• A class is a template for an encapsulated type.
• object is an instance of a class.

Defining a class

public class Date{
	private int day, month, year;
	...
	...
}

• public → To indicate visibility
• Java allows public to be omitted.
• Default visibility is public to package. ( All classes defined in the same directory form part of the same package.)
• day , month , year : Instance variables → In this code, set as private. Usually it is private since making it public breaks encapsulation.

Defining an object

• new creates a new object

• How to set instance variables (Since they are private → cannot be accessed by the object of a class directly)

  • Add methods to update values.
  • this is a reference to current object.

• Initializing objects

  • Use constructors → Special function which is called when an object is created.

• A later constructor can call an earlier one using this.

  public class Date{
  	private int day,month, year;
  
  	public Date(int d, int m, int y){
  		day = d;
  		month = m;
  		year = y;
  	}
  
  	public Date(int d, int m){
  		this(d,m,2021);  // Usage
  	}
  }
  

• If NO constructor is defined Java provides a default constructor with empty arguments.

  • Sets instance variables to sensible defaults:
    • int variables are set to 0
    • float set to 0.0
  • Only valid if no constructors are defined.
    • Otherwise we require an explicit constructor without arguments.

Copy Constructors

• Create a new object from an existing one.

  public class Date{
  	private int day, month, year;
  	
  	public Date(Date d){
  		//Note that the private instance variables of argument are visible.
  		this.day = d.day;
  		this.moth = d.month;
  		this.year = d.year;
  	}
  }
  
  public void UseDate() {
  	Date d1,d2;
  	d1 = new Date(12,4,1954);
  	d2 = new Date(d1);