June 2022
Beginner to intermediate
672 pages
17h 23m
Japanese
Content preview from Head Firstデザインパターン 第2版 ―頭とからだで覚えるデザインパターンの基本
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406
10
章
State
パターンの定義
State
パターンの定義
Stateパターンでは、オブジェクトの内部状態が変化した際にオ
ブジェクトが振る舞いを変更できます。オブジェクトはそのク
ラスを変更したように見えます。
request()
Context
そうです、
State
パターンを実装したのです！
ここではこのパターンは一体何であるかを確認しましょう。
この説明の前半はよく理解できると思います。このパターンは状態を別々のクラスにカプセル化し、現
在の状態を表すオブジェクトに委譲するため、振る舞いは内部状態によって変わります。ガムボールマシ
ンを例に取ると、ガムボールマシンが
NoQuarterState
のときに
25
セントを投入すると、
HasQuarterState
のときに
25
セント投入したとき（マシンは
25
セントを拒否する）とは違う振る舞い（マシンは
25
セン
トを受け付ける）になります。
この定義の後半についてはどうですか？
オブジェクトが「そのクラスを変更したように見える」とは
どういう意味でしょうか？
クライアントの視点から考えてみてください。使っているオブジェクトが完
全にその振る舞いを変更できたら、クライアントにとってはそのオブジェクトが実際に別のクラスから
インスタンス化されたように見えます。しかし実際には、コンポジションを使って単に別の状態オブジェ
クトを参照してクラスの見かけを変えているのです。
それでは、
State
パターンのクラス図を調べてみましょう。
state.
handle()
handle() ...
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.