The code for this article is written in ** JDK 1.8 **.
When comparing objects of your own class, you need to define the comparison logic of what makes it big or small. There are the following two definition methods.
--Implement the Comparable interface in the object class and define it with the compareTo method.
--Define a class that implements the Comparator interface and define it with the compare method.
It can be defined by either method, but there is a guideline for which one to define, so I will explain it.
Consider comparing the Book class with the information in the book.
public class Book {
//title
private String title;
//price
private int price;
//date of issue
private LocalDate dateOfIssue;
}
ComparableImplement the Comparable interface of java.lang and override the compareTo method to define the comparison logic.
For example, if you want to compare with title, implement as follows.
public class Book implements Comparable<Book>{
//title
private String title;
//price
private int price;
//date of issue
private LocalDate dateOfIssue;
@Override
public int compareTo(Book book){
return title.compareTo(book.title);
}
}
The comparison result of title is returned as it is. Sorting the list of Book objects sorts them in ascending order of title.
** Sort example **
Book book1 = new Book();
book1.setTitle("Doraemon");
book1.setPrice(500);
book1.setDateOfIssue(LocalDate.of(2010,10,10));
Book book2 = new Book();
book2.setTitle("Perman");
book2.setPrice(400);
book2.setDateOfIssue(LocalDate.of(2012,10,10));
Book book3 = new Book();
book3.setTitle("Oba Q");
book3.setPrice(300);
book3.setDateOfIssue(LocalDate.of(2014,10,10));
List<Book> bookList = Arrays.asList(book1, book2, book3);
System.out.println("Before sorting:" + bookList);
Collections.sort(bookList);
System.out.println("After sorting:" + bookList);
sort (List <T> list) of java.util.Collections compares and sorts elements by compareTo of the element object.
Execution result
Before sorting:[Book(title=Doraemon, price=500, dateOfIssue=2010-10-10), Book(title=Perman, price=400, dateOfIssue=2012-10-10), Book(title=Oba Q, price=300, dateOfIssue=2014-10-10)]
After sorting:[Book(title=Oba Q, price=300, dateOfIssue=2014-10-10), Book(title=Doraemon, price=500, dateOfIssue=2010-10-10), Book(title=Perman, price=400, dateOfIssue=2012-10-10)]
Note that compareTo can only define ** one comparison logic **.
ComparatorCreate a class that implements the Comparator of java.util and override the compare method to define the comparison logic. For example, if you want to compare with title, implement as follows.
public class BookTitleComparator implements Comparator<Book> {
@Override
public int compare(Book book1, Book book2){
return book1.getTitle().compareTo(book2.getTitle());
}
}
Let's sort using this.
** Sort example **
Book book1 = new Book();
book1.setTitle("Doraemon");
book1.setPrice(500);
book1.setDateOfIssue(LocalDate.of(2010,10,10));
Book book2 = new Book();
book2.setTitle("Perman");
book2.setPrice(400);
book2.setDateOfIssue(LocalDate.of(2012,10,10));
Book book3 = new Book();
book3.setTitle("Oba Q");
book3.setPrice(300);
book3.setDateOfIssue(LocalDate.of(2014,10,10));
List<Book> bookList = Arrays.asList(book1, book2, book3);
System.out.println("Before sorting:" + bookList);
bookList.sort(new BookTitleComparator());
System.out.println("After sorting:" + bookList);
Execution result
Before sorting:[Book(title=Doraemon, price=500, dateOfIssue=2010-10-10), Book(title=Perman, price=400, dateOfIssue=2012-10-10), Book(title=Oba Q, price=300, dateOfIssue=2014-10-10)]
After sorting:[Book(title=Oba Q, price=300, dateOfIssue=2014-10-10), Book(title=Doraemon, price=500, dateOfIssue=2010-10-10), Book(title=Perman, price=400, dateOfIssue=2012-10-10)]
You can create any number of classes that implement Comparator according to ** comparison logic ** as follows.
** Price comparison Comparator **
public class BookPriceComparator implements Comparator<Book> {
@Override
public int compare(Book book1, Book book2){
return Integer.valueOf(book1.getPrice()).compareTo(book2.getPrice());
}
}
** Issue date comparison Comparator **
public class BookDateOfIssueComparator implements Comparator<Book> {
@Override
public int compare(Book book1, Book book2){
return book1.getDateOfIssue().compareTo(book2.getDateOfIssue());
}
}
If you use the static method comparing of Comparator, you don't even need to create a class.
//Title order
bookList.sort(Comparator.comparing(Book::getTitle));
//By price
bookList.sort(Comparator.comparing(Book::getPrice));
//By issue date
bookList.sort(Comparator.comparing(Book::getDateOfIssue));
From the conclusion, I think it is ** basically better to use Comparator **.
Since Comparable can only define one compareTo that belongs to a class, it is said that you should implement the comparison logic that seems most natural for that class. For example, LocalDate seems to be a natural comparison between dates, so compareTo is implemented in the "previous date <after date" logic.
So in which field is it natural to compare in this Book? Is it a title somehow? It seems that, depending on the person, the comparison by publication date may be natural, and if a field such as bookId is added in the future, it will be even more blurry, right? I only have a feeling of a bug.
Comparator can ** clearly specify what the comparison is with the class name **, so you can prevent such blurring. Also, if you want to make a complex comparison such as comparing by publication date if the titles are the same, you can add a separate Comparator implementation class.
--The comparison logic in Comparable should be a natural comparison, but what is natural depends on the situation.
--Comparator can specify the content of comparison logic by the class name to be implemented.
--In Comparable, only one comparison logic can be implemented, but in Comparator, as many as the number of classes can be implemented.
--Basically, let's implement the comparison logic between your own classes with Comparator!
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