Defining Classes I - Java - page 194

designing. A common way to define equals for simple classes of the kind we are
looking at now is to say equals returns true if each instance variable of one object
equals the corresponding instance variable of the other object. This is how we
defined equals in Display 4.7.
If the definition of equals in Display 4.7 seems less than clear, it may help to
rewrite it as follows using the this parameter:

public boolean equals(DateFourthTry otherDate)
{
return ( (( this .month).equals(otherDate.month))
&& ( this .day == otherDate.day) && ( this .year == otherDate.year) );
}

So if date1 and date2 are objects of the class DateFourthTry , then date1.equals
(date2) returns true provided the three instance variables in date1 have values that
are equal to the three instance variables in date2 .
Also, note that the method in the definition of equals that is used to compare months
is not the equals for the class DateFourthTry but the equals for the class String . You
know this because the calling object, which is this.month , is of type String .
(Remember that we use the equals method of the class String because == does not
work correctly for comparing String values. This was discussed in the Pitfall section of
Chapter 3 entitled “Using == with Strings.”)
In Chapter 7, you will see that there are reasons to make the definition of the
equals method a bit more involved. But the spirit of what an equals method should
be is very much like what we are now doing, and it is the best we can do with what we
know so far.
The method toString should be defined so that it returns a String value that rep-
resents the data in the object. One nice thing about the method toString is that it
makes it easy to output an object to the screen. If date is of type DateFourthTry , then
you can output the date to the screen as follows:

toString

System.out.println(date.toString());

In fact, S ystem.out.println was written so that it automatically invokes toString() if
you do not include it. So, the object date can also be output by the following simpler
and equivalent statement:

println used
with objects

System.out.println(date);

This means that the method writeOutput in Display 4.7 is superfluous and could
safely be omitted from the class definition.
If you look at Display 4.8, you will see that toString is also called automatically
when the object is connected to some other string with a +, as in

System.out.println(date1 + " equals " + date2);