Core Java Interview Questions

Question: What is transient variable?

Answer: 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.

Question: Name the containers which uses Border Layout as their default layout?

Answer: Containers which uses Border Layout as their default are: window, Frame and Dialog classes.

 

Question: What do you understand by Synchronization?

Answer: 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.

Example: 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.

   }

}

 

Question: What is Collection API?

Answer: 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.

 

Question: Is Iterator a Class or Interface? What is its use?

Answer: Iterator is an interface which is used to step through the elements of a Collection.

Question: What is similarities/difference between an Abstract class and Interface?

Answer:  Differences are as follows:

Interfaces provide a form of multiple inheritances. 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.

     

Question: How to define an Abstract class?

Answer: A class containing abstract method is called Abstract class. An Abstract class can't be instantiated.

Example of Abstract class:

abstract class testAbstractClass {

  protected String myString;

  public String getMyString() {

  return myString;

  }

  public abstract string anyAbstractFunction();

}

Question: How to define an Interface?

Answer: In Java Interface defines the methods but does not implement them. Interface can include constants. A class that implements the interfaces is bound to implement all the methods defined in Interface.

Example of Interface:

public interface sampleInterface {

  public void functionOne();

  public long CONSTANT_ONE = 1000;

}

Question: Explain the user defined Exceptions?

Answer: User defined Exceptions are the separate Exception classes defined by the user for specific purposed. An user defined can created by simply sub-classing it to the Exception class. This allows custom exceptions to be generated (using throw) and caught in the same way as normal exceptions.

Example:

class myCustomException extends Exception {

   // The class simply has to exist to be an exception

}

 

Question: Explain the new Features of JDBC 2.0 Core API?

Answer: The JDBC 2.0 API includes the complete JDBC API, which includes both core and Optional Package API, and provides inductrial-strength database computing capabilities.

New Features in JDBC 2.0 Core API:

 Scrollable result sets- using new methods in the ResultSet interface allows programmatically move the to particular row or to a position relative to its current position

·         JDBC 2.0 Core API provides the Batch Updates functionality to the java applications.

·         Java applications can now use the ResultSet.updateXXX methods.

·         New data types - interfaces mapping the SQL3 data types

·         Custom  mapping of user-defined types (UTDs)

·         Miscellaneous features, including performance hints, the use of character streams, full precision for java.math.BigDecimal values, additional security, and support for time zones in date, time, and timestamp values.

 

Question: Explain garbage collection?

Answer: Garbage collection is one of the most important feature of Java. Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. User program cann't directly free the object from memory, instead it is the job of the garbage collector to automatically free the objects that are no longer referenced by a program. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in use. I Java on calling System.gc() andRuntime.gc(),  JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected.

Question: How you can force the garbage collection?

Answer: Garbage collection automatic process and can't be forced.

Question: What is OOPS?

Answer: OOP is the common abbreviation for Object-Oriented Programming.

Question: Describe the principles of OOPS.

Answer: There are three main principals of oops which are called Polymorphism, Inheritance and Encapsulation.

Question: Explain the Encapsulation principle.

Answer: Encapsulation is a process of binding or wrapping the data and the codes that operates on the data into a single entity. This keeps the data safe from outside interface and misuse. One way to think about encapsulation is as a protective wrapper that prevents code and data from being arbitrarily accessed by other code defined outside the wrapper.

Question: Explain the Inheritance principle.

Answer: Inheritance is the process by which one object acquires the properties of another object.

Question: Explain the Polymorphism principle.

Answer: The meaning of Polymorphism is something like one name many forms. Polymorphism enables one entity to be used as as general category for different types of actions. The specific action is determined by the exact nature of the situation. The concept of polymorphism can be explained as "one interface, multiple methods".

Question: Explain the different forms of Polymorphism.

Answer: From a practical programming viewpoint, polymorphism exists in three distinct forms in Java:

• ·         Method overloading
• ·         Method overriding through inheritance
• ·         Method overriding through the Java interface

Question: What are Access Specifiers available in Java?

Answer: Access specifiers are keywords that determines the type of access to the member of a class. These are:

• ·         Public
• ·         Protected
• ·         Private
• ·         Defaults

 

Question: Describe the wrapper classes in Java.

Answer: Wrapper class is wrapper around a primitive data type. An instance of a wrapper class contains, or wraps, a primitive value of the corresponding type.

