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Saturday, June 18, 2011

Basic JAVA interview questions

Having covered a lot of topics in .NET, let’s shift our attention to some JAVA questions. In this post, we will cover a few basic JAVA interview questions to get started. We will advance in complexity in later posts.

SO, first, let’s start with constructors.

Question: What is the difference between constructors and other regular methods?

Constructors must have the same name as the class name and cannot return a value. The constructors are called only once per creation of an object while regular methods can be called many times.

Question: What happens if you do not provide a constructor?

Java does not actually require an explicit constructor in the class description. If you do not include a constructor, the Java compiler will create a default constructor in the byte code with an empty argument. If a class includes one or more explicit constructors, the java compiler does not create the default constructor.

Question: Can you call one constructor from another?

Yes, the key is to use the this() construct.

public Shape(int id) {
this.id = id; // "this" means this object
}

public Shape(int id, String type) {
this(id); // calls constructor public Shape(int id)
this.type = type; // "this" means this object
}



Question: How can you invoke the base class constructor in the derived class?


You can use the super() syntax. Note that this has to be the first statement in the function.


public Circle(int id) {
super(id); //must be the very first statement in the constructor.
}



Question: How can you invoke base class methods in the derived class?


To call a regular method in the base class use “super.myMethod( )”. This can be called at any line.


public class Circle extends Shape 
{
public void setUp()
{
super.initialize();
// do circle stuff
}
}



public void reset() throws Throwable
{
try
{
// Do stuff here to rest your object
}
catch (Throwable t) {}
finally
{
super.reset(); //clean up your parent class. Unlike constructors super.regularMethod() can be called at any line.
}
}



Question: Explain static initializers with an example.


When a class is loaded, all blocks that are declared static and don’t have function name (i.e. static initializers) are executed even before the constructors are executed. As the name suggests they are typically used to initialize static fields.


public class StaticInitializer 
{
public static final int x = 5;
public static final int y;
//note that since y is final, it can be initialized only once.

//Static initializer block, which is executed only once when the class is loaded.
static
{
if(x == 5)
y = 10;
else
y = 5;
}

//constructor is called only after static initializer block
public StaticInitializer(){}
}


The code above sets the value of x to 5 and y to 10.

97 comments:

  1. Given stuff and logical thinking are very good!

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  2. When coming to the java point there will be definitely asking the Questions
    on the constructors!So i hope this is the good introduction for the java questions!

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  3. ya this is very very important to the interview, because constructors are the base for the objects creation....

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  4. This comment has been removed by a blog administrator.

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  5. Write a program where the user enters the number of asterisk for the first row printed. The user will then enter another number (that is less than the number entered first). This number will be the number that each row will be decreased by. For example: User will enter 8 first and 3 second (row 1 - 8 asterisks, row 2 - 5 asterisks, row 3 - 2 asterisks. What will be printed is:

    ********
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    You can't print less asterisks than the decrement value (in the example you can't go below 2 because 3 was the decrement number).

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    Replies
    1. This is quite simple:
      static void PrintStars(int startCount, int decrementValue)
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      // decrement for next round
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      }
      else
      {
      Console.WriteLine("Please provide valid input values");
      }
      }

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    3. Hola,


      What a brilliant post I have come across and believe me I have been searching out for this similar kind of post for past a week and hardly came across this.

      I have two methods and they do almost same thing, but at some most inner nested level one of method have additional command.

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    Validate: do not accept numbers less than one for spools ordered. Do not accept a number less than 0 for spools in stock or shipping and handling charges.

    Once an order is printed, ask the user for the next order. Use a sentinel value to stop (q for quit or -1)

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