[Enhancement] Add Strategy pattern

Author: esakik

src/designpattern/strategy/Book.java

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+package designpattern.strategy;
+
+public class Book {
+    private double amount;
+
+    public Book(double amount) {
+        this.amount = amount;
+    }
+
+    public double getAmount() {
+        return this.amount;
+    }
+
+    public void setAmount(double amount) {
+        this.amount = amount;
+    }
+}

src/designpattern/strategy/DiscountStrategy.java

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+package designpattern.strategy;
+
+public class DiscountStrategy implements Strategy {
+    @Override
+    public void discount(Book book) {
+        double amount = book.getAmount();
+        book.setAmount(amount * 0.9);
+    }
+}

src/designpattern/strategy/Main.java

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+package designpattern.strategy;
+
+
+public class Main {
+    public static void main(String[] args) {
+        Book comic = new Book(500);
+        Book technicalBook = new Book(2500);
+
+        Strategy discountStrategy = new DiscountStrategy();
+        Strategy specialDiscountStrategy = new SpecialDiscountStrategy();
+
+        Shop shop = new Shop(discountStrategy);
+        shop.shell(comic);
+        System.out.println("割引後金額は、" + comic.getAmount() + "円");
+
+        shop.setStrategy(specialDiscountStrategy);
+        shop.shell(technicalBook);
+        System.out.println("割引後金額は、" + technicalBook.getAmount() + "円");
+    }
+}

src/designpattern/strategy/Shop.java

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+package designpattern.strategy;
+
+public class Shop {
+    private Strategy strategy;
+
+    public Shop(Strategy strategy) {
+        this.strategy = strategy;
+    }
+
+    public void setStrategy(Strategy strategy) {
+        this.strategy = strategy;
+    }
+
+    public void shell(Book book) {
+        this.strategy.discount(book);
+    }
+}

src/designpattern/strategy/SpecialDiscountStrategy.java

@@ -0,0 +1,9 @@
+package designpattern.strategy;
+
+public class SpecialDiscountStrategy implements Strategy {
+    @Override
+    public void discount(Book book) {
+        double amount = book.getAmount();
+        book.setAmount(amount * 0.7);
+    }
+}

src/designpattern/strategy/Strategy.java

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+package designpattern.strategy;
+
+public interface Strategy {
+    void discount(Book book);
+}

src/designpattern/strategy/package-info.java

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+/**
+ * Strategy パターン
+ *
+ * - 処理アルゴリズムを簡単に切り替えられるようにする
+ * - Command パターンと同様のことができる
+ */
+package designpattern.strategy;