Sorting LinkedHashMap by value using Comparable interface in Java

LinkedHashMap is a general class used to implement the Map interface. Furthermore, it is a subclass of HashMap class, so it can use all methods of HashMap class and perform similar operations.

Java provides multiple methods to sort LinkedHashMap, we will learn how to create it using the Comparable interface and sort it by its value through this article.

Program to sort LinkedHashMap by value

Before jumping directly to the sorting program, let’s look at a few concepts -

LinkedHashMap

As we discussed before, the LinkedHashMap class extends the HashMap class to implement the Map interface. It maintains key-value pairs. Key is an object used to get and receive the value associated with it. It stores the mapped elements in the LinkedList in the order of insertion, i.e. it maintains the insertion order of the elements. Furthermore, whenever we return its elements, it is printed in insertion order.

The general syntax of LinkedHashMap is as follows -

grammar

LinkedHashMap< TypeOfKey, TypeOfValue > nameOfMap = new LinkedHashMap<>();

In the above syntax,

TypeOfKey − Specifies the data type of the key.

TypeOfValue − Specifies the data type of the value to be stored in the map.

nameOfMap − Give your map an appropriate name.

Similar interface

Java provides a variety of sorting algorithms and methods that can help us sort arrays, lists, or any collection. Comparable interface is a very useful additional method when we want to sort custom objects in natural order. For example, it sorts strings lexicographically and numbers numerically. This interface is available in the "java.lang" package.

grammar

class nameOfclass implements Comparable<nameOfclass>

compareTo() method

The Comparable interface only defines a method called "CompareTo" which can be overridden to sort a collection of objects. It provides the ability to compare objects of a class with itself. It returns 0 when the "this" object is equal to the passed object, a positive value if the "this" object is greater, and a negative value otherwise.

grammar

compareTo(nameOfclass nameOfobject); 

Collections.sort() method

The "Collections" class of the collection interface provides a static method named "Collections.sort()" that can sort the elements of a specified collection (such as ArrayList or LinkedList). It is available in the "java.util" package.

grammar

Collections.sort(nameOfcollection);

algorithm

• Step 1 - Create a class 'Cart' that implements the Comparable interface. Inside the class, declare two variables and define a constructor that takes two parameters 'item' and 'price', of type string and double precision float respectively.

• Step 2 - Further, we will convert the object’s data into a string using the “toString()” method. Then, define the "compareTo" method with an object of the "Cart" class as parameter to compare the "this" object with the newly created object.

• Step 3 - Now, in the main() method, declare a 'Cart' object of LinkedHashMap class named 'obj' and use the name 'put()' The built-in method of storing object details into it. 'item' is the key and its corresponding value is 'price'.

• Step 4 - Finally, define an ArrayList collection named "SrtList" to store the sorted elements of the LinkedHashMap. Now, pass "obj" as parameter to "Collections.sort()" method to perform sorting operation by value.

Example

import java.util.*;
import java.lang.*;
public class Cart implements Comparable<Cart> {
   String item;
   double price;
   Cart(String item, double price) {
      // this keyword shows these variables belongs to constructor
      this.item = item; 
      this.price = price;
   }
   // method for converting object into string
   public String toString() {
      return "Item: " + item + ", " + "Price: " + price;
   }
   public String getName() { 
      // to retrieve item name
      return this.item;
   }
   // overriding method
   public int compareTo(Cart comp) {
      if(this.price > comp.price) {
         return 1;
      } else {
         return -1;
      }
   }
   public static void main(String[] args) {
      // Declaring collection LinkedHashMap
      LinkedHashMap<String, Cart> obj = new LinkedHashMap<>();
      // Adding object to the obj map
      Cart obj1 = new Cart("Rice", 59);
      obj.put(obj1.getName(), obj1);
      Cart obj2 = new Cart("Milk", 60);
      obj.put(obj2.getName(), obj2);
      Cart obj3 = new Cart("Bread", 45);
      obj.put(obj3.getName(), obj3);
       // printing details obj map in unsorted order
      System.out.println("Elements of the map: ");
      for (String unKey : obj.keySet()) {
         System.out.println(obj.get(unKey));
      }
      List<Cart> SrtList = new ArrayList<>(obj.values());
      Collections.sort(SrtList); 
      // Sorting the object
      // printing details of obj map in sorted order
      System.out.println("Elements of the newly sorted map: ");
      System.out.println(SrtList);
   }
}

Output

Elements of the map: 
Item: Rice, Price: 59.0
Item: Milk, Price: 60.0
Item: Bread, Price: 45.0
Elements of the newly sorted map: 
[Item: Bread, Price: 45.0, Item: Rice, Price: 59.0, Item: Milk, Price: 60.0]

in conclusion

In Java 1.0 version, a similar interface was introduced for the first time and provided in the "java.lang" package. In this article, we explored the use of LinkedHashMap and the Comparable interface in sorting operations.

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