In this tutorial, it shows the use of java.lang.Comparable andjava.util.Comparator to sort a Java object based on its property value.
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;
}
}
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());
}
}
}
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)
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;
}
}
- this.quantity – compareQuantity is ascending order.
- 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 needComparator. 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);
}
};
}
1. Sort Fruit array based on its “fruitName” property in ascending order.
Arrays.sort(fruits, Fruit.FruitNameComparator);
fruits 1 : Apple, Quantity : 100
fruits 2 : Banana, Quantity : 90
fruits 3 : Orange, Quantity : 80
fruits 4 : Pineapple, Quantity : 70
Arrays.sort(fruits)
fruits 1 : Pineapple, Quantity : 70
fruits 2 : Orange, Quantity : 80
fruits 3 : Banana, Quantity : 90
fruits 4 : Apple, Quantity : 100
Reference
FROM: http://www.mkyong.com/java/java-object-sorting-example-comparable-and-comparator/