Java object sorting example (Comparable and Comparator)

1. Sort an Array

To sort an Array, use the Arrays.sort().

 String[] fruits = new String[] {"Pineapple","Apple", "Orange", "Banana"}; 
 
 Arrays.sort(fruits);
 
 int i=0;
 for(String temp: fruits){
  System.out.println("fruits " + ++i + " : " + temp);
 }

Output

fruits 1 : Apple
fruits 2 : Banana
fruits 3 : Orange
fruits 4 : Pineapple

2. Sort an ArrayList

To sort an ArrayList, use the Collections.sort().

 List<String> fruits = new ArrayList<String>();
 
 fruits.add("Pineapple");
 fruits.add("Apple");
 fruits.add("Orange");
 fruits.add("Banana");
 
 Collections.sort(fruits);
 
 int i=0;
 for(String temp: fruits){
  System.out.println("fruits " + ++i + " : " + temp);
 }

Output

fruits 1 : Apple
fruits 2 : Banana
fruits 3 : Orange
fruits 4 : Pineapple

3. Sort an Object with Comparable

How about a Java Object? Let create a Fruit class:

public class Fruit{
 
 private String fruitName;
 private String fruitDesc;
 private int quantity;
 
 public Fruit(String fruitName, String fruitDesc, int quantity) {
  super();
  this.fruitName = fruitName;
  this.fruitDesc = fruitDesc;
  this.quantity = quantity;
 }
 
 public String getFruitName() {
  return fruitName;
 }
 public void setFruitName(String fruitName) {
  this.fruitName = fruitName;
 }
 public String getFruitDesc() {
  return fruitDesc;
 }
 public void setFruitDesc(String fruitDesc) {
  this.fruitDesc = fruitDesc;
 }
 public int getQuantity() {
  return quantity;
 }
 public void setQuantity(int quantity) {
  this.quantity = quantity;
 }
}

To sort it, you may think of Arrays.sort() again, see below example :

package com.mkyong.common.action;
 
import java.util.Arrays;
 
public class SortFruitObject{
 
 public static void main(String args[]){
 
  Fruit[] fruits = new Fruit[4];
 
  Fruit pineappale = new Fruit("Pineapple", "Pineapple description",70); 
  Fruit apple = new Fruit("Apple", "Apple description",100); 
  Fruit orange = new Fruit("Orange", "Orange description",80); 
  Fruit banana = new Fruit("Banana", "Banana description",90); 
 
  fruits[0]=pineappale;
  fruits[1]=apple;
  fruits[2]=orange;
  fruits[3]=banana;
 
  Arrays.sort(fruits);
 
  int i=0;
  for(Fruit temp: fruits){
     System.out.println("fruits " + ++i + " : " + temp.getFruitName() + 
   ", Quantity : " + temp.getQuantity());
  }
 
 } 
}

Nice try, but, what you expect the Arrays.sort() will do? You didn’t even mention what to sort in the Fruit class. So, it will hits the following error :

Exception in thread "main" java.lang.ClassCastException: 
com.mkyong.common.Fruit cannot be cast to java.lang.Comparable
 at java.util.Arrays.mergeSort(Unknown Source)
 at java.util.Arrays.sort(Unknown Source)

To sort an Object by its property, you have to make the Object implement the Comparable interface and override the compareTo() method. Lets see the new Fruit class again.

public class Fruit implements Comparable<Fruit>{
 
 private String fruitName;
 private String fruitDesc;
 private int quantity;
 
 public Fruit(String fruitName, String fruitDesc, int quantity) {
  super();
  this.fruitName = fruitName;
  this.fruitDesc = fruitDesc;
  this.quantity = quantity;
 }
 
 public String getFruitName() {
  return fruitName;
 }
 public void setFruitName(String fruitName) {
  this.fruitName = fruitName;
 }
 public String getFruitDesc() {
  return fruitDesc;
 }
 public void setFruitDesc(String fruitDesc) {
  this.fruitDesc = fruitDesc;
 }
 public int getQuantity() {
  return quantity;
 }
 public void setQuantity(int quantity) {
  this.quantity = quantity;
 }
 
 public int compareTo(Fruit compareFruit) {
 
  int compareQuantity = ((Fruit) compareFruit).getQuantity(); 
 
  //ascending order
  return this.quantity - compareQuantity;
 
  //descending order
  //return compareQuantity - this.quantity;
 
 } 
}

The new Fruit class implemented the Comparable interface, and overrided the compareTo() method to compare its quantity property in ascending order.

