Java Collections Java Java API
Interface:
java.util.Comparator
Methods:
static <T,U extends Comparable<? super U>> Comparator<T> comparing(
Function<? super T,? extends U> keyExtractor)
This static method returns an instance of
Comparator
, that compares type T objects according to the specified key extractor function.
static <T,U> Comparator<T> comparing(Function<? super T,? extends U> keyExtractor,
Comparator<? super U> keyComparator)
This static method returns an instance of
Comparator
that compares type T objects according to the specified function (a compare key extractor) and a comparator (the
extracted key comparator).
Examples
 package com.logicbig.example.comparator;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
public class ComparingExample {
public static void main(String... args) {
List<Customer> list = createExampleCustomers();
System.out.printf("before sort: %s%n", list);
Collections.sort(list, Comparator.comparing(Customer::getEmail));
System.out.printf("after sort: %s%n", list);
}
private static String getEmailDomain(String email) {
int i = email.indexOf("@");
if (i != -1) {
return email.substring(i + 1);
}
return "";
}
private static List<Customer> createExampleCustomers() {
return Arrays.asList(
new Customer("sara@example.com", "Sara"),
new Customer("john@example.com", "John"),
new Customer("mike@example.com", "Mike")
);
}
private static class Customer {
private String email;
private String name;
public Customer(String email, String name) {
this.email = email;
this.name = name;
}
public String getEmail() {
return email;
}
public String getName() {
return name;
}
@Override
public String toString() {
return "Customer{" +
"email='" + email + '\'' +
", name='" + name + '\'' +
'}';
}
}
}
Outputbefore sort: [Customer{email='sara@example.com', name='Sara'}, Customer{email='john@example.com', name='John'}, Customer{email='mike@example.com', name='Mike'}]
after sort: [Customer{email='john@example.com', name='John'}, Customer{email='mike@example.com', name='Mike'}, Customer{email='sara@example.com', name='Sara'}]
package com.logicbig.example.comparator;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
public class ComparingExample2 {
public static void main(String... args) {
List<Customer> list = createExampleCustomers();
System.out.printf("before sort: %s%n", list);
Collections.sort(list, Comparator.comparing(Customer::getEmail, getEmailDomainComparator()));
System.out.printf("after sort: %s%n", list);
}
public static Comparator<String> getEmailDomainComparator(){
return (email1, email2) -> {
String emailDomain1 = getEmailDomain(email1);
String emailDomain2 = getEmailDomain(email2);
return emailDomain1.compareTo(emailDomain2);
};
}
private static String getEmailDomain(String email) {
int i = email.indexOf("@");
if (i != -1) {
return email.substring(i + 1);
}
return "";
}
private static List<Customer> createExampleCustomers() {
return Arrays.asList(
new Customer("sara@example.com", "Sara"),
new Customer("john@example.com", "John"),
new Customer("mike@example.com", "Mike")
);
}
private static class Customer {
private String email;
private String name;
public Customer(String email, String name) {
this.email = email;
this.name = name;
}
public String getEmail() {
return email;
}
public String getName() {
return name;
}
@Override
public String toString() {
return "Customer{" +
"email='" + email + '\'' +
", name='" + name + '\'' +
'}';
}
}
}
Outputbefore sort: [Customer{email='sara@example.com', name='Sara'}, Customer{email='john@example.com', name='John'}, Customer{email='mike@example.com', name='Mike'}]
after sort: [Customer{email='sara@example.com', name='Sara'}, Customer{email='john@example.com', name='John'}, Customer{email='mike@example.com', name='Mike'}]
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