Exception Handling & try-with-resources

7. Practice Problems

1. Catch a NumberFormatException when parsing "abc" and print a friendly message.
   "abc" parse করতে গিয়ে NumberFormatException ধরে একটি বন্ধুত্বপূর্ণ message দিন।
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   Main.java

   class Main {
       public static void main(String[] args) {
           try {
               int n = Integer.parseInt("abc");
               System.out.println(n);
           } catch (NumberFormatException ex) {
               System.out.println("Please enter a valid number.");
           }
       }
   }

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2. Write a custom unchecked exception AgeInvalidException and throw it when age is negative.
   নিজে AgeInvalidException তৈরি করুন; age নেগেটিভ হলে throw করুন।
   ✨ Show Answer

   Main.java

   class Main {
       static class AgeInvalidException extends RuntimeException {
           AgeInvalidException(String msg) { super(msg); }
       }
       static void setAge(int a) {
           if (a < 0) throw new AgeInvalidException("age cannot be negative");
           System.out.println("age = " + a);
       }
       public static void main(String[] args) {
           setAge(22);
           try { setAge(-3); }
           catch (AgeInvalidException ex) { System.out.println("caught: " + ex.getMessage()); }
       }
   }

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3. Demonstrate try-with-resources with two AutoCloseable resources — show they close in reverse order.
   দুটি AutoCloseable resource দিয়ে try-with-resources দেখান; reverse order-এ close হয় এটা প্রমাণ করুন।
   ✨ Show Answer

   Main.java

   class Main {
       static class R implements AutoCloseable {
           String n;
           R(String n) { this.n = n; System.out.println("open " + n); }
           @Override public void close() { System.out.println("close " + n); }
       }
       public static void main(String[] args) {
           try (R a = new R("A"); R b = new R("B")) {
               System.out.println("inside block");
           }
       }
   }

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4. Explain when to use finally versus try-with-resources.
   finally কবে, try-with-resources কবে — পার্থক্য ব্যাখ্যা করুন।
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   Answer: Use try-with-resources whenever the resource implements AutoCloseable — files, sockets, DB connections, streams. It is shorter, exception-safe, and automatically suppresses exceptions from close() without losing the primary exception. Keep finally only for cleanup that is not a closable resource — e.g., restoring a ThreadLocal, logging a metric, or releasing a manual lock that doesn't implement AutoCloseable.

   Resource যদি AutoCloseable হয় (file, socket, DB connection, stream) — সব সময় try-with-resources। এটি ছোট, exception-safe, close()-এর exception-কে suppress করে মূল exception সংরক্ষণ করে। finally রাখুন শুধু সেই cleanup-এর জন্য যা resource নয় — যেমন ThreadLocal reset, metric log, বা manual lock release যেটি AutoCloseable নয়।

5. Wrap a lower-level IOException into a domain ReportFailedException preserving the cause.
   নিচু স্তরের IOException-কে domain-level ReportFailedException-এ wrap করুন; cause হারাবেন না।
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   Main.java

   import java.io.*;
   
   class Main {
       static class ReportFailedException extends RuntimeException {
           ReportFailedException(String m, Throwable cause) { super(m, cause); }
       }
   
       static void buildReport() {
           try {
               throw new IOException("disk full");
           } catch (IOException ex) {
               throw new ReportFailedException("could not write daily report", ex);
           }
       }
   
       public static void main(String[] args) {
           try {
               buildReport();
           } catch (ReportFailedException ex) {
               System.out.println("top level: " + ex.getMessage());
               System.out.println("root cause: " + ex.getCause());
           }
       }
   }

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