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#95 Updated and clarified Command pattern example

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This commit is contained in:
Ilkka Seppala
2015-06-30 17:52:53 +03:00
parent fb446c2991
commit db6ec3cc1a
9 changed files with 224 additions and 237 deletions
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83
command/src/main/java/com/iluwatar/command/App.java
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@@ -1,36 +1,47 @@
package com.iluwatar.command;
/**
*
* In Command pattern actions are objects that can be executed and undone.
*
* In this example the commands are the spells cast by the wizard on the goblin.
*
*/
public class App {
public static void main(String[] args) {
Wizard wizard = new Wizard();
Goblin goblin = new Goblin();
goblin.printStatus();
wizard.castSpell(new ShrinkSpell(), goblin);
goblin.printStatus();
wizard.castSpell(new InvisibilitySpell(), goblin);
goblin.printStatus();
wizard.undoLastSpell();
goblin.printStatus();
wizard.undoLastSpell();
goblin.printStatus();
wizard.redoLastSpell();
goblin.printStatus();
wizard.redoLastSpell();
goblin.printStatus();
}
}
package com.iluwatar.command;
/**
*
* In Command pattern actions are objects that can be executed and undone.
*
* Four terms always associated with the command pattern are command, receiver, invoker and client. A command
* object (spell) knows about receiver (target) and invokes a method of the receiver. Values for parameters of
* the receiver method are stored in the command. The receiver then does the work. An invoker object (wizard)
* knows how to execute a command, and optionally does bookkeeping about the command execution. The invoker
* does not know anything about a concrete command, it knows only about command interface. Both an invoker object
* and several command objects are held by a client object (app). The client decides which commands to execute at
* which points. To execute a command, it passes the command object to the invoker object.
*
* In other words, in this example the wizard casts spells on the goblin. The wizard keeps track of the previous
* spells cast, so it is easy to undo them. In addition, the wizard keeps track of the spells undone, so they
* can be redone.
*
*
*/
public class App {
public static void main(String[] args) {
Wizard wizard = new Wizard();
Goblin goblin = new Goblin();
goblin.printStatus();
wizard.castSpell(new ShrinkSpell(), goblin);
goblin.printStatus();
wizard.castSpell(new InvisibilitySpell(), goblin);
goblin.printStatus();
wizard.undoLastSpell();
goblin.printStatus();
wizard.undoLastSpell();
goblin.printStatus();
wizard.redoLastSpell();
goblin.printStatus();
wizard.redoLastSpell();
goblin.printStatus();
}
}

35
command/src/main/java/com/iluwatar/command/Goblin.java
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@@ -1,15 +1,20 @@
package com.iluwatar.command;
public class Goblin extends Target {
public Goblin() {
setSize(Size.NORMAL);
setVisibility(Visibility.VISIBLE);
}
@Override
public String toString() {
return "Goblin";
}
}
package com.iluwatar.command;
/**
*
* Goblin is the target of the spells
*
*/
public class Goblin extends Target {
public Goblin() {
setSize(Size.NORMAL);
setVisibility(Visibility.VISIBLE);
}
@Override
public String toString() {
return "Goblin";
}
}

67
command/src/main/java/com/iluwatar/command/InvisibilitySpell.java
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@@ -1,31 +1,36 @@
package com.iluwatar.command;
public class InvisibilitySpell extends Command {
private Target target;
@Override
public void execute(Target target) {
target.setVisibility(Visibility.INVISIBLE);
this.target = target;
}
@Override
public void undo() {
if (target != null) {
target.setVisibility(Visibility.VISIBLE);
}
}
@Override
public void redo() {
if (target != null) {
target.setVisibility(Visibility.INVISIBLE);
}
}
@Override
public String toString() {
return "Invisibility spell";
}
}
package com.iluwatar.command;
/**
*
* InvisibilitySpell is a concrete command
*
*/
public class InvisibilitySpell extends Command {
private Target target;
@Override
public void execute(Target target) {
target.setVisibility(Visibility.INVISIBLE);
this.target = target;
}
@Override
public void undo() {
if (target != null) {
target.setVisibility(Visibility.VISIBLE);
}
}
@Override
public void redo() {
if (target != null) {
target.setVisibility(Visibility.INVISIBLE);
}
}
@Override
public String toString() {
return "Invisibility spell";
}
}

71
command/src/main/java/com/iluwatar/command/ShrinkSpell.java
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@@ -1,33 +1,38 @@
package com.iluwatar.command;
public class ShrinkSpell extends Command {
private Size oldSize;
private Target target;
@Override
public void execute(Target target) {
oldSize = target.getSize();
target.setSize(Size.SMALL);
this.target = target;
}
@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";
}
}
package com.iluwatar.command;
/**
*
* ShrinkSpell is a concrete command
*
*/
public class ShrinkSpell extends Command {
private Size oldSize;
private Target target;
@Override
public void execute(Target target) {
oldSize = target.getSize();
target.setSize(Size.SMALL);
this.target = target;
}
@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";
}
}

92
command/src/main/java/com/iluwatar/command/Wizard.java
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@@ -1,45 +1,47 @@
package com.iluwatar.command;
import java.util.Deque;
import java.util.LinkedList;
public class Wizard extends Target {
private Deque<Command> undoStack = new LinkedList<>();
private Deque<Command> redoStack = new LinkedList<>();
public Wizard() {
setSize(Size.NORMAL);
setVisibility(Visibility.VISIBLE);
}
public void castSpell(Command command, Target target) {
System.out.println(this + " casts " + command + " at " + target);
command.execute(target);
undoStack.offerLast(command);
}
public void undoLastSpell() {
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";
}
}
package com.iluwatar.command;
import java.util.Deque;
import java.util.LinkedList;
/**
*
* Wizard is the invoker of the commands
*
*/
public class Wizard {
private Deque<Command> undoStack = new LinkedList<>();
private Deque<Command> redoStack = new LinkedList<>();
public Wizard() {
}
public void castSpell(Command command, Target target) {
System.out.println(this + " casts " + command + " at " + target);
command.execute(target);
undoStack.offerLast(command);
}
public void undoLastSpell() {
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";
}
}