Following table lists the primitive types and the corresponding wrapper classes:

Primitive	Wrapper
boolean	java.lang.Boolean
byte	java.lang.Byte
char	java.lang.Character
double	java.lang.Double
float	java.lang.Float
int	java.lang.Integer
long	java.lang.Long
short	java.lang.Short
void	java.lang.Void

Question: Read the following program:

public class test {

public static void main(String [] args) {

  int x = 3;

  int y = 1;

   if (x = y)

   System.out.println("Not equal");

  else

  System.out.println("Equal");

 }

}

What is the result?

   A. The output is ?Equal?

   B. The output in ?Not Equal?

   C. An error at " if (x = y)" causes compilation to fall.

   D. The program executes but no output is show on console.

Answer: C

Question: what is the class variables ?

Answer: When we create a number of objects of the same class, then each object will share a common copy of variables. That means that there is only one copy per class, no matter how many objects are created from it. Class variables or static variables are declared with the static keyword in a class, but mind it that it should be declared outside outside a class. These variables are stored in static memory. Class variables are mostly used for constants, variable that never change its initial value. Static variables are always called by the class name. This variable is created when the program starts i.e. it is created before the instance is created of class by using new operator and gets destroyed when the programs stops. The scope of the class variable is same a instance variable. The class variable can be defined anywhere at class level with the keyword static. It initial value is same as instance variable. When the class variable is defined as int then it's initial value is by default zero, when declared boolean its default value is false and null for object references. Class variables are associated with the class, rather than with any object.

Question: What is the difference between the instanceof and getclass, these two are same or not?

Answer: instanceof is a operator, not a function while getClass is a method of java.lang.Object class. Consider a condition where we use

if(o.getClass().getName().equals("java.lang.Math")){ }

This method only checks if the classname we have passed is equal to java.lang.Math. The class java.lang.Math is loaded by the bootstrap ClassLoader.

This class is an abstract class.

This class loader is responsible for loading classes. Every Class object contains a reference to the ClassLoader that defines. getClass() method returns the runtime class of an object. It fetches the java instance of the given fully qualified type name. The code we have written is not necessary, because we should not compare getClass.getName().

The reason behind it is that if the two different class loaders load the same class but for the JVM, it will consider both classes as different classes so, we can't compare their names. It can only gives the implementing class but can't compare a interface, but instanceof operator can.

The instanceof operator compares an object to a specified type. We can use it to test if an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface.

We should try to use instanceof operator in place of getClass() method. Remember instanceof opeator and getClass are not same.

Try this example; it will help you to better understand the difference between the two.

interface one{

}

class Two implements one {

}

class Three implements one {

}

public class Test {

public static void main(String args[]) {

one test1 = new Two();

one test2 = new Three();

System.out.println(test1 instanceof one); //true

System.out.println(test2 instanceof one); //true

System.out.println(Test.getClass().equals(test2.getClass())); //false

}

}

Threads in JAVA

A thread is a basic processing unit to which an operating system allocates processor time,and more than one thread can be executing code inside a process.

Every Java program has at least one thread, the thread that executes the Java program. It is created when you invoke the static main method of your Java class.

There are two ways to create a thread.

Extend the java.lang.Thread class Implement the java.lang.Runnable interface.

Once you have a Thread object, you call its start method to start the thread. When a thread is started, its run method is executed. Once the run method returns or throws an exception, the thread dies and will be garbage-collected.

Every Thread has a state and a Thread can be in one of these six states.

new. A state in which a thread has not been started. runnable. A state in which a thread is executing. blocked. A state in which a thread is waiting for a lock to access an object. waiting. A state in which a thread is waiting indefinitely for another thread to perform an action. timed__waiting. A state in which a thread is waiting for up to a specified period of time for another thread to perform an action. terminated. A state in which a thread has exited.

The values that represent these states are encapsulated in the java.lang.Thread.State enum. The members of this enum are NEW, RUNNABLE, BLOCKED, WAITING, TIMED__WAITING,  and TERMINATED.

Static Keyword in Java

While you are programming you would want to use some class members independently of any object of that class. Normally a class member is accessed with the help of the object of that class. However, it is possible to create class members that can be used by itself. To create such a member, the keyword statichas to precede its declaration.

When a class member is declared asstatic, it can be accessed before any object of that class is created and without reference to any object. Both methods and variable can be declared as static. The best example to understand is our main() which is declared static. Its is static because it must be called before any object exist.

Instance variables declared as static are, generally global variables.

