Espresso: A Concentrated Introduction to Java
Summary: We consider the standard methods that all (or most) objects provide.
Prerequisites: Basics of Java.
Contents:
In Java, all template classes provide a variety of standard methods.
By standard method, I mean that it is a standard that you
include the method. You have already encountered one such method,
toString
.
Why does Java have standard methods? Because programmers benefit
from knowing that certain methods are there. For example, you can
easily convert any object to a string with the toString
method, and you don't even have to bother to look it up. Similarly,
you can compare any two objects in the same class for equality with
the equals
method.
Java has two groups of standard methods. Some must be there
for every class. These methods include toString
and equals
. Others you may choose whether or
not to include, and you specify in an interesting way that you
have included them. These methods include clone
and compareTo
.
What happens if you don't write the standard methods? Java supplies its own, and they don't always work the way you would expect them to work.
One of the simplest methods that most classes provide is the
toString
method, which has the signature
public String toString()
As the name of the method suggests, this method converts the object to a string. Why would we want to convert an object to a string? Most typically, so that we can print the object (to the screen, to a file).
The toString
method is so fundamental, that Java
uses it implicitly in a number of cases. For example, whenever
you try to print an object, Java calls the toString
method, even if you haven't explicitly listed it in your call.
Similarly, if you try too concatenate another object with a
string using the +
operation, Java converts the
object to a string using the toString
method.
What happens when you don't write the toString
method?
Java supplies a default version that prints the class of the object
followed by the address of the object in memory. (Yes, it's a strange
default, but it's about as good as anything.)
Another simple standard method is equals
, which you
use to determine whether one object is naturally
the
same as another object. You first have a responsibility to
determine what naturally
means. For example, are
two decimal numbers the same if they are not precise equal?
Are two fractions the same if one is simplified and one is
not? Are two vectors in two-space the same if one has an
angle of 360 degrees more than the other? Are two vectors
in two-space the same if they have the same radius, and their
angles differ by a very small amount (less than 1/1000 of
a degree)?
The standard signature for the equals
method
is
public boolean equals(Object other)
Note that it is somewhat difficult to compare a specific object to a general object. So, what do you do? My standard solution
is to provide a second form of equals
that takes
the specific kind of object as a parameter, and then, in the
first equals
, call the second while casting the
object to that type. (Recall that you cast a value to another
type by prefacing the name of the value with the name of the
type in parentheses.)
For example, in the Fraction
class, we might write
public boolean equals(Object other) { return this.equals((Fraction) other); } // equals(Object) public boolean equals(Fraction other) { return this.numerator.equals(other.numerator) && this.denominator.equals(other.denominator); } // equals(Fraction)
What happens if the cast fails? Java throws a
ClassCastException
. How do you then write
the equals(Object)
method? There are two
choices. You can catch that exception, or you can check
whether the cast is safe with the instanceof
operation.
Using the prior strategy, you might write
public boolean equals(Object other) { try { return this.equals((Fraction) other); } catch (ClassCastException cce) { return false; // If other isn't a fraction, it's not equal. } } // equals(Object)
Using the latter strategy, you might write
public boolean equals(Object other) { if (other instanceof Fraction) { return this.equals((Fraction) other); } else { return false; } } // equals(Object)
Of course, you should express that more concisely as
public booleans equals(Object other) { return (other instanceof Fraction) && this.equals((Fraction) other); } // equals(Object)
If you don't bother to write equals
, Java provides a
default version that returns true
only if the two values
share the same memory locations.
The equals
method provides the simplest form of comparison.
At times, we need more complex comparison, such as when we want to put
a sequence of values in order from smallest to largest. For such situations,
objects may (but need not) provide the compareTo
method,
which you may have used with some of the standard classes, such as
java.math.BigInteger
.
This method has the signature
public int compareTo(Class other)
where Class is typically the class you're defining. For example,
in the Fraction
class, we would write
public boolean compareTo(Fraction other)
The this.compareTo(other)
method returns
this
naturally precedes other
;
this.equals(other)
;
other
naturally precedes this
How should you decide if one object naturally precedes
another? That's up to you. What if you can't choose such a
relationship? Then you shouldn't bother implementing
compareTo
. What if there are many possible
relationships, as in the case of of students, who you might compare
by name, by age, by student ID, by height, by GPA, or by something
completely different? Then yoou need to pick one as the default
or choose no default. In a subsequent reading, we'll consider how
to handle multiple comparisons.
The ordering given by compareTo
should be transitive
and reflexive. When we say that it is transitive, we mean that if a.compareTo(b)
returns a
negative number and b.compareTo(c)
returns a negative
number, then a.compareTo(c)
should also return a negative
number. (We can say something similar when it reutrns positive
numbers.) When we say that it is reflexive, we mean that if
a.compareTo(b)
returns a negative number, then
b.compareTo(a)
should return a positive number (and vice
versa).
Because this standard method is not always implementable (that is, there
is sometimes no natural ordering), you need not include it. If you do
you should add the following line to the header of your class
implements Comparable<Class>
You must also import java.util.Comparable
.
For example,
import java.util.Comparable; public class Fraction implements Comparable<Fraction>
Note that for the compareTo
method, you need not
follow the "two method" strategy that you had to use in
equals
. Only one compareTo
,
which compares to objects in the same class, is all that
is necessary.
At times, you have one copy of an object and you need another copy. For
example, you may have created a StringBuffer
and want
to keep the original and make a copy that you will modify. To support
such situations, Java encourages you to provide a method called clone
.
The signature of this method is
public Object clone()
You may find it strange that clone
returns an
Object
rather than explicitly returning a member of
the specified class. This form of return was all that was supported
in an early version of Java (that is, there was no way to have multiple
methods with the same name and parameter types, but different return
types), and it seems to have been retained.
Because clone
returns an object, you need to cast
the return value. For example, the Java compiler will complain
about
Fraction frac2 = frac1.clone();
and insist that you instead write
Fraction frac2 = (Fraction) frac1.clone();
Although clone
is standard, it is also optional. If you
supply the method, you should indicate that your class
implements Cloneable
. For example,
public class Fraction implements Cloneable, Comparable<Fraction>
The last of the standard methods is somewhat strange. The
hashCode
method returns some integer that represents
this object
. What integer should you return? It's up to you.
The two general rules are that
equals
) method
should have the same value returned by hashCode
.
Of course, it is impossible to guarantee that unequal objects have different numbers, since there are typically more objects than there are integers. You should simply try to give unequal objects different numbers.
Why does Java include the hashCode
method as a standard
method? That method is very useful for hash tables, which we will
consider later in the semester. (The designers of Java made some
strange decisions as to what to make default. Some folks find this
one of the stranger ones.)
Do you have to write the hashCode
method? It's not
a bad idea. The Java standard suggests that if you write equals
,
then you must write hashCode
.
What is the default behavior of hashCode
? By default
hashCode
returns some value computed from the memory
location of the object. Hence, two equal values are unlikely to have
the same hash code.
Wednesday, 22 February 2006 [Samuel A. Rebelsky]
equals
section.
Sunday, 26 Feburary 2006 [Samuel A. Rebelsky]
Monday, 27 February 2006 [Samuel A. Rebelsky]
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