Comparators

Comparators

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

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()

Comparators

Comparable interface

Good practices

Links

Comparators

Links

Collection.sort or stream.sorted()

Links