Boge Anne Lise Grzech pierworodny 04 Dziecko miĹoĹci
ustawa_o_transporcie_drogowym_wersja_obowiazujaca_od_01.04.2009_r.
Leigh Lora Twelve Quickies of Christmas 04 Sarah's Seduction
Jeffries Sabrina Taniec zmysĹĂłw Stare panny Swanlea 04
James Alan Gardner [League Of Peoples 04] Hunted
Glen Cook Black Company 04 Silver_Spike
04. Oakley Natasha KrĂłlewski rod Ksiezniczka i magnat
Glen Cook Garrett 04 Old Tin Sorrows
Draka 04 Drakon, S. M. Stirling
04 Pora Na Milosc
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all of the fields in the class. And for good measure, throw in the toString() and hashCode() methods.
148
Understanding Comparable
The source code for just such a class is shown in Listing 11-1.
Listing 11-1: Creating a Comparable object.
import java.util.*;
public class Employee implements Comparable {
String department, name;
public Employee(String department, String name) {
this.department = department;
this.name = name;
}
public String getDepartment() {
return department;
}
public String getName() {
return name;
}
public String toString() {
return "[dept=" + department +
",name=" + name + "]";
}
public int compareTo(Object obj) {
Employee emp = (Employee)obj;
int deptComp = department.compareTo(emp.getDepartment());
return ((deptComp == 0) ?
name.compareTo(emp.getName()) :
deptComp);
}
public boolean equals(Object obj) {
if (!(obj instanceof Employee)) {
return false;
}
Employee emp = (Employee)obj;
return department.equals(emp.getDepartment()) &&
name.equals(emp.getName());
}
public int hashCode() {
return 31*department.hashCode() + name.hashCode();
}
}
Tip When defining the compareTo() method, be sure not to overload the method by declaring it as having an
argument of the specific object type you want to compare to. It won't be called naturally as it isn't part of
the Comparable interface.
Listing 11-2 demonstrates the usage of the comparable employee by creating a company with a bunch of
employees. For simplicity's sake, names are held as one field of the form "Lastname, Firstname" to ensure that
they are ordered properly in one field.
Listing 11-2: Testing our Comparable object.
149
Comparator Basics
import java.util.*;
public class Company {
public static void main (String args[]) {
Employee emps[] = {
new Employee("Finance", "Degree, Debbie"),
new Employee("Finance", "Grade, Geri"),
new Employee("Finance", "Extent, Ester"),
new Employee("Engineering", "Measure, Mary"),
new Employee("Engineering", "Amount, Anastasia"),
new Employee("Engineering", "Ratio, Ringo"),
new Employee("Sales", "Stint, Sarah"),
new Employee("Sales", "Pitch, Paula"),
new Employee("Support", "Rate, Rhoda"),
};
Set set = new TreeSet(Arrays.asList(emps));
System.out.println(set);
}
}
Running the program displays the sorted company roster on one really long line. Notice how the elements are
sorted by department first and name second.
[[dept=Engineering,name=Amount, Anastasia], [dept=Engineering,name=Measure,
Mary], [dept=Engineering,name=Ratio, Ringo], [dept=Finance,name=Degree,
Debbie], [dept=Finance,name=Extent, Ester], [dept=Finance,name=Grade, Geri],
[dept=Sales,name=Pitch, Paula], [dept=Sales,name=Stint, Sarah],
[dept=Support,name=Rate, Rhoda]]
Comparator Basics
While Comparable objects are able to compare multiple instances of themselves, a Comparator is a class that
can compare two other objects. When you don't like the natural ordering of a class or your class doesn't
implement Comparable, you can provide a custom ordering by creating a class that implements Comparator.
Table 11-3 lists the methods of the Comparator interface.
Table 11-3: Summary of the Comparator Interface
VARIABLE/METHOD NAME VERSION DESCRIPTION
compare() 1.2 Checks for ordering between two
elements.
equals() 1.2 Checks for equality with another
Comparator.
Understanding Comparator
Of the two methods in the interface, you actually only have to implement one.
The compare() method functions similarly to the compareTo() method, however, both objects to compare
must be passed in as arguments:
150
Using Comparator
public int compare(Object obj1, Object obj2)
The equals() method has nothing to do with comparing elements for sorting. It is meant to compare the
Comparator to another Comparator:
public boolean equals(Object obj)
As this method is already implemented in the Object class, your Comparator does not have to override the
behavior. However, if you wish to provide a way for different comparators to be equal, override the method.
It's possible that different implementations impose the same order on elements, but in different manners.
There is one predefined Comparator already implemented for you. If you wish to sort elements in their reverse [ Pobierz całość w formacie PDF ]
