Example of Collections.sort in Java
May 18, 2013
Comparable, all the elements of the List must implement Comparable interface. When using Comparator, we need to create a class implementing Comparator and the instance will be passed in Collections.sort() as an argument.
Collections.sort() with Comparable
sort() method sorts a given List into ascending order. The elements of the list must be comparable.
According to java doc, the method syntax of sort() with Comparable is given below.
public static <T extends Comparable<? super T>> void sort(List<T> list)
SortWithComparable.java
package com.concretepage.util;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
public class SortWithComparable {
public static void main(String[] args) {
Person a1 = new Person("Ram");
Person a2 = new Person("Shyam");
Person a3 = new Person("Rahim");
List<Person> list = new ArrayList<Person>();
list.add(a1);
list.add(a2);
list.add(a3);
Collections.sort(list);
for (Person a : list) {
System.out.println(a.getName());
}
}
}
class Person implements Comparable<Person> {
private String name;
public Person(String name) {
this.name = name;
}
@Override
public int compareTo(Person o) {
return name.compareTo(o.name);
}
public String getName() {
return name;
}
}
Rahim Ram Shyam
Collections.sort() with Comparator
sort() can also sort a List on the basis of the given Comparator. According to java doc, the method syntax of sort() with Comparator is given below.
public static <T> void sort(List<T> list, Comparator<? super T> c)
SortWithComparator.java
package com.concretepage.util;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
public class SortWithComparator {
public static void main(String[] args) {
User a1 = new User("Ram");
User a2 = new User("Shyam");
User a3 = new User("Rahim");
List<User> list = new ArrayList<User>();
list.add(a1);
list.add(a2);
list.add(a3);
Collections.sort(list, new UserComparator());
for (User a : list) {
System.out.println(a.getName());
}
}
}
class User {
private String name;
public User(String name) {
this.name = name;
}
public String getName() {
return name;
}
}
package com.concretepage.util;
import java.util.Comparator;
public class UserComparator implements Comparator<User> {
@Override
public int compare(User o1, User o2) {
return o1.getName().compareTo(o2.getName());
}
}
Rahim Ram Shyam
