#95 Updated and clarified Command pattern example
This commit is contained in:
Ilkka Seppala
@ -1,36 +1,47 @@
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package com.iluwatar.command;
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/**
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*
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* In Command pattern actions are objects that can be executed and undone.
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*
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* In this example the commands are the spells cast by the wizard on the goblin.
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*
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*/
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public class App {
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public static void main(String[] args) {
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Wizard wizard = new Wizard();
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Goblin goblin = new Goblin();
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goblin.printStatus();
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wizard.castSpell(new ShrinkSpell(), goblin);
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goblin.printStatus();
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wizard.castSpell(new InvisibilitySpell(), goblin);
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goblin.printStatus();
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wizard.undoLastSpell();
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goblin.printStatus();
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wizard.undoLastSpell();
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goblin.printStatus();
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wizard.redoLastSpell();
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goblin.printStatus();
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wizard.redoLastSpell();
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goblin.printStatus();
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}
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}
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package com.iluwatar.command;
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/**
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*
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* In Command pattern actions are objects that can be executed and undone.
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*
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* Four terms always associated with the command pattern are command, receiver, invoker and client. A command
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* object (spell) knows about receiver (target) and invokes a method of the receiver. Values for parameters of
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* the receiver method are stored in the command. The receiver then does the work. An invoker object (wizard)
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* knows how to execute a command, and optionally does bookkeeping about the command execution. The invoker
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* does not know anything about a concrete command, it knows only about command interface. Both an invoker object
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* and several command objects are held by a client object (app). The client decides which commands to execute at
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* which points. To execute a command, it passes the command object to the invoker object.
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*
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* In other words, in this example the wizard casts spells on the goblin. The wizard keeps track of the previous
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* spells cast, so it is easy to undo them. In addition, the wizard keeps track of the spells undone, so they
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* can be redone.
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*
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*
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*/
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public class App {
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public static void main(String[] args) {
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Wizard wizard = new Wizard();
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Goblin goblin = new Goblin();
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goblin.printStatus();
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wizard.castSpell(new ShrinkSpell(), goblin);
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goblin.printStatus();
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wizard.castSpell(new InvisibilitySpell(), goblin);
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goblin.printStatus();
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wizard.undoLastSpell();
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goblin.printStatus();
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wizard.undoLastSpell();
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goblin.printStatus();
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wizard.redoLastSpell();
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goblin.printStatus();
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wizard.redoLastSpell();
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goblin.printStatus();
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}
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}
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@ -1,15 +1,20 @@
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package com.iluwatar.command;
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public class Goblin extends Target {
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public Goblin() {
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setSize(Size.NORMAL);
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setVisibility(Visibility.VISIBLE);
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}
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@Override
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public String toString() {
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return "Goblin";
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}
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}
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package com.iluwatar.command;
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/**
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*
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* Goblin is the target of the spells
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*
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*/
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public class Goblin extends Target {
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public Goblin() {
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setSize(Size.NORMAL);
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setVisibility(Visibility.VISIBLE);
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}
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@Override
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public String toString() {
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return "Goblin";
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}
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}
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@ -1,31 +1,36 @@
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package com.iluwatar.command;
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public class InvisibilitySpell extends Command {
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private Target target;
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@Override
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public void execute(Target target) {
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target.setVisibility(Visibility.INVISIBLE);
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this.target = target;
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}
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@Override
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public void undo() {
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if (target != null) {
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target.setVisibility(Visibility.VISIBLE);
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}
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}
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@Override
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public void redo() {
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if (target != null) {
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target.setVisibility(Visibility.INVISIBLE);
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}
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}
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@Override
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public String toString() {
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return "Invisibility spell";
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}
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}
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package com.iluwatar.command;
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/**
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*
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* InvisibilitySpell is a concrete command
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*
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*/
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public class InvisibilitySpell extends Command {
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private Target target;
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@Override
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public void execute(Target target) {
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target.setVisibility(Visibility.INVISIBLE);
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this.target = target;
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}
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@Override
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public void undo() {
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if (target != null) {
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target.setVisibility(Visibility.VISIBLE);
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}
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}