Methods declared as static have several restrictions :


They can only call other static methods
They must only access static data.
They cannot use this or super in anyway (Super is a keyword used in Inheritance ).Below is a code that will help you understand the use of static. In the example discussed all methods and variables are declared as static. So as an when you execute this code the 1st static declaration gets executed, then 2nd and so on.


// Demonstrate static variables, methods, and blocks.
class ExampleStatic
{
 static int a = 5;
 static int b;
 static void setValMeth(int x)
 {
 System.out.println("x = " + x);
 System.out.println("a = " + a);
 System.out.println("b = " + b);
 }
static
{
System.out.println("Static block initialized.");
 b = a * 5;
 }
 public static void main(String args[])
{
 setValMeth(50);
 }
}



As soon as the ExampleStatic class is loaded, all of the static statements are run. First, a is set to 5, then the static block executes (printing a message), and finally, b is initialized to a * 5 or 25. Then main( ) is called, which calls meth( ), passing 50 to x. The three println( ) statements refer to the two static variables a and b, as well as to the local variable x.


Note It is illegal to refer to any instance variables inside of a static method.
Here is the output of the program:


Static block initialized.
x = 50
a = 5
b = 25

Static methods and variables can be used independently of the object. To do so you only need to specify the classname followed by the dot operator and method Ex :

classname.method();


The above method is same as the calling of non-static methods through object reference variable. The static variable can also be accessed in the same way.

Here is an example in which there is static method callMe() and the static variablesa,b are explained outside of their class.
class Check1
{
 static int a = 42;
 static int b = 99;
 static void callme()
{
 System.out.println("a = " + a);
 }
 }
 class Check2
{
 public static void main(String args[])
{
 Check1.callme();
 System.out.println("b = " + Check1.b);
 }
 }

OUTPUT :


a = 42
b = 99

Exceptions in Java

An exception is a problem that arises during the execution of a program. An exception can occur for many different reasons, including the following:

1. A user has entered invalid data.
2. A file that needs to be opened cannot be found.
3. A network connection has been lost in the middle of communications, or the JVM has run out of memory.

Some of these exceptions are caused by user error, others by programmer error, and others by physical resources that have failed in some manner.
To understand how exception handling works in Java, you need to understand the three categories of exceptions:

Checked exceptions:

1. A checked exception is an exception that is typically a user error or a problem that cannot be foreseen by the programmer. For example, if a file is to be opened, but the file cannot be found, an exception occurs. These exceptions cannot simply be ignored at the time of compilation.
2. Run time exceptions: A run time exception is an exception that occurs that probably could have been avoided by the programmer. As opposed to checked exceptions, run time exceptions are ignored at the time of compilation.
3. Errors: These are not exceptions at all, but problems that arise beyond the control of the user or the programmer. Errors are typically ignored in your code because you can rarely do anything about an error. For example, if a stack overflow occurs, an error will arise. They are also ignored at the time of compilation.

Exception Hierarchy:
All exception classes are subtypes of the java.lang.Exception class. The exception class is a subclass of the Throwable class. Other than the exception class there is another subclass called Error which is derived from the Throwable class.

Errors are not normally trapped form the Java programs. These conditions normally happen in case of severe failures, which are not handled by the java programs. Errors are generated to indicate errors generated by the run time environment. Example : JVM is out of Memory. Normally programs cannot recover from errors.

The Exception class has two main subclasses : IOException class and RuntimeException Class.

Exception Sub-Classes

Exceptions Methods :

Following is the list of important methods available in the Throwable class.

SN	Methods with Description
1	public String getMessage() Returns a detailed message about the exception that has occurred. This message is initialized in the Throwable constructor.
2	public Throwable getCause() Returns the cause of the exception as represented by a Throwable object.
3	public String toString() Returns the name of the class concatenated with the result of getMessage()
4	public void printStackTrace() Prints the result of toString() along with the stack trace to System.err, the error output stream.
5	public StackTraceElement [] getStackTrace() Returns an array containing each element on the stack trace. The element at index 0 represents the top of the call stack, and the last element in the array represents the method at the bottom of the call stack.
6	public Throwable fillInStackTrace() Fills the stack trace of this Throwable object with the current stack trace, adding to any previous information in the stack trace.

Common Exceptions :
In java it is possible to define two catergories of Exceptions and Errors.

1. JVM Exceptions: These are exceptions/errors that are exclusively or logically thrown by the JVM. Examples : NullPointerException, ArrayIndexOutOfBoundsException, ClassCastException,
2. Programmatic exceptions . These exceptions are thrown explicitly by the application or the API programmers Examples: IllegalArgumentException, IllegalStateException.