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Core Java Interview Questions and Answers

  Difficulty Level: AllBeginnerIntermediateExpert

Ques 1. What is phantom memory?

Ques 2. Does JVM maintain a cache by itself? Does the JVM allocate objects in heap? Is this the OS heap or the heap maintained by the JVM? Why?

Ques 3. What is reflection API? How are they implemented?

Ques 4. How can I swap two variables without using a third variable?

Ques 5. Explain working of Java Virtual Machine (JVM)?

Ques 6. What is dead lock in thread?

Ques 7. When is static variable loaded? Is it at compile time or runtime? When exactly a static block is loaded in Java?

Ques 8. What is thread pool in java?

Ques 9. What is concurrency in java?

Ques 10. What is reason of NoClassDefFoundError in Java?

Ques 11. How to resolve NoClassDefFoundError?

Ques 12. Difference between ClassNotFoundException and NoClassDefFoundError in Java?

Ques 13. What is java.lang.OutOfMemoryError in Java?

Ques 14. Types of OutOfMemoryError in Java.

Ques 15. Difference between "java.lang.OutOfMemoryError: Java heap space" and "java.lang.OutOfMemoryError: PermGen space"

Ques 16. How HashMap works in Java?

Ques 17. What is the difference between Synchronized Collection classes and Concurrent Collection Classes ? When to use what ?

Ques 18. How to use Comparator and Comparable in Java? With example.

Ques 19. Why String is immutable or final in Java?

Ques 20. What is Dictionary Class in Java?

Ques 21. What is Reference Handler Thread in Java?

Ques 22. What is Main thread in Java?

Ques 23. What is Signal Dispatcher thread in Java?

Ques 24. In Java, you can create a String object as below : String str = "abc"; & String str = new String("abc");  Why cant a button object be created as : Button bt = "abc"? Why is it compulsory to create a button object as: Button bt = new Button("abc"); Why this is not compulsory in String's case?

Ques 25. Can you call one constructor from another if a class has multiple constructors?

Ques 26. What are some alternatives to inheritance?

Ques 27. When can an object reference be cast to an interface reference?

Ques 28. What is the algorithm used in Thread scheduling?

Ques 29. What are the threads will start, when you start the java program?

Ques 30. What are the approaches that you will follow for making a program very efficient?

Ques 31. What is an object's lock and which object's have locks?

Ques 32. There are two classes: A and B. The class B need to inform a class A when some important event has happened. What Java technique would you use to implement it?

Ques 33. What is hash-collision in Hashtable and how it is handled in Java?

Ques 34. Can an inner class declared inside of a method access local variables of this method?

Ques 35. When you think about optimization, what is the best way to findout the time/memory consuming process?

Ques 36. How do I convert a numeric IP address like 192.18.97.39 into a hostname like java.sun.com?

Ques 37. What is the advantage of the event-delegation model over the earlier event-inheritance model?