Update file samplecode.java

Summary by Bito

This pull request adds a parallel execution feature to samplecode.java by integrating a fixed thread pool to manage concurrent tasks, addressing concurrency through task submission, result aggregation, and robust exception handling. It also introduces a comprehensive collection of Java bug examples in app.java for educational purposes, with updates ensuring proper shutdown procedures for the executor to prevent resource leakage and fixing iterator usage bugs.

Detailed Changes

• Fixes incorrect iterator usage in app.java by replacing for-each loop with iterator.remove() to prevent ConcurrentModificationException during list modification
• Adds TimeUnit import in samplecode.java to enable timeout-based executor shutdown and termination handling
• Introduces parallel task execution using ExecutorService with fixed thread pool in samplecode.java
• Implements result collection and aggregation from concurrent Future objects in samplecode.java
• Includes exception handling for task execution failures in samplecode.java