Core Java Most Commonly Asked Interview Questions.

  • What are the differences between JDK, JRE, JVM, JIT?
  • Which JDK version you used in your project?
  • What are the Java 5, 7, 8 features?
  • How to migrate the java 7 project to java 8 version?
  • How to write Documentation Comment?
  • What is Class & Object?
  • What is OOPS and what are their pillars?What is Abstraction, PolymorphismInheritance and Encapsulation?
  • How did you implemented OOPS concept in your project?
  • What is static in java?
  • What is final in java?
  • Why can't a local variable have default value?
  • What is Constructor and what are the differences between Constructor and Method?
  • What is Constructor Chaining in Java?
  • What are the things happening internally at memory level when we create an object in java? Explain Object creation steps.
  • What is Static Polymorphism or Compile Time Polymorphism or Overloading or Early binding?
  • What is Dynamic Polymorphism or Run Time Polymorphism or Overriding or Late Binding?
  • What are the differences between Overloading and Overriding?
  • Can we have different return type in overloading/overriding?
  • Can we override static method ? Justify for YES or NO.
  • How to restrict the methods from overriding?
  • What are the difference between Interface and Abstract Class?
  • Why Abstract Classes are there in Java?
  • What is static import?
  • What is path, classpathWhat are the differences between them? How to set the path and classpath?
  • What is jar-war-ear? Differences among them? How to create them? How to execute them?
  • What is Diamond problem in Multiple Interitance?
  • What is Has-A and Is-A relationship in java?
  • Difference between Association vs Inheritance and Aggregation vs Composition.
  • What is Coupling and Cohesion?
  • What are the importance of Inner class in java, why did vendors design it?
  • What is hashcode?
  • Difference between equals method and == operator?
  • What is cloning, Types of cloning,Difference between them Write the code to implement Deep Clone?
  • What is garbage collection in java, Explain in detail.
  • What are the differences between String, StringBuilder & StringBuffer?
  • What is the difference between creating the string object with new operator and without new operator?
  • What is String Constant Pool and Why String uses it?
  • Why String is final, Immutable?
  • Name the five important methods of String that you used so frequently.
  • What is Auto - Boxing & Auto - Unboxing?

  • What are BigInteger and BigDecimal classes in java? 
  • What are the differences among Heap, Stack and Method Area?
  • What is design pattern?
  • How to create singleton design pattern?
  • What is Immutable Class and how to design Immutable class?
  • How to break the singleton design pattern?
  • What is class loader, How many types of class loaders are available in java?
  • In how many ways we can create the object of class?
  • What is difference between Compile Time and Runtime Exception?
  • How to create user defined exception?
  • What are the differences between final, finally & finalize?
  • What are the differences between throw & throws?
  • Which is  preferable between finally block and finalize method and why?
  • Explain the Exception throwing rules while overriding the methods.
  • In how many ways we can create the thread? Which one is preferable and why?
  • Explain the thread life cycle.
  • What are the differences between Sleep method and Wait method?
  • Why wait and notify methods are available in Object class while being used in context of Thread Class?
  • What is Synchronization? In how many ways we can apply Synchronization?Explain in detail.
  • What is Deadlock? write the code for Deadlock.
  • What is Consumer & Producer Problem? Write the code for the same. 
  • What is thread pooling? explain it with codes?
  • What is join method?Write the code to explain.
  • What is Daemon Thread? How to create Daemon Thread?
  • What is difference between stop & yield method in java?
  • What is volatile keyword in java?
  • What is collection? Draw the hierarchy of collection.
  • What are the differences between Collection, Collection Framework & Collections?
  • Which collection you used in your project and why?
  • How does Set collection rejects the duplicate elements?
  • What are the differences between:
  •    Array vs ArrayList.             
  •    ArrayList vs LinkedList.
  •    List vs Set.
  •    HashSet vs TreeSet.
  •    Iterator vs ListIterator.
  •    Comparable vs Comparator.
  • What is Hashing Technique, How to avoid Collision in Hashing?
  • Explain the internal Implementation of HashMap & LinkedHashMap?
  • What are the differences between peak & poll of Queue?
  • What is Concurrent Collection?
  • What is Generic?
  • What is Serialization? What is Transient? Can we serialize static variable?
  • What is Reflection?
  • What are the differences between creating the object using  newInstance method  and new operator?

Coding Round.
  • Prime Number Checking.
  • One Sorting Code.
  • Second Highest Number.
  • Star Pattern Printing Program.
  • Factorial.
  • Reverse String.
  • Reverse Number.
  • Count the occurrence of each character in given string.
  • Consumer Producer Code.
  • Dead Lock Code.
  • Palindrome.
  • Armstrong.
  • Creation of Thread Pool.
  • Temperature Conversion.
Various Company's Interview Questions Link below.
For Answers click on this link Get Answer


  1. Thank you for the list, very helpful...

  2. The questions what ever listed, i have just followed blindly and went for my first interview, finally i cracked the and I'm in. Its all because of you.

    1. Hi Ranjitha,
      Thank you so much for the feedback.
      Glad to know that it helped you alot.


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