Java Interview Questions and Answers

Jave

1. What does a well-written OO program look like?
A well-written OO program exhibits recurring structures that promote abstraction, flexibility, modularity and elegance.
2. Can you have virtual functions in Java?
Yes, all functions in Java are virtual by default. This is actually a pseudo trick question because the word "virtual" is not part of the naming convention in Java (as it is in C++, C-sharp and VB.NET), so this would be a foreign concept for someone who has only coded in Java. Virtual functions or virtual methods are functions or methods that will be redefined in derived classes.
3. Jack developed a program by using a Map container to hold key/value pairs. He wanted to make a change to the map. He decided to make a clone of the map in order to save the original data on side. What do you think of it?
If Jack made a clone of the map, any changes to the clone or the original map would be seen on both maps, because the clone of Map is a shallow copy. So Jack made a wrong decision.
4. What is more advisable to create a thread, by implementing a Runnable interface or by extending Thread class?
Strategically speaking, threads created by implementing Runnable interface are more advisable. If you create a thread by extending a thread class, you cannot extend any other class. If you create a thread by implementing Runnable interface, you save a space for your class to extend another class now or in future.
1. What does a well-written OO program look like?
A well-written OO program exhibits recurring structures that promote abstraction, flexibility, modularity and elegance.
5. What is NullPointerException and how to handle it?
When an object is not initialized, the default value is null. When the following things happen, the NullPointerException is thrown: --Calling the instance method of a null object. --Accessing or modifying the field of a null object. --Taking the length of a null as if it were an array. --Accessing or modifying the slots of null as if it were an array. --Throwing null as if it were a Throwable value. The NullPointerException is a runtime exception. The best practice is to catch such exception even if it is not required by language design.
6. An application needs to load a library before it starts to run, how to code?
One option is to use a static block to load a library before anything is called. For example, class Test { static { System.loadLibrary("path-to-library-file"); } .... } When you call new Test(), the static block will be called first before any initialization happens. Note that the static block position may matter.
7. What is a platform?
A platform is the hardware or software environment in which a program runs. Most platforms can be described as a combination of the operating system and hardware, like Windows 2000 and XP, Linux, Solaris, and MacOS.
8. What is transient variable?
Transient variable can't be serialize. For example if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can't be written to the stream instead when the class is retrieved from the ObjectStream the value of the variable becomes null.
9. Name the containers which uses Border Layout as their default layout?
Transient variable can't be serialize. For example if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can't be written to the stream instead when the class is retrieved from the ObjectStream the value of the variable becomes null.
10. What do you understand by Synchronization?
Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access one resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value.
Synchronization prevents such type of data corruption.
E.g. Synchronizing a function:
public synchronized void Method1 () {
// Appropriate method-related code.
}
E.g. Synchronizing a block of code inside a function:
public myFunction (){
synchronized (this) {
// Synchronized code here.
}
}

11. What is Collection API?
The Collection API is a set of classes and interfaces that support operation on collections of objects. These classes and interfaces are more flexible, more powerful, and more regular than the vectors, arrays, and hashtables if effectively replaces.
Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap.
Example of interfaces: Collection, Set, List and Map.
12. Is Iterator a Class or Interface? What is its use?
Iterator is an interface which is used to step through the elements of a Collection.
13. What is similarities/difference between an Abstract class and Interface?
Differences are as follows:
Interfaces provide a form of multiple inheritance. A class can extend only one other class. Interfaces are limited to public methods and constants with no implementation. Abstract classes can have a partial implementation, protected parts, static methods, etc.
A Class may implement several interfaces. But in case of abstract class, a class may extend only one abstract class. Interfaces are slow as it requires extra indirection to to find corresponding method in in the actual class. Abstract classes are fast.
Similarities:

Neither Abstract classes or Interface can be instantiated.

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