📚 Main Topics

1. Try-Catch-Finally Blocks

   • Creating try blocks without catch or finally.
   • Behavior of return statements in try and finally blocks.
   • Nested try-catch blocks.
   • When finally blocks may not execute.

2. Java Keywords

   • Differences between final, finally, and finalize.

3. Collections and Concurrency

   • Handling exceptions in collections.
   • Differences between HashMap, ConcurrentHashMap, and SynchronizedHashMap.
   • Null keys and values in HashMap and TreeMap.

4. Singleton Design Pattern

   • Different ways to implement Singleton classes.
   • Thread safety in Singleton implementations.

5. Java Inheritance and Interfaces

   • Multiple inheritance in Java.
   • Differences between abstract classes and interfaces.
   • Functional interfaces and their properties.

6. Threading in Java

   • Returning values from threads using Callable.
   • Handling multiple exceptions in a single catch block.

7. Immutable Classes

   • How to create immutable classes and their importance.

8. Sealed Classes

   • Introduction to sealed classes and their benefits in inheritance control.

✨ Key Takeaways

• Try-Catch-FinallyYou can create a try block without a catch block if a finally block is present. In Java 8 and later, try-with-resources can be used without catch or finally.
• Return BehaviorIn a try-finally structure, the return statement in the finally block takes precedence over the try block.
• Null HandlingHashMap allows one null key and multiple null values, while TreeMap does not allow null keys unless handled in custom classes.
• Singleton PatternsVarious methods exist to implement Singleton, including eager initialization, lazy initialization, and using enums for thread safety.
• Functional InterfacesCan extend other interfaces but must adhere to the rule of having only one abstract method.
• ThreadingUse Callable for returning values from threads, as Runnable does not support return values.
• Immutable ClassesMust be declared final, with private final fields and no setters to ensure immutability.
• Sealed ClassesIntroduced in Java 17, they restrict which classes can extend them, providing better control over inheritance.

🧠 Lessons

• Understanding the nuances of Java's exception handling and concurrency can significantly impact your performance in interviews.
• Familiarity with the latest Java features (like sealed classes and records) is crucial for modern Java development.
• Knowing the differences between collections and their thread safety is essential for writing robust applications.
• Mastering the Singleton pattern and immutability principles can help in designing efficient and safe Java applications.