Writing Text Editor in Java

To showcase the Command pattern, we'll be creating a basic implementation of a text editor. This includes the ability to add content(type it on the page), backspace to remove content, and the ability to undo and redo our actions.

We'll start out with a Document class that will contain the text data and actions to add and remove content:

The Document Class:

public class Document
{
   String content;

   public Document()
   {
       content = "";
   }

   public void PrintDocument()
   {
       System.out.println(content);
   }

   public void AddContent(String content)
   {
       this.content += content;
   }

   public int Length()
   {
       return content.length();
   }

   public String CharAt(int index)
   {
       return String.valueOf(content.charAt(index));
   }

   public void RemoveContent(int numChars)
   {
       if(numChars < 0) return;
       if(numChars > content.length() - 1)
       {
           content = "";
           return;
       }

       //content = content.Remove(content.Length - numChars);
       content = content.substring(0,  content.length() - numChars);
       
   }
}

Next, we'll want to create an interface for our commands. We'll want to have the usual execute method, in addition to undo and redo methods:

IUndoableCommand

public interface IUndoableCommand
{
   void Execute();
   void Undo();
   void Redo();
}

AddContentCommand

public class AddContentCommand implements IUndoableCommand
{
   Document document;
   String content;

   public AddContentCommand(Document document, String content)
   {
       this.document = document;
       this.content = content;
   }

   public void Execute()
   {
       this.document.AddContent(content);
   }

   public void Undo()
   {
       this.document.RemoveContent(content.length());
   }

   public void Redo()
   {
       this.Execute();
   }
}

BackspaceCommand

public class BackspaceCommand implements IUndoableCommand
{
   Document document;
   String removedChar;

   public BackspaceCommand(Document document)
   {
       this.document = document;
       removedChar = null;
   }

   public void Execute()
   {
       removedChar = document.CharAt(document.Length() - 1);
       this.document.RemoveContent(1);
   }

   public void Undo()
   {
       this.document.AddContent(this.removedChar);
   }

   public void Redo()
   {
       this.Execute();
   }
   
}

Lastly, we'll want to create a Document writer that will hold onto a list of our actions in order to undo and redo inputs:

DocumentWriter

import java.util.*;

public class DocumentWriter
{
   Stack<IUndoableCommand> undoStack;
   Stack<IUndoableCommand> redoStack;

   Document document;

   public DocumentWriter(Document document)
   {
       this.document = document;
       this.undoStack = new Stack<IUndoableCommand>();
       this.redoStack = new Stack<IUndoableCommand>();
   }

   public void Write(String content)
   {
       IUndoableCommand command = new AddContentCommand(this.document, content);

       this.undoStack.push(command);
       this.redoStack.clear();

       command.Execute();
   }

   public void Backspace()
   {
       if(this.document.Length() == 0) return;

       IUndoableCommand command  = new BackspaceCommand(this.document);
       this.undoStack.push(command);
       this.redoStack.clear();

       command.Execute();
   }

   public void Undo()
   {
       if(this.undoStack.size() > 0)
       {
           IUndoableCommand command = this.undoStack.pop();
           this.redoStack.push(command);
           command.Undo();
       }
   }

   public void Redo()
   {
       if(this.redoStack.size() > 0)
       {
           IUndoableCommand command = this.redoStack.pop();
           this.undoStack.push(command);
           command.Redo();
       }
   }
}

Demo:

public class Solution
{
   public static void Main(String[] args)
   {
       
       Document doc = new Document();
       DocumentWriter writer = new DocumentWriter(doc);

       // Write to the document:
       writer.Write("Hello world!");

       writer.Backspace();
       doc.PrintDocument(); // Hello world
       writer.Undo();
       doc.PrintDocument(); // Hello world!
       writer.Redo();
       doc.PrintDocument(); // Hello world

       writer.Write(" Goodbye!"); 
       doc.PrintDocument(); // Hello world Goodbye!
       writer.Undo();
       doc.PrintDocument(); // Hello world
       writer.Undo();
       doc.PrintDocument(); // Hello world!

   }
}