Frequently asked questions on java programming language part-1

Frequently asked questions on java programming language part-1

Question: How to prevent a class from inheritance without using final modifier

        

Answer:  A class can be prevented from inheritance by defining a private
constructor 

class Foo

{

// it is a private constructor

// so cant be called from outside the class Foo

private Foo()

{

}

}

class Doo extends Foo // it is wrong

{

}

reason of this : the constructor of sub-class automatically invokes the constructor of super-class (using super keyword), and we know that private things cant be accessed outside the class

Question: What is copy constructor and how to implement it in java

Answer:  A copy constructor takes an object as an input and initialize new object

by coping its data into new object 

[object] ==> [copy-constructor] ==> [new-object] 

  example,

   class Foo

{

// a non static variable

int i;

// a no argument constructor

// i will not call this constructor in my code

// it is just defined

Foo()

{

i = 1;

}

// a paremeterized constructor

Foo(int i)

{

this.i = i;

}

// this is a copy constructor

// here we are taking a reference-variable 'f1' of class Foo

Foo(Foo f)

{

// copy the values of object 'f' one by one inside 

// the current object (which is being initialized)

// 'this' means current object

this.i = f.i;

}

// define main method             

public static void main(String[] args)

{

// create an object 'f1' by using paremeterized constructor

// look here, we are passing int value inside constructor

Foo f1 = new Foo(100);

// create an object 'f2' by using copy constructor

// look here, we are passing object-name 'f1' inside constructor

Foo f2 = new Foo(f1);

// note: the object 'f1' and 'f2' will have same data but with 

// different address

}

}