Top 30 Phone Tech Interview Questions on Java

1. Why String is immutable in Java? (Security, String pool implementation, see morehere)
2. Can abstract class have constructor in Java? (Yes, detailed answer is here)
3. Which two methods is overridden by an Object, intended to be used as key in HashMap?
(equals and hashCode, read more)

4. Difference between wait and sleep in Java?(wait release lock, sleep keep it, for details seehere)

5. Difference between List and Set in Java(List is ordered, allows duplicates and indexed, Set is unordered, don't allow duplicates, for more detailed answer, seethis post)

6. How do you make a class Immutable in Java?(Make it final, final fields without setter, state is only set in constructor, no leak of internal reference, copy data for mutable members,read more)

7. Which data type you should used to represent currency in Java? (long or BigDecimal, if you say double, you need to convince them about rounding and how do you avoid floating point issues. for more detailed discussion, see thispost)

8. When to use abstract class and interface in Java? (Use interface for type declaration, use abstract class if evolution is concern, for few more points, seethis post)

9. Difference between Hashtableand HashMap in Java? (former is thread-safe and doesn't allow null, later is not thread-safe, former is also slow because of whole locking of Map, whileHashMap is fast because of no locking,read more)

10. Difference between ArrayListand LinkedList in Java? (former is fast, backed by array, while later is backed by linked-list, queue, former also supports index based access at O(1), while later provides search at cost of O(n) time, for in-depth discussion, seehere)

11. Difference between Overloading and Overriding in Java?( former take place at compile time, later happens at runtime, only virtual method can be overridden, static, final and private method can't be overridden in Java. for more in-depth discussion, seethis post)

12. What kind of reference types are exists in Java? Differences?(Strong reference, Weak references, Soft reference and Phantom reference. Except strong, all other reference allows object to be garbage collected. For example, if an object hash only weak reference, than it's eligible for GC, if program needs space)

13. Difference between checked and unchecked exception in Java? (former is checked by compiler and it's handling is enforced by mandating try-catch or try-finally block. Later is not checked by compiler but can be caught using try-catch or try-finally block. For example, java.io.IOException,java.sql.SQLException are checked exception, whilejava.lang.NullPointerException andjava.lang.ArrayIndexOutOfBoundsException are example of unchecked exception in Java, for better answer seehere)

14. Does Java array is instance of Object? (Yes, and this is stark difference from array in C/C++, though it doesn't have any method, it has an attribute called length, which denotes size of array, seehere to know more about array in Java)

15. Does List<Number> can hold Integers?  (Yes)
16. Can we pass ArrayList<Number> to a method which acceptsList<Number> in Java? (Yes)
17. Can we pass ArrayList<Integer> to a method which acceptsList<Number>? (No) How to fix that? (use wildcards e.g.List<? extends Number> to know more about bounded and unbounded wildcards and other generics questions see thispost)

18. What is volatile variable in Java? (guarantees happens before relationship, variable's value is read by main memory, for detail answer seehere)

19. Difference between CountDownLatchand CyclicBarrier in Java? (former can not be reused once count reaches zero, while later can be reused even after barrier is broken, for in-depth discussion, seethis post)

20. Does BlockingQueue is thread-safe? (Yes, take() andput() method of this class guarantees thread-safety, no need to externally synchronize this class for adding and retrieving objects, here is an example of this class to solveproducer consumer problem in Java)

21. Why wait and notify method should be called in loop? (to prevent doing task, if condition is not true and thread is awake due to false alarms, checking conditions in loop ensures that processing is only done when business logic allows)

22. What is difference between "abc".equals(unknown string) and unknown.equals("abc")? (former is safe from NullPointerException,see here for more best practices and tips to avoid NPE in Java)

23. What is marker or tag interface in Java? (an interface, which presence means an instruction for JVM or compiler e.g.Serializable, from Java 5 onwards Annotation is better suited for this job, to learn more and answer in detail see thisdiscussion)

24. Difference between Serializableand Externalizableinterface in Java? (later provides more control over serialization process, and allow you to define custom binary format for your object, later is also suited for performance sensitive application, seehere for more details)

25. Can Enum types implement interface in Java? (Yes)
26. Can Enum extend class in Java? (No, because Java allows a class to only extend one class and enum by default extendsjava.lang.Enum, see here for moreEnum interview questions)

27. How to prevent your class from being subclassed? (Make it final or make constructor private)
28. Can we override Static method in Java? Compilation error? (No, it can only be hidden, no compilation error)
29. Which design pattern have you used recently? (give any example except Singleton and MVC e.g. Decorator, Strategy or Factory pattern)
30. Difference between StringBufferand StringBuilder in Java? ( former is synchronized, and slow, while later is not synchronized and fast, for more details seethis post)

Read more: http://javarevisited.blogspot.com/2014/02/top-30-java-phone-interview-questions.html#ixzz2xJnP2TiO