Comparators

• Comparators

  • Comparable interface

    • Links

  • Comparators

    • Links

    • Collection.sort or stream.sorted()

  • Links

Comparable interface

• One method to implement, compareTo

  • Takes another other object of same class that implements it and returns an int

  • signature: public int compareTo(T o);

• Why Use

  • The objects created from the class has a natural ordering

  • allows classes like collections to use it when doing sort for example.

  • use for any class with alphanumeric or chronological ordering

• Rules

  • return 0 if both objects are equal

  • return less than one, when the current object is less than the object passed into the compareTo method

  • return greater than one, when the current object is more than the object passed into the compareTo method

• Using a comparator in compareTo

  • See github link????

• Use comparator constructor method

  • can use like a builder, and chain multiple compares and in the order you want them to compare to.

  • see github??

• Using a field's object compareTo method in the current object

  • exmaple:

  public class Duck implements Comparable<Duck> {
    public String name;
  
    public Duck(String name) {
        this.name = name;
      }
  
    public int compareTo(Duck d) {
      return name.compareTo(d.name);
    }
  }

  • Using the field name of duck objects to compare with each other.

  • name is a String Type, and String implements Comparable. thus can use compareTo from String

  • Another Example

    • see github ???

  • Note all classes/types will have an implementation of comparable, so will not be able to call it's compareTo method

• Using own method

  • Example

  public class Duck implements Comparable<Duck> {
    public int age;
  
    public Duck(int age) {
        this.age = age;
      }
  
    public int compareTo(Duck d) {
      return age - d.age;
    }
  }

  • comparing on the age of duck, but it's field is a primitive and does not have a compareTo method

    • could create an integer wrapper class and compareTo on that, as Integer implements compareTo

  • see github ???

  • Avoid this way of doing it

    • arithemtic overflow issues

• Use of > and < and ==

  • Avoid as verbose and error prone

  • see github???

• Comparing multiple fields

  • see githubg???

• Static compare method

  • see github???

• Comparable and equals

  • make them consistent

  • override equals() and set it to true when comparable has two objects equal returns 0

  • If not possible, write a comment saying this inconsistent

• Good practices

Links

• https://docs.oracle.com/javase/8/docs/api/java/lang/Comparable.html

Comparators

• reversed

  • instead of Comparator COMPARATOR = (x1,x2) -> x2.price - x1.price; to ????

• lambda

  • Comparator c = (d1, d2) -> d1.age - d2.age;

  • refactor to use comparator constructor method

    • Comparator COMPARATOR = (x1,x2) -> x1.price - x2.price; to Comparator.comparingInt(x -> x.price);

• Old way

  • Example >???

• Chainging comparing

  • http://www.baeldung.com/java-8-comparator-comparing

• passing into sorted() and sort()

Links

• https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html

• https://www.javabrahman.com/java-8/the-complete-java-8-comparator-tutorial-with-examples/

Collection.sort or stream.sorted()

Links

• http://www.baeldung.com/java-comparator-comparable

• https://www.youtube.com/watch?v=DXPjjVnUvgA

• https://www.youtube.com/watch?v=oAp4GYprVHM

• https://www.youtube.com/watch?v=lFbBI85oTnY Write Efficient Bug-free and Simple Comparators in Java - JEP Café episode 17

• https://dzone.com/articles/sorting-java-arraylist

• https://www.journaldev.com/780/comparable-and-comparator-in-java-example