Top Interview Questions

You need to override equals() and hashcode() methods of a class whose objects you want to use as Key in a hashmap.
Immutable classes are Java classes whose objects cannot be modified once they are created. Any modification in immutable object will result into new object. For example 'isString' is immutable in Java. Mostly immutable are also final in Java in order to prevent sub class from overriding methods in Java which can compromise on immutability. You can achieve same functionality by making a member which is not final but private and cannot be modified except in constructor.
When we create a string with new() operator, it's created in heap and not added into string pool whereas string created using literal is added in the string pool itself which exists in PermGen area of heap.

String s = new String("Test");


The above shown example will not put the object in string pool and so we need to call String.intern() method which is used to put them into string pool explicitly. This will only happen when you make a string object as string literal e.g. String s = "Test".Java will automatically add this into the string pool.
This is a classic Java question which some people find it quite tricky whereas some consider it very easy. StringBuilder in Java was introduced in Java 5 and the only difference between Stringbuffer and StringBuilder is that Stringbuffer methods are synchronized while the latter one is non-synchronized. Please refer 'StringBuilder vs StringBuffer' to know more such more differences.
This is another good question related to Java asked in interviews. This kind of question is mainly asked by Amazon and other such equivalent companies.
Difference between Vector and Arraylist is one of the most common Core Java Interview questions you will come across in Collection. This question is mostly asked at an initial stage by the Interviewers before testing the deep knowledge regarding Collection. Vector and ArrayList classes are implemented using dynamically resizable array that enables fast random access and list traversal similar to that while using an ordinary array. ArrayList supports dynamic arrays in which changes can be made as per requirement that is ArrayList can be dynamically increased or decreased in size.
This is one of the toughest questions of Java usually asked in an interview which is open for all. A friend of mine was unaware of the answer to this question and also did not hesitate in sharing this with me. According to me stored procedure returns an error code only if some operation fails but if stored procedure itself fails than catching SQLException is the only choice left.
There is a bit difference when looking at exception handling. If your tasks throw an exception and also if it has been submitted then this exception will go to the uncaught exception handler (if you don't provide one explicitly, then the default one will just print the stack trace to System.err). If you have submitted the task with any thrown exception, checked exception or not, it then becomes part of the task's return status. For a task that has been submitted with submit and it terminates with an exception, then Future.get will again throw this exception, wrapped in an ExecutionException.
Abstract Factory provides an extended level of abstraction. Consider different factories where each is extended from an Abstract Factory and becomes responsible for creation of different hierarchies of objects based on the type of factory. E.g. AbstractFactory extended by AutomobileFactory, UserFactory, RoleFactory etc. Each individual factory would be responsible for creation of objects in that particular genre.
Singleton in Java is a class which has just one instance in whole Java application. For instance java.lang.Runtime is a Singleton class. Creating Singleton was initially tricky in Java 4 but it became easier with inception of Enum by Java 5. Please refer my article 'How to create thread-safe Singleton in Java' for more details on writing Singleton using enum and double checked locking which serves the purpose of this Java interview question.
This core Java question is follow up of previous question. The candidate is expected to write Java singleton using double checked locking. Remember to use volatile variable in order to make Singleton thread safe.
It is a bit tricky but you can perform this task using the 'while' loop and 'for' loop.
We can override hashcode whenever necessary and especially when we want to do an equality check or when we want to use our object as key in HashMap.
You will not be able to recover your object from HashMap if that is used as key in HashMap.
Critical section is better because if we lock the whole method than every time when someone calls this method it will have to wait even though we are not creating any object.
When we create string with new() operator it gets created in heap and is not added into string pool while String created using literal operator are added in the String pool itself which exists in Perm area of heap.
This is a good question and is open to all. According to me a poor hashcode function will result into frequent collisions in HashMap which eventually increases time for adding an object into Hash Map.
This is another good question. The get() method will go into an infinite loop during concurrent access and re-sizing.
Java Memory Model defines the legal interaction of threads with the memory in a real computer system. In another way we can say that it describes which behaviors are legal in multi-threaded code. It determines when a thread can reliably see the write method done on variables by other threads. It defines semantics for volatile, final & synchronized which guarantees the visibility of memory operations across the Threads.

Let's first discuss about Memory Barrier which is the base for the further discussions. There are two type of memory barrier instructions in JMM - read barriers and write barrier.

A read barrier invalidates the local memory (cache, registers, etc) and then reads the contents from the main memory, so that changes made by other threads becomes visible to the current Thread. A write barrier flushes out the contents of the processor's local memory to the main memory, so that changes made by the current Thread becomes visible to the other threads.

JMM semantics for synchronized
When a thread acquires monitor of an object, by entering into a synchronized block of code, it performs a read barrier (invalidates the local memory and reads from the heap instead). Similarly exiting from a synchronized block as part of releasing the associated monitor, it performs a write barrier (flushes changes to the main memory) Thus modifications to a shared state using synchronized block by one Thread, is guaranteed to be visible to subsequent synchronized reads by other threads. This guarantee is provided by JMM in presence of synchronized code block.

JMM semantics for Volatile fields
Read & write to volatile variables have same memory semantics as that of acquiring and releasing a monitor using synchronized code block. So the visibility of volatile field is guaranteed by the JMM. Moreover afterwards Java 1.5, volatile reads and writes are not reorderable with any other memory operations (volatile and non-volatile both). Thus when Thread A writes to a volatile variable V, and afterwards Thread B reads from variable V, any variable values that were visible to A at the time V was written are guaranteed now to be visible to B.

Let's try to understand the same using the following code


Data data = null;
volatile boolean flag = false;

Thread A
-------------
data = new Data();
flag = true; <-- writing to volatile will flush data as well as flag to main memory

Thread B
-------------
if(flag==true){ <-- as="" barrier="" data.="" flag="" font="" for="" from="" perform="" read="" reading="" volatile="" well="" will="">
use data; < !--- data is guaranteed to visible even though it is not declared volatile because of the JMM semantics of volatile flag.
}
This is a very common tricky Java question and its considered tricky because many programmers think that 'finally' block always gets executed. This question challenges the concept by putting return statement in 'try' or 'catch' block or calling System.exit from 'try' or 'catch' block. The answer to this question in Java is that 'finally' block will execute even if you put 'return' statement in 'try' block or 'catch' block but 'finally' block won't run if you call System.exit form 'try' or 'catch'.
This is another common tricky Java question. As mentioned before method overriding is a good topic to ask tricky questions in Java. One cannot override private or static method in Java. If you create a similar method with same return type and same method arguments then it is called method hiding.
The difficult part of this Java questions is how does the HashMap works in Java which is also a popular topic to prompt confusing and tricky question in Java. If we put the same key again then it will replace the old mapping because HashMap doesn't allow duplicate keys.
One can throw super class of RuntimeException in overridden method but you cannot do the same if its checked Exception.
public int compareTo(Object o)
{
Employee emp = (Employee) emp;
return this.id - o.id;
}
If one is not well-versed in writing multi-threading code then this becomes a really tricky question for him. This Java question can be tricky even for experienced and senior programmers, who are not really exposed to deadlock and race conditions. Key point here is order, that is if one acquires resources in a particular order and release resources in reverse order one can easily prevent deadlock.
This concept was introduced from Java 5. The main difference between both of them is that you can reuse CyclicBarrier even if Barrier is broken but you cannot reuse CountDownLatch in Java. You can refer CyclicBarrier vs CountDownLatch in Java for more differences.
You cannot access static variable in non-static context in Java. You can refer further as to why you cannot access non-static variable from static method to learn more about this tricky Java question.