Java Reference
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addition steps “spread out” the range of hash codes and dramatically reduce the
likelihood that two unequal Circle objects have the same code. Efective Java by
Joshua Bloch (Addison Wesley) includes a helpful recipe for constructing efficient
hashCode() methods like this one.
Example 5-1 includes a compareTo() method. This method is defined by the
java.lang.Comparable interface rather than by Object , but it is such a common
method to implement that we include it in this section. The purpose of Comparable
and its compareTo() method is to allow instances of a class to be compared to each
other in the way that the < , <= , > , and >= operators compare numbers. If a class
implements Comparable , we can say that one instance is less than, greater than, or
equal to another instance. This also means that instances of a Comparable class can
be sorted.
Because compareTo() is not declared by the Object class, it is up to each individual
class to determine whether and how its instances should be ordered and to include a
compareTo() method that implements that ordering. The ordering defined by
Example 5-1 compares Circle objects as if they were words on a page. Circles are
first ordered from top to bottom: circles with larger y coordinates are less than cir‐
cles with smaller y coordinates. If two circles have the same y coordinate, they are
ordered from left to right. A circle with a smaller x coordinate is less than a circle
with a larger x coordinate. Finally, if two circles have the same x and y coordinates,
they are compared by radius. The circle with the smaller radius is smaller. Notice
that under this ordering, two circles are equal only if all three of their fields are
equal. This means that the ordering defined by compareTo() is consistent with the
equality defined by equals() . This is very desirable (but not strictly required).
The compareTo() method returns an int value that requires further explanation.
compareTo() should return a negative number if the this object is less than the
object passed to it. It should return 0 if the two objects are equal. And compareTo()
should return a positive number if this is greater than the method argument.
Object defines a method named clone() whose purpose is to return an object with
fields set identically to those of the current object. This is an unusual method for
two reasons. First, it works only if the class implements the java.lang.Cloneable
interface. Cloneable does not define any methods (it is a marker interface) so
implementing it is simply a matter of listing it in the implements clause of the class
signature. The other unusual feature of clone() is that it is declared protected .
Therefore, if you want your object to be cloneable by other classes, you must imple‐
ment Cloneable and override the clone() method, making it public .
The Circle class of Example 5-1 does not implement Cloneable ; instead it provides
a copy constructor for making copies of Circle objects:
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