The compareTo() method is hard to explain, in integer sorting, just remember

1. this.quantity – compareQuantity is ascending order.
2. compareQuantity – this.quantity is descending order.

To understand more about compareTo() method, read this Comparable documentation.

Run it again, now the Fruits array is sort by its quantity in ascending order.

fruits 1 : Pineapple, Quantity : 70
fruits 2 : Orange, Quantity : 80
fruits 3 : Banana, Quantity : 90
fruits 4 : Apple, Quantity : 100

4. Sort an Object with Comparator

How about sorting with Fruit’s “fruitName” or “Quantity”? The Comparable interface is only allow to sort a single property. To sort with multiple properties, you need Comparator. See the new updated Fruit class again :

import java.util.Comparator;
 
public class Fruit implements Comparable<Fruit>{
 
 private String fruitName;
 private String fruitDesc;
 private int quantity;
 
 public Fruit(String fruitName, String fruitDesc, int quantity) {
  super();
  this.fruitName = fruitName;
  this.fruitDesc = fruitDesc;
  this.quantity = quantity;
 }
 
 public String getFruitName() {
  return fruitName;
 }
 public void setFruitName(String fruitName) {
  this.fruitName = fruitName;
 }
 public String getFruitDesc() {
  return fruitDesc;
 }
 public void setFruitDesc(String fruitDesc) {
  this.fruitDesc = fruitDesc;
 }
 public int getQuantity() {
  return quantity;
 }
 public void setQuantity(int quantity) {
  this.quantity = quantity;
 }
 
 public int compareTo(Fruit compareFruit) {
 
  int compareQuantity = ((Fruit) compareFruit).getQuantity(); 
 
  //ascending order
  return this.quantity - compareQuantity;
 
  //descending order
  //return compareQuantity - this.quantity;
 
 }
 
 public static Comparator<Fruit> FruitNameComparator 
                          = new Comparator<Fruit>() {
 
     public int compare(Fruit fruit1, Fruit fruit2) {
 
       String fruitName1 = fruit1.getFruitName().toUpperCase();
       String fruitName2 = fruit2.getFruitName().toUpperCase();
 
       //ascending order
       return fruitName1.compareTo(fruitName2);
 
       //descending order
       //return fruitName2.compareTo(fruitName1);
     }
 
 };
}

The Fruit class contains a static FruitNameComparator method to compare the “fruitName”. Now the Fruit object is able to sort with either “quantity” or “fruitName” property. Run it again.
1. Sort Fruit array based on its “fruitName” property in ascending order.

Arrays.sort(fruits, Fruit.FruitNameComparator);

Output

fruits 1 : Apple, Quantity : 100
fruits 2 : Banana, Quantity : 90
fruits 3 : Orange, Quantity : 80
fruits 4 : Pineapple, Quantity : 70

2. Sort Fruit array based on its “quantity” property in ascending order.

Arrays.sort(fruits)

Output

fruits 1 : Pineapple, Quantity : 70
fruits 2 : Orange, Quantity : 80
fruits 3 : Banana, Quantity : 90
fruits 4 : Apple, Quantity : 100

The java.lang.Comparable and java.util.Comparator are powerful but take time to understand and make use of it, may be it’s due to the lacking of detail example.

My thoughts…

In future, Arrays class should provides more generic and handy method – Arrays.sort(Object, String, flag).
To sort a object array by its “fruitName” in ascending order.

Arrays.sort(fruits, fruitName, Arrays.ASCENDING);

To sort a object array by its “quantity” in ascending order.

Arrays.sort(fruits, quantity, Arrays.DESCENDING);