How does abstraction help in hiding the implementation details in Java?

Question

Abstraction is a process of hiding the implementation details and showing only functionality to the user.

Another way, it shows only important things to the user and hides the internal details. So below is an example where an abstract class is made and abstract methods are overridden. But the thing i didn't understand is how it is hiding the implementation details?

abstract class Bank
{    
   abstract int getRateOfInterest();    
} 

class SBI extends Bank
{    
 int getRateOfInterest()
  {
   return 7;
   }    
  }

class PNB extends Bank  
{    
 int getRateOfInterest()
   { 
    return 8;
   }    
 }    

class TestBank{    
public static void main(String args[])
{    
 Bank b;   
 b=new SBI();  
 System.out.println("Rate of Interest is: "+b.getRateOfInterest()+" %");    
 b=new PNB();  
 System.out.println("Rate of Interest is: "+b.getRateOfInterest()+" %");    
 }
 }     

Answer 1

The abstraction in your code is the abstract class itself:

abstract class Bank {    
   abstract int getRateOfInterest();    
} 

and the rest is the implementation (and the implementation details), specifically: classes PNB and SBI

But the thing i didn't understand is how it is hiding the implementation details?

Imagine you have a bank comparison engine, which is represented by the BankComparisonEngine class. It will just take a Bank (abstract class) as an argument, then get its interest rate and save it to its internal database, like so:

class BankComparisonEngine {
  public void saveInterestRateOf(Bank bank) {
    int rate = bank.getRateOfInterest();
    // Save it somwehere to reuse later 
  }
}

How are the implementation details hidden exactly? Well, BankComparisonEngine does not know how getRateOfInterest() method of a concrete implementation of Bank works (that is: PNB.getRateOfInterest() or SBI.getRateOfInterest() is implemented). It only knows it is a method that returns an int (and that it should return an interest rate). The implementation details are hidden inside the concrete classes that extend abstract class Bank.

Answer 2

When a client has to use your object, the client need not import your class or have your class definition in his jar, he/she can just import the abstract class or interface and accept your object as an argument, like explained in bank example. So client will still be able to use your object with parent reference but can't actually see (or need) the functional implementation of the class. Hope this cleared your doubt.