Design Patterns in Java — The Practical GoF

8. Practice Problems

Try each before revealing.

1. Implement a Singleton Logger using the enum idiom and log three lines.
   Enum idiom দিয়ে একটি Singleton Logger বানান এবং তিনটি log line লিখুন।
   ✨ Show Answer

   Main.java

   enum Logger {
       INSTANCE;
       void log(String msg) { System.out.println("[LOG] " + msg); }
   }
   
   class Main {
       public static void main(String[] args) {
           Logger.INSTANCE.log("app started");
           Logger.INSTANCE.log("user login");
           Logger.INSTANCE.log("payment received");
       }
   }

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2. Write a ShapeFactory that returns a Circle, Square, or Triangle based on a string key, each printing its area formula.
   একটি ShapeFactory লিখুন যা string অনুযায়ী Circle/Square/Triangle ফেরত দেবে এবং প্রত্যেকে তার area formula প্রিন্ট করবে।
   ✨ Show Answer

   Main.java

   interface Shape { void area(); }
   class Circle   implements Shape { public void area(){System.out.println("π r²");} }
   class Square   implements Shape { public void area(){System.out.println("s²");} }
   class Triangle implements Shape { public void area(){System.out.println("½ b h");} }
   
   class ShapeFactory {
       static Shape of(String k) {
           return switch(k) {
               case "circle"   -> new Circle();
               case "square"   -> new Square();
               case "triangle" -> new Triangle();
               default           -> throw new IllegalArgumentException(k);
           };
       }
   }
   
   class Main {
       public static void main(String[] args) {
           ShapeFactory.of("circle").area();
           ShapeFactory.of("square").area();
           ShapeFactory.of("triangle").area();
       }
   }

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3. Write a Builder for a User with required name, optional email and phone.
   একটি User-এর Builder বানান — required name, optional email ও phone।
   ✨ Show Answer

   Main.java

   class User {
       private final String name, email, phone;
       private User(Builder b){name=b.n;email=b.e;phone=b.p;}
       public String toString(){return name+"/"+email+"/"+phone;}
       static class Builder {
           String n, e="—", p="—";
           Builder(String name){n=name;}
           Builder email(String x){e=x;return this;}
           Builder phone(String x){p=x;return this;}
           User build(){return new User(this);}
       }
   }
   
   class Main {
       public static void main(String[] args) {
           User u = new User.Builder("Fatima").email("f@abcltech.com").build();
           System.out.println(u);
       }
   }

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4. Is "every Singleton" always a good idea? List two problems.
   সব সময় Singleton কি ভালো? দুটি সমস্যা লিখুন।
   ✨ Show Answer

   Answer: (1) Singletons are effectively global state — they make testing hard because tests cannot swap them for a fake without static hacks. (2) They hide dependencies: a class that reaches for Config.INSTANCE inside itself looks independent but is actually tightly coupled. Modern dependency-injection (Spring) gives you one-instance semantics without the static baggage.

5. Rewrite the Strategy example so that instead of BiFunction, two algorithms are enum members with an abstract apply(int, int).
   Strategy উদাহরণটি এমনভাবে লিখুন যাতে দুটি algorithm enum member হয় — apply(int, int) abstract method সহ।
   ✨ Show Answer

   Main.java

   enum Discount {
       PERCENT { public int apply(int pr, int d) { return pr - (pr * d / 100); } },
       FLAT    { public int apply(int pr, int d) { return pr - d; } };
       public abstract int apply(int pr, int d);
   }
   
   class Main {
       public static void main(String[] args) {
           System.out.println(Discount.PERCENT.apply(1000, 10));
           System.out.println(Discount.FLAT.apply(1000, 150));
       }
   }

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