Java comparable and comparator example 2016-12-13 06:09
There are two ways to sort multiple objects. Create a Comparator or make objects implement Comparable interface. In this blog I will show you how to sort multiple objects.
1. Create a Comparator
User
public class User {
private int age;
private String username;
public User(String username, int age) {
this.username = username;
this.age = age;
}
@Override
public String toString() {
return "User{" +
"age=" + age +
", username='" + username + '\'' +
'}';
}
//getter and setter methods
}
Comparator
public class ComparatorByAge implements Comparator<User> {
@Override
public int compare(User o1, User o2) {
int age1 = o1.getAge();
int age2 = o2.getAge();
return (age1 < age2) ? -1 : ((age1 == age2) ? 0 : 1);
}
}
Use comparator to sort objects.
List<User> userList = new LinkedList<>();
userList.add(new User("Henry", 27));
userList.add(new User("Justin", 26));
userList.add(new User("Charles", 32));
System.out.println("before sorted:" + userList);
Collections.sort(userList, new ComparatorByAge());
System.out.println("after sorted:" + userList);
The output is like following.
before sorted:[ComparableUser{username='Henry', age=27}, ComparableUser{username='Justin', age=26}, ComparableUser{username='Charles', age=32}]
after sorted:[ComparableUser{username='Justin', age=26}, ComparableUser{username='Henry', age=27}, ComparableUser{username='Charles', age=32}]
If you use JDK1.8+ you can write Comparator with lambda expression.
List<User> userList = new LinkedList<>();
userList.add(new User("Henry", 27));
userList.add(new User("Justin", 26));
userList.add(new User("Charles", 32));
System.out.println("before sorted:" + userList);
Collections.sort(userList, (o1, o2) -> (o1.getAge() < o2.getAge()) ? -1 : ((o1.getAge() == o2.getAge()) ? 0 : 1));
System.out.println("after sorted:" + userList);
2. Make objects implement Comparable
ComparableUser
public class ComparableUser implements Comparable<ComparableUser> {
public ComparableUser(String username, Integer age) {
this.username = username;
this.age = age;
}
private String username;
private Integer age;
//getter and setter methods
@Override
public int compareTo(ComparableUser o) {
return this.age.compareTo(o.age);
}
@Override
public String toString() {
return "ComparableUser{" +
"username='" + username + '\'' +
", age=" + age +
'}';
}
}
You can use Collections.sort() to sort objects which implements Comparable interface.
List<ComparableUser> comparableUsers = new LinkedList<>();
comparableUsers.add(new ComparableUser("Henry", 27));
comparableUsers.add(new ComparableUser("Justin", 26));
comparableUsers.add(new ComparableUser("Charles", 32));
System.out.println("before sorted:" + comparableUsers);
Collections.sort(comparableUsers);
System.out.println("after sorted:" + comparableUsers);
The output is like the following.
before sorted:[User{age=27, username='Henry'}, User{age=26, username='Justin'}, User{age=32, username='Charles'}]
after sorted:[User{age=26, username='Justin'}, User{age=27, username='Henry'}, User{age=32, username='Charles'}]
Objects will be sorted automatically when you put them into TreeSet.
Set<ComparableUser> userSet = new TreeSet<>();
userSet.add(new ComparableUser("Henry", 27));
userSet.add(new ComparableUser("Justin", 26));
userSet.add(new ComparableUser("Charles", 32));
System.out.println(userSet);
The output is like following.
[ComparableUser{username='Justin', age=26}, ComparableUser{username='Henry', age=27}, ComparableUser{username='Charles', age=32}]