{"id":205,"date":"2006-09-14T11:20:23","date_gmt":"2006-09-14T11:20:23","guid":{"rendered":"http:\/\/kubasek.com\/blog\/pragmatic_craftsman\/?p=205"},"modified":"2006-09-14T11:20:23","modified_gmt":"2006-09-14T11:20:23","slug":"the-decorator-pattern","status":"publish","type":"post","link":"https:\/\/pragmaticcraftsman.kubasek.com\/2006\/09\/14\/the-decorator-pattern\/","title":{"rendered":"The Decorator Pattern"},"content":{"rendered":"

I’ve been inthe dark as far as the Decorator pattern is concerned. I knew, inprinciple, how it works, but I can see that my understanding was veryincomplete. Plus, I never had a chance to useit. While reading the Head First Design Patterns<\/em>book,<\/span> Idiscovered something basic that: when you wrap an object several timesand then call a method on it, it will be called as many times as it waswrapped. The trick? Keepa reference to the object — you’re creating a chain. <\/span><\/p>\n

<font  <\/font><\/font><\/p>\n

Let me gothrough this step by step. By example. This is <\/span>thebook’s example, the Starbuzz Coffee decorator.<\/span><\/p>\n

<font  <\/font><\/font><\/p>\n

A simpleinterface.<\/span><\/p>\n

 <\/font><\/p>\n

<fontpublic<\/span><\/b> interface<\/span><\/b>Beverage {<\/span><\/font><\/p>\n

<fontpublic<\/span><\/b> String getDescription();<\/span><\/font><\/p>\n

<fontpublic<\/span><\/b> BigDecimal getCost();<\/span><\/font><\/p>\n

<font}<\/span><\/font><\/p>\n

We have severalcoffee types, Dark Roast being one of them.<\/span><\/p>\n

<fontpublic<\/span><\/b> class<\/span><\/b>DarkRoast implements<\/span><\/b>Beverage {<\/span><\/font><\/p>\n

<font  <\/span>public<\/span><\/b> BigDecimal getCost() {<\/span><\/font><\/p>\n

<font     <\/span>return<\/span><\/b> new<\/span><\/b>BigDecimal(“1.55”<\/span>);<\/span><\/font><\/p>\n

<font    }<\/font><\/span><\/p>\n

<font  <\/font><\/span>public<\/span><\/b> String getDescription() {<\/span><\/font><\/p>\n

<font    <\/span>return<\/span><\/b>“Dark Roast”<\/span>;<\/span><\/font><\/p>\n

<font  <\/span>}<\/span><\/font><\/p>\n

<font}<\/span><\/font><\/p>\n

When orderingcoffee, you can pick a coffee type (Dark Roast, Latte, etc), and youcan also add on to the coffee. For instance, you can add a whip creamon top, or add a shot of expresso to it. Which, of course, adds to theprice. You could extend Beverage with all of the types, but that’s toomany classes. <\/span>Here’swhere the decorator pattern comes into play. <\/span><\/p>\n

Here’sthe solution. You define the AddOnDecorator and extend it with thedifferent add ons.<\/span><\/p>\n

<font\/** It’sjust an empty class *\/<\/span><\/font><\/b><\/p>\n

<fontpublic<\/span><\/b> abstract<\/span><\/b>class<\/span><\/b>AddOnDecorator implements<\/span><\/b>Beverage { }<\/span><\/font><\/p>\n

<font <\/font><\/font><\/span><\/p>\n

<fontpublic<\/span><\/strong> class<\/span><\/b>Expresso extends<\/span><\/b>AddOnDecorator {<\/span><\/font><\/p>\n

<fontBeveragebeverage<\/span>;<\/span><\/font><\/p>\n

<fontpublic<\/span><\/b> Expresso(Beverage beverage) {<\/span><\/font><\/p>\n

<font  <\/span>this<\/span><\/b>.beverage<\/span> =beverage;<\/span><\/font><\/p>\n

<font}<\/span><\/font><\/p>\n

<fontpublic<\/span><\/b> BigDecimal getCost() {<\/span><\/font><\/p>\n

<font  <\/span>return<\/span><\/b> new<\/span><\/b>BigDecimal(“0.25”<\/span>).add(beverage<\/span>.getCost());<\/span><\/font><\/p>\n

<font}<\/span><\/font><\/p>\n

<fontpublic<\/span><\/b> String getDescription() {<\/span><\/font><\/p>\n

<font  <\/span>return<\/span><\/b> beverage<\/span>.getDescription()+ “, with Expresso”<\/span>;<\/span><\/font><\/p>\n

<font }<\/font><\/span><\/p>\n

}<\/span><\/font><\/p>\n

<font <\/font><\/font><\/p>\n

<fontpublic<\/span><\/b> class<\/span><\/b>Whip extends<\/span><\/b>AddOnDecorator {<\/span><\/font><\/p>\n