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@Override
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public void redo() {
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if (target != null) {
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target.setVisibility(Visibility.INVISIBLE);
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}
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}
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@Override
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public String toString() {
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return "Invisibility spell";
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}
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}
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@ -1,33 +1,38 @@
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package com.iluwatar.command;
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public class ShrinkSpell extends Command {
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private Size oldSize;
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private Target target;
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@Override
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public void execute(Target target) {
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oldSize = target.getSize();
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target.setSize(Size.SMALL);
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this.target = target;
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}
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@Override
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public void undo() {
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if (oldSize != null && target != null) {
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Size temp = target.getSize();
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target.setSize(oldSize);
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oldSize = temp;
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}
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}
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@Override
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public void redo() {
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undo();
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}
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@Override
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public String toString() {
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return "Shrink spell";
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}
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}
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package com.iluwatar.command;
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/**
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*
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* ShrinkSpell is a concrete command
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*
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*/
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public class ShrinkSpell extends Command {
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private Size oldSize;
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private Target target;
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@Override
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public void execute(Target target) {
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oldSize = target.getSize();
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target.setSize(Size.SMALL);
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this.target = target;
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}
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@Override
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public void undo() {
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if (oldSize != null && target != null) {
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Size temp = target.getSize();
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target.setSize(oldSize);
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oldSize = temp;
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}
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}
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@Override
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public void redo() {
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undo();
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}
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@Override
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public String toString() {
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return "Shrink spell";
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}
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}
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@ -1,45 +1,47 @@
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package com.iluwatar.command;
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import java.util.Deque;
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import java.util.LinkedList;
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public class Wizard extends Target {
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private Deque<Command> undoStack = new LinkedList<>();
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private Deque<Command> redoStack = new LinkedList<>();
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public Wizard() {
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setSize(Size.NORMAL);
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setVisibility(Visibility.VISIBLE);
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}
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public void castSpell(Command command, Target target) {
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System.out.println(this + " casts " + command + " at " + target);
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command.execute(target);
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undoStack.offerLast(command);
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}
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public void undoLastSpell() {
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if (!undoStack.isEmpty()) {
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Command previousSpell = undoStack.pollLast();
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redoStack.offerLast(previousSpell);
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System.out.println(this + " undoes " + previousSpell);
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previousSpell.undo();
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}
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}
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public void redoLastSpell() {
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if (!redoStack.isEmpty()) {
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Command previousSpell = redoStack.pollLast();
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undoStack.offerLast(previousSpell);
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System.out.println(this + " redoes " + previousSpell);
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previousSpell.redo();
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}
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}
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@Override
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public String toString() {
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return "Wizard";
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}
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}
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package com.iluwatar.command;
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import java.util.Deque;
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import java.util.LinkedList;
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/**
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*
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* Wizard is the invoker of the commands
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*
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*/
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public class Wizard {
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private Deque<Command> undoStack = new LinkedList<>();
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private Deque<Command> redoStack = new LinkedList<>();
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public Wizard() {
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}
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public void castSpell(Command command, Target target) {
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System.out.println(this + " casts " + command + " at " + target);
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command.execute(target);
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undoStack.offerLast(command);
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}
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public void undoLastSpell() {
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if (!undoStack.isEmpty()) {
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Command previousSpell = undoStack.pollLast();
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redoStack.offerLast(previousSpell);
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System.out.println(this + " undoes " + previousSpell);
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previousSpell.undo();
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}
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}
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public void redoLastSpell() {
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if (!redoStack.isEmpty()) {
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Command previousSpell = redoStack.pollLast();
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undoStack.offerLast(previousSpell);
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System.out.println(this + " redoes " + previousSpell);
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previousSpell.redo();
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}
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}
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@Override
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public String toString() {
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return "Wizard";
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}
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}
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