Coding Practice to avoid error while using Object references

Let's understand this by example


import java.util.Date;

import java.util.Date;

public class Refree {

    public static void main(String args[]) {

        Date date = new Date();
        Dater coach = new Dater(date);
        System.out.println(coach.date);
        date.setYear(1111);
        System.out.println(coach.date);

    }

}

class Dater {
    public Date date;

    Dater(Date date) {
        this.date = date;
    }
}







This program will print

Mon Dec 02 16:33:48 IST 2013
Mon Dec 02 16:33:48 IST 3011

But in line date.setYear(1111); we only want to change the value of date and not coach instance But this code is changing values of date as well as coach.

So how can we avoid that :

Let's see another version of same program


import java.util.Date;

public class Refree {

    public static void main(String args[]) {

        Date date = new Date();
        Dater coach = new Dater(date);
        System.out.println(coach.date);
        date.setYear(1111);
        System.out.println(coach.date);

    }

}

class Dater {
    public Date date;

    Dater(Date date) {
        this.date = new Date(date.getTime());
    }
}

-->

This program will print

Mon Dec 02 16:36:36 IST 2013
Mon Dec 02 16:36:36 IST 2013


So that's what we wanted..