add redo function on command pattern

command/src/main/java/com/iluwatar/App.java

@@ -20,7 +20,17 @@ public class App {
 
 		wizard.castSpell(new InvisibilitySpell(), goblin);
 		goblin.printStatus();
+
 		wizard.undoLastSpell();
 		goblin.printStatus();
+
+		wizard.undoLastSpell();
+		goblin.printStatus();
+
+		wizard.redoLastSpell();
+		goblin.printStatus();
+
+		wizard.redoLastSpell();
+		goblin.printStatus();
 	}
 }

command/src/main/java/com/iluwatar/Command.java

@@ -11,6 +11,8 @@ public abstract class Command {
 
 	public abstract void undo();
 
+	public abstract void redo();
+
 	@Override
 	public abstract String toString();
 

command/src/main/java/com/iluwatar/InvisibilitySpell.java

@@ -17,6 +17,13 @@ public class InvisibilitySpell extends Command {
 		}
 	}
 
+	@Override
+	public void redo() {
+		if (target != null) {
+			target.setVisibility(Visibility.INVISIBLE);
+		}
+	}
+
 	@Override
 	public String toString() {
 		return "Invisibility spell";

command/src/main/java/com/iluwatar/ShrinkSpell.java

@@ -15,10 +15,17 @@ public class ShrinkSpell extends Command {
 	@Override
 	public void undo() {
 		if (oldSize != null && target != null) {
+			Size temp = target.getSize();
 			target.setSize(oldSize);
+			oldSize = temp;
 		}
 	}
 
+	@Override
+	public void redo() {
+		undo();
+	}
+
 	@Override
 	public String toString() {
 		return "Shrink spell";

command/src/main/java/com/iluwatar/Target.java

@@ -31,7 +31,8 @@ public abstract class Target {
 	public abstract String toString();
 
 	public void printStatus() {
-		System.out.println(String.format("%s, size=%s visibility=%s", this,
+		System.out.println(String.format("%s, [size=%s] [visibility=%s]", this,
 				getSize(), getVisibility()));
+		System.out.println();
 	}
 }

command/src/main/java/com/iluwatar/Wizard.java

@@ -1,8 +1,12 @@
 package com.iluwatar;
 
+import java.util.Deque;
+import java.util.LinkedList;
+
 public class Wizard extends Target {
 
-	private Command previousSpell;
+	private Deque<Command> undoStack = new LinkedList<>();
+	private Deque<Command> redoStack = new LinkedList<>();
 
 	public Wizard() {
 		setSize(Size.NORMAL);
@@ -12,16 +16,27 @@ public class Wizard extends Target {
 	public void castSpell(Command command, Target target) {
 		System.out.println(this + " casts " + command + " at " + target);
 		command.execute(target);
-		previousSpell = command;
+		undoStack.offerLast(command);
 	}
 
 	public void undoLastSpell() {
-		if (previousSpell != null) {
+		if (!undoStack.isEmpty()) {
+			Command previousSpell = undoStack.pollLast();
+			redoStack.offerLast(previousSpell);
 			System.out.println(this + " undoes " + previousSpell);
 			previousSpell.undo();
 		}
 	}
 
+	public void redoLastSpell() {
+		if (!redoStack.isEmpty()) {
+			Command previousSpell = redoStack.pollLast();
+			undoStack.offerLast(previousSpell);
+			System.out.println(this + " redoes " + previousSpell);
+			previousSpell.redo();
+		}
+	}
+
 	@Override
 	public String toString() {
 		return "Wizard";