Popular Java Interview Questions for Freshers Interview (0-3 years of java experience)
Core OOP Concepts
- Is Java pure Object Oriented Programming Language ? Check Answer
- What is difference between String, StringBuilder and StringBuffer ?
- Is it possible for a Java method to inline swap two int or integer values ? Discuss pass-by-reference vs pass-by-value ?
- What is difference between JRE and JDK
- What is difference between 32bit and 64bit JDK ? Does size of primitive type changes between these two variants ?
- How Garbage Collection Works ? What are different Space partitions inside JVM (PermGen, Eden Space, etc)
- If a method throws Nullpointer exception in super class, can subclass method override it to throw RuntimeException ?
Answer : http://javainterviews.scribbleit.in/core-java/java-method-override-rules-for-runtime-exceptions
Q. What is the criteria for a class to be eligible for key in Hashmap ?Answer : http://javainterviews.scribbleit.in/core-java/class-criteria-for-making-it-hashmap-key
Q. What are benefits of making a class Immutable ? How will you make a class Immutable ?Answer : http://javainterviews.scribbleit.in/core-java/concept-of-immutable-class-in-java-concurrency
Q. What is difference between ArrayList and Vector? Which one shall be preferred?Answer : http://javainterviews.scribbleit.in/core-java/vector-vs-arraylist-java-collections
Q. Discuss how Hashtable works ?Answer : http://javainterviews.scribbleit.in/core-java/discuss-internals-of-concurrent-hashmap-in-java
Q. Write code to remove entries from hashmap while iterating over it ?Answer : http://javainterviews.scribbleit.in/core-java/add-remove-entries-from-hashmap-during-iteration
Q. What is difference between fail-safe and fail-fast iterators ? Give example of each in Java Collections framework ?Answer : http://javainterviews.scribbleit.in/core-java/fail-safe-vs-fail-fast-iterator-in-java-collections
Concurrency & multi-threading
- The difference between sleep and wait in Java?
- Difference between Callable and Runnable interface ? Check Answer
- Explain visibility and atomicity problems faced in multi-threaded environment?
- What is use of Volatile variables in Java ? Explain working of volatile with respect to register cache & main heap memory?
- Can volatile keyword be used to make a non-atomic operation as atomic?
- What is difference between wait() and sleep() method in Java?
- How will you implement a thread-safe singleton in Java?
- Which one of the following would be easy to write? thread-safe code for for 10 threads or 2 threads?
- How do you call wait() method? using if block or loop? Why? Provide a example for your explanation.
- How do you take thread dump in Java on Windows and Unix machine?
- What is a thread local variable in Java?
- Write wait-notify code for producer-consumer problem?
- What is an immutable object? How and why do you create an Immutable object in Java?
- How will you join two Tables A and B where data in Table B is
optional, i.e. even if no matching record exists in Table B, rows of
Table A shall appear in the query result ?
Answer : Hint is Left Outer Join between A and B
- How will you find nth highest salary from Employee Table using SQL ?