Let us see the basic interfaces and its
methods:
public interface java.lang.Comparable{
public
abstract int compareTo(java.lang.Object);
}
public interface java.util.Comparator{
public
abstract int compare(java.lang.Object, java.lang.Object);
public
abstract boolean equals(java.lang.Object);
}
Let us observe the following code :
Here
i am trying to sort some group of stirng objects using Arrays.sort() method and
collections.sort() method respectively it sorted and gave me the proper output
....but when i tried to sort group of employee objects i got the exception as :Exception in thread "main" java.lang.ClassCastException: Employee cannot be cast to java.lang.Comparable
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
public class Test {
public static void main(String[] args) {
String
names []={"Apple","Avacado","Apricot","Grapes","Gooseberries"};
Arrays.sort(names);
System.out.println("Using Arrays.sort()");
for (String lString
: names) {
System.out.println("Fruit :"+lString);
}
List
fruits = new ArrayList();
for (String lString
: names) {
fruits.add(lString);
}
Collections.sort(fruits);
System.out.println("Using Collections.sort()");
for (String lString
: fruits) {
System.out.println("Fruit :"+lString);
}
Employee
emp[] = new Employee[5];
emp[0]
= new Employee("R001", "Robin", 45000);
emp[1]
= new Employee("R012", "Antony", 55000);
emp[2]
= new Employee("R011", "Williams", 75000);
emp[3]
= new Employee("R121", "John", 65000);
emp[4]
= new Employee("R061", "James", 35000);
Arrays.sort(emp);
for(Employee e:emp){
System.out.println("Employee :id :"+e.getEmpId()+" \t Name
:"+e.getEmpName()+"\t sal :"+e.getEmpSal());
}
}
}
Output:
Using Arrays.sort()
Fruit :Apple
Fruit :Apricot
Fruit :Avacado
Fruit :Gooseberries
Fruit :Grapes
Using Collections.sort()
Fruit :Apple
Fruit :Apricot
Fruit :Avacado
Fruit :Gooseberries
Fruit :Grapes
Exception in thread "main" java.lang.ClassCastException: Employee cannot be cast to java.lang.Comparable
at
java.util.Arrays.mergeSort(Unknown Source)
at
java.util.Arrays.sort(Unknown Source)
at Test.main(Test.java:40)
When trying to sort Employee objects the error because
Arrays.sort()
See the method :
public static void
sort(java.lang.Object[]);
All elements in the
array must implement the Comparable interface.Then only it can do
natural ordering of elements.
So i changed my Employee class as below :
import java.util.Comparator;
public class Employee implements
Comparable {
private String mEmpId;
private String mEmpName;
private double mEmpSal;
public Employee(String pEmpId, String pEmpName, double pEmpSal) {
super();
mEmpId = pEmpId;
mEmpName = pEmpName;
mEmpSal = pEmpSal;
}
public String getEmpId() {
return mEmpId;
}
public void setEmpId(String pEmpId) {
mEmpId = pEmpId;
}
public String getEmpName() {
return mEmpName;
}
public void setEmpName(String pEmpName) {
mEmpName = pEmpName;
}
public double getEmpSal() {
return mEmpSal;
}
public void setEmpSal(double
pEmpSal) {
mEmpSal = pEmpSal;
}
@Override
public int compareTo(Employee empObj) {
double empSal = empObj.getEmpSal();
return empSal > mEmpSal ? 0 : 1;
}
/*
* This is to compare based on empName
*/
public static Comparator EmployeeNameComparator = new Comparator() {
@Override
public int compare(Employee e1, Employee e2) {
return e1.getEmpName().compareTo(e2.getEmpName());
}
};
/*
* This is to compare based on employee id
*/
public static Comparator EmployeeIdComparator = new Comparator() {
@Override
public int compare(Employee e1, Employee e2) {
return e1.getEmpId().compareTo(e2.getEmpId());
}
};
}
Here with this class you can observe i have implemented method
public abstract int
compareTo(java.lang.Object);
This method will
automatically have current object and then gets the next object to compare.
This method i have overriden to sort employee based on their
salary.
Now coming to Comparator interface you
can observe
public abstract int compare(java.lang.Object,
java.lang.Object);
It takes two arguments
obj1,obj2.
When ever you want to add any other sorting techniques for your
custom objects you can use this.
Here in my class i have created two anonymous classes to sort
employee objects based on their name and employeeId.
import java.util.Arrays;
public class EmpMain {
public static void main(String[] args) {
Employee
emp[] = new Employee[5];
emp[0] = new Employee("R001", "Robin", 45000);
emp[1] = new Employee("R012", "Antony", 55000);
emp[2] = new Employee("R011", "Williams", 75000);
emp[3] = new Employee("R121", "John", 65000);
emp[4] = new Employee("R061", "James", 35000);
Arrays.sort(emp);
System.out.println("Without
any comparator : ");
for(Employee e:emp){
System.out.println("Employee
:id :"+e.getEmpId()+" \t Name :"+e.getEmpName()+"\t
sal :"+e.getEmpSal());
}
Arrays.sort(emp,Employee.EmployeeIdComparator);
System.out.println("With
empId comparator : ");
for(Employee e:emp){
System.out.println("Employee
:id :"+e.getEmpId()+" \t Name :"+e.getEmpName()+"\t
sal :"+e.getEmpSal());
}
}
}
Without any comparator :
Employee :id :R061 Name :James
sal :35000.0
Employee :id :R001 Name :Robin
sal :45000.0
Employee :id :R012 Name :Antony
sal :55000.0
Employee :id :R121 Name :John
sal :65000.0
Employee :id :R011 Name :Williams
sal :75000.0
With empId comparator :
Employee :id :R001 Name :Robin
sal :45000.0
Employee :id :R011 Name :Williams
sal :75000.0
Employee :id :R012 Name :Antony
sal :55000.0
Employee :id :R061 Name :James
sal :35000.0
Employee :id :R121 Name :John
sal :65000.0
This also u can use :
List empList = new
ArrayList();
empList.add(emp[0]);
empList.add(emp[1]);
empList.add(emp[2]);
empList.add(emp[3]);
empList.add(emp[4]);
Collections.sort(empList);
for (Employee lEmployee : empList) {
System.out.println("Employee
:id :"+lEmployee.getEmpId()+" \t Name :"+lEmployee.getEmpName()+"\t sal :"+lEmployee.getEmpSal());
}
Collections.sort(empList,Employee.EmployeeNameComparator);
for (Employee lEmployee : empList) {
System.out.println("Employee
:id :"+lEmployee.getEmpId()+" \t Name :"+lEmployee.getEmpName()+"\t sal :"+lEmployee.getEmpSal());
}
Conclusion :
Use Comparable for natural sort ordering and use Comparator for
any custom sorting .....
Always Arrays.sort() and Collections.sort() can sort only objects
of type comparable.
