How to implement the Java comparable interface?

Question

I am not sure how to implement a comparable interface into my abstract class. I have the following example code that I am using to try and get my head around it:

public class Animal{     public String name;     public int yearDiscovered;     public String population;      public Animal(String name, int yearDiscovered, String population){         this.name = name;         this.yearDiscovered = yearDiscovered;         this.population = population; }      public String toString(){         String s = "Animal name: "+ name+"\nYear Discovered: "+yearDiscovered+"\nPopulation: "+population;         return s;     } } 

I have a test class that will create objects of type Animal however I want to have a comparable interface inside this class so that older years of discovery rank higher than low. I have no idea on how to go about this though.

Answer 1

You just have to define that Animal implements Comparable<Animal> i.e. public class Animal implements Comparable<Animal>. And then you have to implement the compareTo(Animal other) method that way you like it.

@Override public int compareTo(Animal other) {     return Integer.compare(this.year_discovered, other.year_discovered); } 

Using this implementation of compareTo, animals with a higher year_discovered will get ordered higher. I hope you get the idea of Comparable and compareTo with this example.

Answer 2

You need to:

• Add implements Comparable<Animal> to the class declaration; and
• Implement a int compareTo( Animal a ) method to perform the comparisons.

Like this:

public class Animal implements Comparable<Animal>{     public String name;     public int year_discovered;      public String population;       public Animal(String name, int year_discovered, String population){         this.name = name;         this.year_discovered = year_discovered;         this.population = population;     }      public String toString(){      String s = "Animal name: "+ name+"\nYear Discovered: "+year_discovered+"\nPopulation: "+population;      return s;     }      @Override     public int compareTo( final Animal o) {         return Integer.compare(this.year_discovered, o.year_discovered);     } }