // (I can only guess on what your code currently looks like...) command.execute(); } public boolean canUndo() { return nextPointer > 0; } public void undo() { if(canUndo()) { nextPointer--; Command commandToUndo = commands.get(nextPointer); // Undo the command, or return it to whatever called this to be undone, or something command.undo(); } else { throw new IllegalStateExcpetion("Cannot undo"); } } public boolean canRedo() { return nextPointer < commands.size(); } public void redo() { if(canRedo()) { commandToDo = commands.get(nextPointer); nextPointer++; // Do the command, or return it to whatever called this to be re-done, or something commandToDo.execute(); } else { throw new IllegalStateException("Cannot redo"); } } } 

If I...

 interface Command { /* execute / undo etc */ } public class AddFriendCommand implements Command { private String friendName; // ... other fields, constructor / getters etc ... public void execute() { // Actually do it... System.out.println("Added friend " + name); } public void undo() { // Undo it... System.out.println("Removed friend " + name); } } public class RemoveFriendCommand implements Command { private String friendName; // ... other fields, constructor / getters etc ... public void execute() { // Actually do it, maybe throw exception if friend does not exist? // (that would have to be a runtime exception unless you want the interface method to throw stuff); System.out.println("Removed friend " + name); } public void undo() { // Undo it... System.out.println("Added friend " + name); } } 

You can repeat the above sequence using ...

 CommandStack stack = new CommandStack(); stack.doCommand(new AddFriendCommand("Jim")); stack.doCommand(new AddFriendCommand("Bill")); stack.doCommand(new AddFriendCommand("Jill")); stack.doCommand(new RemoveFreindCommand("Jim")); stack.undo(); stack.undo(); 

If you have now made a new command (via doCommand), it will forget that you have ever added “Jill” or deleted “Jim”, but instead will now remember the new command and the rest of the command history, which has not been canceled.

Hope this helps.

Answer 2

You misunderstand how a team template works. You want to have a separate List your Commands , where each instance of the command represents an action .

So you would like to have something like:

 List<Command> actionStack; 

and then enter the material e.g.

 public class AddCommand implements Command { private final void List<FacebookUser> userList; private final void FacebookUser newUser; public AddCommand(List<FacebookUser> userList, FacebookUser newUser) { this.userList = userList; this.newUser = newUser; } @Override public void undo() { userList.remove(newUser); } @Override public void redo() { userList.add(newUser); } }