To return the reference type object cautiously in the accessor

One of the three characteristics of object-oriented is encapsulation, the domain (variable) in the class, as far as possible using private, if you really want to access, you can provide get and set, to avoid the damage of encapsulation.

However, if a reference type object is returned in a get accessor, it may also be modified by the outside world, thereby damaging the encapsulation. Such as:

public class Employee {
	private Date hireday;
	Public Date Gethireday () {return
		this.hireday;
	}
	
	private String name;
	Public Employee (String name) {
		this.name = name;
	}
	public boolean equals [Employee other] {return
		name.equalsignorecase (other.name);
	}
}

		Employee Zhang3 = new Employee ("Zhang3");
		Date d = zhang3.gethireday ();
		D.settime (D.gettime ()-10000000);

This is something I had never thought of before.