Question

[Java] Please test your code in the link I provide before you post your answer.

The output should be looked like exact same as the tester.

http://www.codecheck.it/files/17050415451csldwjahxt2kwn73ahd6vukt

Thank you.

Write a simplified application to illustrate the use of an undo button for a word processor. It keeps a history of all items and allows the user to undo the last.

Write a class UndoStack.

Implement using a Stack of Strings. The constructor will create an empty Stack to keep the history.

A UndoStack has these methods:

public void add(String phrase) adds the phase to the history of the UndoStack

public String undo() removes the last thing added or null if there is nothing in the history to undo. Note that this method violates the rule that a method is not both a mutator and an accessor. Also trying to remove an item form an empty stack causes an exception

public void undoAll() removes all items from the history.

Use the following file:

**********************************************

UndoStackRunner.java

public class UndoStackRunner
{

   public static void main(String[] args)
   {
      UndoStack words = new UndoStack();
      words.add("Mary had a");
      words.add("tiny lamp");
      words.undo();
      words.add("little lamb.");
      words.add("I's ");
      words.undo();
      words.add("It's");
      words.add("fleece");
      words.add("was");
      words.add("white as snow.");
      
      String removed;
      while ((removed = words.undo()) != null)
      {
         System.out.println(removed);
      }

      words.add("End of story");
      words.undoAll();
      
      if ((words.undo()) == null)
      {
         System.out.println("Can not undo. History is empty.");
      }
   }

}

**********************************************

Answer 1

import java.util.*;
public class UndoStack {
private Stack st;
UndoStack(){
st = new Stack(); //Creating empty stack
}
public void add( String a) {
st.push(new String(a)); //adding element to stack

}
public String undo() {
if(!st.empty()) {
String a = (String) st.pop(); // returns the string that has been removed or undo
return a;
}
return null;

}

public void undoAll(){
while(!st.empty())
st.pop(); //deleting all elements in the stack
}
}

In this we have used data structure Stack. Because in undo we follow LIFO approach. In this program also we are doing same and Stack inbuild method pop and push helps us to make it more convinient.

Answer 2

Answer:

Program code screen shot:

UndoStack.Java import the Stack and Scanner classes fron the util package import java.util. public class UndoStack // Declare the Stack predefine object for Stack private Stack<String stack: constructor UndoStack () // initialize the stack stacknew Stack<String> ); // add0 method to push the string a value into /stack public void add (String a) / adding element to stack stack.push (new String (a)) // undo) method to public String undo () I condition to check whether the stack is empty or not fif empty, then pop the value Erom the stack if (stack.empty) / returns the string that has been // removed from the stack String a = 3tack.popl); // return the removed stack value return a: otherwise return null value return null: /I undoAl1 method is used to empty the atack public void undoAll ) loop to pop all the elements from the stack untii the stack is empty while (stack.enpty )) // deleting all elements in the atack stack.pop ) public class UndoStackRunner public static void main (String[] args) UndoStack wordsnew UndoStack) words.add ("Mary had a") words.add ("tinylap) words.undo ) words.add ("little lanb." words.add("I'3" words.undo ) words.add("It 's" words.add("fleece" words.add ("was") words.add ("white as snow." String removed; while ( (removedwords.undo O) !- null) System.out.println(removed) words.add ("End of story": words.undoAli 0: if ((worda .undo ( )) 一null) System.out.println("Can not undo. History is empty.");

Sample Output:

<terminated> UndoStackRunner [Java Application] C:\Program white as snow. was Ileece It's little lamb Mary had a Can not undo. History is empty

Program code to copy:

// UndoStack.java

// import the Stack and Scanner classes from the util package

import java.util.*;

public class UndoStack

{

     // Declare the Stack predefine object for Stack

     private Stack<String> stack;

     // constructor

     UndoStack()

     {

          // initialize the stack

          stack = new Stack<String>();

     }

     // add() method to push the string a value into

     // stack

     public void add(String a)

     {

          // adding element to stack

          stack.push(new String(a));

     }

     // undo() method to

     public String undo()

     {

          // condition to check whether the stack is empty or not

          // if empty, then pop the value from the stack

          if (!stack.empty())

          {

              // returns the string that has been

              // removed from the stack

              String a = stack.pop();

                            

              // return the removed stack value

              return a;

          }

         

          // otherwise return null value

          return null;

     }

     // undoAll() method is used to empty the stack

     public void undoAll()

     {

          // loop to pop all the elements from the stack

          // until the stack is empty

          while (!stack.empty())

          {

              // deleting all elements in the stack

              stack.pop();

          }

     }

}

public class UndoStackRunner

{

   public static void main(String[] args)

   {

      UndoStack words = new UndoStack();

      words.add("Mary had a");

      words.add("tiny lamp");

      words.undo();

      words.add("little lamb.");

      words.add("I's ");

      words.undo();

      words.add("It's");

      words.add("fleece");

      words.add("was");

      words.add("white as snow.");

     

      String removed;

      while ((removed = words.undo()) != null)

      {

         System.out.println(removed);

      }

      words.add("End of story");

      words.undoAll();

     

      if ((words.undo()) == null)

      {

         System.out.println("Can not undo. History is empty.");

      }

   }

}