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Java Questionnaire II

What is the use of bin and lib in JDK?

1. Bin contains all tools like javac, appletviewer etc whereas lib contains API and all the packages.

2. Bin folder contains the binaries - the actual executable programs whereas the lib folder contains the libraries upon which the binaries are built.

What is method overloading and method overriding?

A method is said to be overloaded if it has same name but different signatures. Consider an example below

class ClassWithOverrideMethods
{
	public void printchar()
	{		
		System.out.println("this is the method without parameter");
	}

	public void printchar(char str)
	{
		System.out.println(str);
	}
}

class MainClass
{
	public static void main(String args[])
	{
		ClassWithOverrideMethods clsObject = new ClassWithOverrideMethods();
		clsObject.printchar();
		clsObject.printchar('a');
	}
}

Output

this is the method without parameter

a

Here 2 methods with the same name i.e. printchar but different signature are there in the same class.

Overriding means overriding the functionality of existing method of a super class from the method of a sub class. Consider an example below

class Shape
{
	public void draw()
             {
             	System.out.println("I can be of any shape");
	}
}
  
class Rectangle extends Shape
{
	public void draw()
             {
             	System.out.println("My shape is rectangular");	
	}
}

class MainClass
{
	public static void main(String args[])
             {
              	Rectangle r=new Rectangle();
                          r.draw();
	}
}

Output

My shape is rectangular

In the example above the method draw is overridden in the sub class Rectangle.

What is the difference between this() and super()?

this keyword refers to the current object. Consider the example below

class thisKeywordDemo
{
	int a=100;
	
	void disp()
	{
		int a=10;
		System.out.println("a"+a);
		System.out.println("a"+this.a);
	}

	public static void main(String args[])
	{
		thisKeywordDemo t=new thisKeywordDemo();
		t.disp();
	}
}

Output

a10

a100

So here this.a belongs to the current object i.e. disp() method.

super keyword refers to the object of super class. Consider an example

class superKeywordDemo
{
   void disp()
   {
       System.out.println("I m inside parent class");
   }
}
class SubClass extends superKeywordDemo
{
   void disp()
   {
       System.out.println("I m inside child class");
   }

   void CallBothMethods()
   {
      disp(); 
      super.disp();
   }
   
}

class MainClass
{
   public static void main(String aa[])
   {
      SubClass s = new SubClass(); 
      s.CallBothMethods();
   }
}

Output

I m inside child class

I m inside parent class

What is inner class?

The class which is defined within other class is called inner class. Consider an example

class Outer
{
   class Inner 
   {
      void disp()
      {
         System.out.println("I m inside Inner class disp");
      }
   }
   void CallInnerClass()
   {
      Inner n = new Inner();
      n.disp();
   }
   
   
}

class MainClass
{
   public static void main(String aa[])
   {
      Outer o = new Outer();
      o.CallInnerClass();
   }
}

Here class Inner is defined within Outer class.

What is the difference between checked exceptions and unchecked exception?

Checked exceptions are those which can be handled through the code. For example if a program tries to read a file and the file does not exists on the specified location then you can add following code to handle this exception

try
{}
catch(FileNotFoundException e)
{}

Unchecked exceptions are those which cannot be handled though the code. For example if your application is running on a computer and if the computer doesn't have sufficient memory at certain point of time to run the application then you will get system out of memory exception which you cannot handle though code.

What is a byte code in Java?

Byte codes are compiled format of Java programs (files with .class extension). Once the code is converted into byte code it is transferred to JVM for execution.

What is the difference between Vector and Arraylist in Java?

      1. Vector is thread safe where as arraylist is not thread safe.

      2. Arraylist is quicker in performance as compared to vector.

      3. Vector has the default size of 10 whereas arraylist has not default size.

      4. Arraylist don't define any increment size while vector does.

      5. Arraylist can be seen directly without any iterator while vector requires an iterator to display all it's content.

What is immutable object in Java?

Immutable objects are those whose state cannot be changed once constructed. For example string class is an immutable object. Consider a code sample

String ABC = "test";
ABC = "2344";

In the first line of code an object ABC for storing string value 'test' is created and points to a particular memory location. In the second line another memory location for storing string value '2344' is created and the object ABC is modified to point to a new location. So the value of the first location is not changed once created. So string is an immutable object.

How to prevent a method from being overridden?

final method can prevent a method from being overridden. Consider an example below

class BaseClass
{
	public final void Display()
	{
		System.out.println("This is the method of base class");
	}
}

class ChildClass extends BaseClass
{	
	public void Display()
	{
		System.out.println("This is the method of child class");
	}
}

class MainClass
{
	public static void main(String args[])
	{		
		ChildClass c = new ChildClass();
		c.Display();
	}
}

When you execute this method you will get following errors

D:\MainClass.java:11: error: Display() in ChildClass cannot override Display() in BaseClass

public final void Display()

overridden method is final

1 error

What is the difference between process and thread?

      • A process contains multiple threads

      • A processor executes threads not processes

      • Different processes can't work under same memory location. But the threads can work under the same memory area.

      • A process can be executed as a single entity whereas thread cannot be executed as a single entity.

Posted By -

Karan Gupta

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