DataSection76

// Guide to Data Structures
// Section 7.6
// Copyright 2018, by J.T. Streib and T. Soma

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

class UndoButton extends JFrame implements ActionListener {
private JButton blue, red, green, undo;
private StackRefGeneric<String> undoStack
       = new StackRefGeneric<String>();

public static void main(String[] args) {
  UndoButton frame = new UndoButton();
  frame.setVisible(true);
}

public UndoButton() {
  Container contentPane = getContentPane();

  // set the frame size
  setSize(300, 200);

  // set the content pane properties
  contentPane.setLayout(null);
  contentPane.setBackground(Color.white);

  // create and place four buttons on the frame's content pane
  blue = new JButton("Blue");
  blue.setBounds(100, 10, 100, 30);
  getContentPane().add(blue);

  red = new JButton("Red");
  red.setBounds(100, 50, 100, 30);
  getContentPane().add(red);

  green = new JButton("Green");
  green.setBounds(100, 90, 100, 30);
  getContentPane().add(green);

  undo = new JButton("Undo");
  undo.setBounds(100, 130, 100, 30);
  getContentPane().add(undo);

  // associate an action listener to the four buttons
  blue.addActionListener(this);
  red.addActionListener(this);
  green.addActionListener(this);
  undo.addActionListener(this);

  // exit upon closing the frame
  setDefaultCloseOperation(EXIT_ON_CLOSE);
}

public void actionPerformed(ActionEvent event) {
  String buttonText, color;
  JButton clickedButton = (JButton) event.getSource();
  buttonText = clickedButton.getText();

  if(buttonText.equals("Undo"))
     if(undoStack.empty())
        JOptionPane.showMessageDialog(null, "cannot undo");
     else {
        undoStack.pop();
        if(undoStack.empty())
           JOptionPane.showMessageDialog(null, "cannot undo");
        else {
           color = undoStack.pop();
           setBackgroundColor(color);
        }
     }
  else
     setBackgroundColor(buttonText);
}

private void setBackgroundColor(String color) {
  if(color.equals("Blue")) {
     getContentPane().setBackground(Color.blue);
     undoStack.push("Blue");
  }
  else
     if(color.equals("Red")) {
        getContentPane().setBackground(Color.red);
        undoStack.push("Red");
     }
     else
        if(color.equals("Green")) {
           getContentPane().setBackground(Color.green);
           undoStack.push("Green");
        }
}
}

public class StackRefGeneric<T> implements StackGeneric<T> {
  private NodeGeneric<T> top;

  public StackRefGeneric(){
     top = null;
  }

  public boolean empty() {
     // insert code
  }

  public boolean full() {
     return false;
  }

  public T pop() {
     T item = null;
     // insert code
  }

  public void push(T item) {
     NodeGeneric<T> newp = new NodeGeneric<T>(item);
     newp.setNext(top);
     top = newp;
  }

}

public interface StackGeneric<T> {
  public boolean empty();

  public boolean full();

  public T pop();

  public void push(T item);
}

public class NodeGeneric<T> {
  private T data;
  private NodeGeneric<T> next;

  public NodeGeneric() {
     this(null);
  }

  public NodeGeneric(T data) {
     this.data = data;
     next = null;
  }

  public T getData() {
     return data;
  }

  public void setData(T data) {
     this.data = data;
  }

  public NodeGeneric getNext() {
     return next;
  }

  public void setNext(NodeGeneric<T> next) {
     this.next = next;
  }
}