Question

public class MyDate {
    private int day = 1;
    private int month = 1;
    private int year = 2000;

    public MyDate(int day, int month, int year) 
    {
        this.day = day;
        this.month = month;
        this.year = year;
    }

    public MyDate(MyDate date) 
    {
        this.day = date.day;
        this.month = date.month;
        this.year = date.year;
    }

    /*
        here why do we need to use Class name before the method name?
    */
    public MyDate addDays(int moreDays) 
    {
        // "this" is referring to which object and why?
        MyDate newDate = new MyDate(this);
        newDate.day = newDate.day + moreDays;

        // Not Yet Implemented: wrap around code...
        return newDate;
    }

    public String toString()
    {
        return "" + day + "-" + month + "-" + year;
    }
}

Answer 1

this will refer to the current object instance that you will create. Inside any java methods, this will always hold a reference to its object instance.

An example –

MyDate myDate = new MyDate(10, 10, 2012);
myDate.addDays(10);

The line that you were wondering about will point to the newDate object that is created here. The line –

MyDate newDate = new MyDate(this);

will use this constructor –

public MyDate(MyDate date) {
    this.day = date.day;
    this.month = date.month;
    this.year = date.year;
}

to create and return a new object, passing it a reference to the current object instance, so that it can copy its day, month and year values.