Tracing Objects - Intro to Java Programming Udacity English Japanese

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Now that you've seen a lot of code for implementing methods. Let me show you a

メソッドを実行するためのコードを多数見てきました

technique that can help you understand what a method implementation does. It's

そのメソッドの実行とは何かを理解する方法をお教えします

called the object tracing technique. So, let's say that you have a, a couple of methods that do some work that may not be so easy to understand. Here's what

オブジェクトトレーシングという技術です何をどうするのか理解が難しいメソッドがいくつかあったとします

you do. Pick some scenario, that exercises these methods. Like here, I'm making

そのような場合はこれらのメソッドを働かせるためのシナリオを作ってみます

a car, I add gas, I drive it, and I drive it a bit more. And now we want to

例えば車を作ってガソリンを補充し走行するというものです

understand, what do these methods calls do to the car. It's very low tech. Get yourself an index card or a napkin or some other piece of paper. Make a column

これらのメソッドの呼び出しが車に何をするか知りたいとします原始的ですがメモ用紙など何か紙を用意してください

for each instance variable. So, let's get started. I put the paper clip so we

各インスタンス変数のマスを作りますでは始めましょう　このクリップでどこを見ているか目印にします

can see where we are. We construct a car, that initialized the instance

まず車を作って各インスタンス変数を設定します

variables. I didn't have room to put in the code for the constructor but

コンストラクタのコードを挿入するスペースはありませんでしたね？

hopefully you remember. The gas and tank and miles driven are initialized to

ガソリンの残量と走行距離は初期値をゼロに

zero. The miles per gallon with the parameter of the constructor. Let's move

1ガロンあたりの走行量はコンストラクタのパラメータにします

on. Now we enter the add gas method here. Amount is thirty. Gas in tank is

次にaddGasメソッドを記入します量は30としタンク内のガソリン量はゼロです

zero. So, the effective set gas in tank is set to thirty. Moving on. Now we

そのためこの数式によりタンク内のガソリン量は30になります

drive ten miles. So, distance is ten. And since this method is a bit longer and it has another local variable here, it makes sense to make another card for it.

次にここで10マイル走行しました距離は10ですねこのメソッドは長くて別のローカル変数もあるのでもう1枚紙が必要です

Or actually, since I ran out of simulated cards, I'll use a napkin. So the

紙がなければ紙ナプキンでも使ってください

distance parameter is 10, over here. We update miles driven, previously it was 0. We add 10. That's now 10. Now here we compute the gas consumed. That's another local variable. I'll put it on my napkin. It's the distance divided by miles per gallon. 10 divided by 10. So, we consumed a gallon. Now we can look

距離のパラメータである10をここに入れると走行距離はゼロから10マイルに更新されますではガソリンの消費量を算出しますここでまた別のローカル変数です走行距離を1ガロン当たりの走行距離で割ります 10÷10で1ガロン消費したことになります

at this statement here. Gas in tank was 30. Gas consumed was 1. The difference

この命令文を見るとタンク内のガソリンは30です消費量は1ガロンで差は29です

is 29. So, that should now by moved into gas in tank. Now, we're at the end of this method call, and the local variable here and the parameter variable go

タンク内のガソリン量が29となりますメソッドの呼び出しも終了ですこのローカル変数とパラメータ変数もなくなります

away. So, we can just crumple up our napkin here. These variables are no longer there once we exited this method. Of course, the instance variables, the long-term memory of the object, they stay around. Now here we are on the next

では紙もなくしてしまいましょうこれらの変数はメソッドが終了するとなくなりますインスタンス変数は長期記憶のオブジェクトなのでずっと残ります

method call, we're calling the drive method again. That means that this local variable, and this one here, will get recreated. Of course this time the

では次のメソッドの呼び出しです Driveメソッドを再度呼び出しますこことここのローカル変数が新しくなります

distance is 20. Now let's see, miles driven is, miles driven plus distance.

今回の距離は20ですね milesDriven＝milesDriven＋distanceです

Previously it was 10. Now we have 20, now it becomes 30. Gas consumed is

先ほどは距離が10でしたが今は20なので計30になりました

distance over miles driven, that's 20 by 10 or two. Gas in tank was 29 minus two is 27. So, now we've executed the three steps in this method. This method

ガソリン消費量は距離×走行距離の20÷10＝2 タンク内のガソリンは29なので29－2＝27 ここまでメソッドの3つのステップを実行しましたこのメソッドは終了です

is now finished. And once again, the local variables go away. And we're done,

そして再度ローカル変数はなくなります

and this is the remaining state of the object. What's the point? By doing this

そしてこれが残ったオブジェクトの命令文です

exercise, you gain an understanding of what the statements in the methods do to the instance variables of an object. Sarah will walk you through an example on

この作業によりメソッドの中の命令文がオブジェクトのインスタンス変数に行うことがわかりますこのあとサラが例を見せてくれます

how you can use this technique to find a bug in a method implementation.

バグを見つけるためにこの方法をどう使うか一緒に練習しましょう

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