How to implement the Java comparable interface?

问题:

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.

回答1:

You just have to define that Animal implements Comparable i.e. public class Animal implements Comparable. 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.

回答2:

You need to:

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

Like this:

public class Animal implements Comparable{     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);     } } 

回答3:

While you are in it, I suggest to remember some key facts about compareTo() methods

1. CompareTo must be in consistent with equals method e.g. if two objects are equal via equals() , there compareTo() must return zero otherwise if those objects are stored in SortedSet or SortedMap they will not behave properly.

2. CompareTo() must throw NullPointerException if current object get compared to null object as opposed to equals() which return false on such scenario.

Read more: http://javarevisited.blogspot.com/2011/11/how-to-override-compareto-method-in.html#ixzz4B4EMGha3

回答4:

Implement Comparable interface in your class and provide implementation of int compareTo(Animal other) method in your class.See This Post

回答5:

You would need to implement the interface and define the compareTo() method. For a good tutorial go to - Tutorials point link or MyKongLink

回答6:

Possible alternative from the source code of Integer.compare method which requires API Version 19 is :

public int compareTo(Animal other) { return Integer.valueOf(this.year_discovered).compareTo(other.year_discovered); }

This alternative does not require you to use API version 19.

回答7:

Emp class needs to implement Comaparable interface so we need to Override its compateTo method.

import java.util.ArrayList; import java.util.Collections;   class Emp implements Comparable{      int empid;     String name;      Emp(int empid,String name){          this.empid = empid;            this.name = name;      }       @Override     public String toString(){         return empid+" "+name;     }      @Override     public int compareTo(Emp o) {       if(this.empid==o.empid){        return 0;       }       else if(this.empid 

---

public class JavaApplication1 {       public static void main(String[] args) {      ArrayList a= new ArrayList();      a.add(new Emp(10,"Mahadev"));       a.add(new Emp(50,"Ashish"));       a.add(new Emp(40,"Amit"));       Collections.sort(a);       for(Emp id:a){       System.out.println(id);       }     }  } 

回答8:

Use a Comparator...

    public class AnimalAgeComparator implements Comparator {  @Override public int compare(Animal a1, Animal a2) {   ... } } 

回答9:

This thing can easily be done by implementing a public class that implements Comparable. This will allow you to use compareTo method which can be used with any other object to which you wish to compare.

for example you can implement it in this way:

public String compareTo(Animal oth)  {     return String.compare(this.population, oth.population); } 

I think this might solve your purpose.

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