<fontBeveragebeverage<\/span>;<\/span><\/font><\/p>\n

<fontpublic<\/span><\/b> Whip(Beverage beverage) {<\/span><\/font><\/p>\n

<fontthis<\/span><\/b>.beverage<\/span> =beverage;<\/span><\/font><\/p>\n

<font }<\/font><\/span><\/p>\n

 <\/font><\/p>\n

<fontpublic<\/span><\/b> BigDecimal getCost() {<\/span><\/font><\/p>\n

<fontreturn<\/span><\/b> new<\/span><\/b>BigDecimal(“0.10”<\/span>).add(beverage<\/span>.getCost());<\/span><\/font><\/p>\n

<font }<\/font><\/span><\/p>\n

 <\/font><\/p>\n

<fontpublic<\/span><\/b> String getDescription() {<\/span><\/font><\/p>\n

<fontreturn<\/span><\/b> beverage<\/span>.getDescription()+ “, Whipped”<\/span>;<\/span><\/font><\/p>\n

<font}<\/span><\/font><\/p>\n

<font}<\/span><\/font><\/p>\n

<font <\/font><\/font><\/p>\n

<fontpublic<\/span><\/b> class<\/span><\/b>StarbuzzCoffee {<\/span><\/font><\/p>\n

<fontpublic<\/span><\/b> static<\/span><\/b>void<\/span><\/b>main(String[] args) {<\/span><\/font><\/p>\n

<fontBeveragedarkRoast = new<\/span><\/b>DarkRoast();<\/span><\/font><\/p>\n

<fontdarkRoast= new<\/span><\/b>Expresso(darkRoast);<\/span><\/font><\/p>\n

<fontdarkRoast= new<\/span><\/b>Whip(darkRoast);<\/span><\/font><\/p>\n

<fontSystem.out<\/span><\/i>.println(darkRoast.getDescription()+ ” costs “<\/span><\/span><\/font><\/p>\n

<font+darkRoast.getCost());<\/span><\/font><\/p>\n

<font}<\/span><\/font><\/p>\n

<font}<\/span><\/font><\/p>\n

WhenI first looked at the code, I was confused. I thought that newWhip(…) would just override it,right?<\/span> <\/span><\/p>\n

Thisis what gets printed: Dark Roast, with Expresso, Whippedcosts 1.90<\/em><\/span><\/p>\n

You get the beverage you want(DarkRoast), you pass it to the Expresso wrapper, which in turn passesit to the Whip wrapper. <\/span><\/span><\/p>\n

Firstthe E<\/span>xpressowrapper is called, itreceives the dark roast beverage. Then the dark roast is passed againto the Whip wrapper. <\/span>If you look closely (and this isa little confusing), <\/span>when an Expresso is <\/span>instantiated,<\/span> it receives the beverage andmakes a local copy (so it is never lost). <\/span>Essentially, a chain is made.When the action on the final object is made, the chain is traversed andthe beverage object is passed around. <\/span>That’s how<\/span> this patternworks. (Wow, I learn something (basic) every day. :-))<\/span><\/p>\n

The Decoratorpattern is cool. It rocks. \ud83d\ude42It lets you keep adding functionality and still keep the objectscohesive (as you’re not bloating the objects). No coupling as well.Nice.<\/span><\/span><\/p>\n

<\/font><\/p>\n

Reference<\/strong>
Head First Design Patterns<\/a>, the example above was taken from the book<\/p>\n","protected":false},"excerpt":{"rendered":"

I’ve been inthe dark as far as the Decorator pattern is concerned. I knew, inprinciple, how it works, but I can see that my understanding was veryincomplete. Plus, I never had […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[2],"tags":[],"class_list":["post-205","post","type-post","status-publish","format-standard","hentry","category-architecture-design"],"_links":{"self":[{"href":"https:\/\/pragmaticcraftsman.kubasek.com\/wp-json\/wp\/v2\/posts\/205","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pragmaticcraftsman.kubasek.com\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/pragmaticcraftsman.kubasek.com\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/pragmaticcraftsman.kubasek.com\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/pragmaticcraftsman.kubasek.com\/wp-json\/wp\/v2\/comments?post=205"}],"version-history":[{"count":0,"href":"https:\/\/pragmaticcraftsman.kubasek.com\/wp-json\/wp\/v2\/posts\/205\/revisions"}],"wp:attachment":[{"href":"https:\/\/pragmaticcraftsman.kubasek.com\/wp-json\/wp\/v2\/media?parent=205"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/pragmaticcraftsman.kubasek.com\/wp-json\/wp\/v2\/categories?post=205"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/pragmaticcraftsman.kubasek.com\/wp-json\/wp\/v2\/tags?post=205"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}