[4] Intro to Object-oriented programming - Access control and about the Object class

The following are my notes on OOP course (Object-oriented programming) based on Kunal Kashwaha’s lectures on YouTube (link in the reference section)

Access control / Access modifiers

4 Types of access modifiers:

public: accessible everywhere

private: only accessible in the same class (file)

default: when a specific access modifier is not specified.

protected: useful when inheritance is involved

Purpose of having access control:

For security, restricting access to the data members

`protected` with inheritance

The access level of a protected modifier is within the package and outside the package through child class.

Parent class


package packageOne;
public class Base
{
    protected void display(){
        System.out.println("in Base");
    }
}

Child class


package packageTwo;
public class Derived extends packageOne.Base{

    public void show(){
        new Base().display();       // this is not working
        new Derived().display();    // is working
        display(); // is working
    }
}

new Base().display() is not working because the caller's (the this instance) class is not defined in the same package so the protected method “display” cannot be accessed

Access modifiers with method overriding

If you are overriding any method, overridden method (i.e. declared in subclass) must not be more restrictive.


class A{  
	protected void msg() { 
		System.out.println("Hello java");
	}  
}  
  
public class Simple extends A{  
void msg(){System.out.println("Hello java");}//Compile Time Error  

 public static void main(String args[]){  
   Simple obj=new Simple();  
   obj.msg();  
   }  
}

The default modifier is more restrictive than protected. That is why, there is a compile-time error.

Object Class in Java

The Object class is the parent class of all the classes in java by default.

It provides multiple methods which are as follows (and these functions can be overridden)

toString() method

It is always recommended to override the toString() method to get our own String representation of Object.


// Default behavior of toString() is to print class name, then
// @, then unsigned hexadecimal representation of the hash code
// of the object

public String toString()
{
    return getClass().getName() + "@" + Integer.toHexString(hashCode());
}

hashCode() method

It returns distinct integers for distinct objects. It converts the internal address of the object to an integer by using an algorithm. The main advantage of saving objects based on hash code is that searching becomes easy.

equals(Object obj) method

finalize() method

This method is called just before an object is garbage collected. Called only once on a object even though the object might be collected multiple times by garbage collector