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How to Create TreeMap Objects using Comparable Interface in Java?
TreeMap is a class of Java Collection Framework that implements NavigableMap Interface. It stores the elements of the map in a tree structure and provides an efficient alternative to store the key-value pairs in sorted order. Note that while creating objects of TreeMap we need to use the comparable interface so that we can maintain the sorting order of its elements. In this article, we are going to discuss a few Java programs to create TreeMap objects using comparable interface.
Java Program to create TreeMap Objects using Comparable Interface
Before jumping to the Java program directly, let's have a brief look at the basics of Comparable Interface.
Comparable Interface
This interface is useful when we want to sort custom objects by their natural ordering. For example, It sorts strings in dictionary order and numerics in numerical order. This interface is available in 'java.lang' package. Generally, classes and interfaces defined in this package are by default available for our use therefore, it is not necessary to import this package explicitly.
Syntax
class nameOfclass implements Comparable
Here, class is the keyword that creates a Class and implements is the keyword that enables the use of features provided by an Interface.
compareTo()
The Comparable interface defines only a single method named 'CompareTo' that can be overridden in order to sort the collection of objects. It gives the power to compare the objects of a class to itself. It returns 0 when 'this' object is equal to the passed object, a positive value if 'this' object is greater otherwise a negative value.
Syntax
compareTo(nameOfclass nameOfobject);
Since a TreeMap stores its element in the form of the key-value pairs in sorted order, therefore, we need to use the Comparable Interface otherwise we will encounter a ClassCastException.
Example
The following example illustrates the use of Comparable Interface in creating TreeMap.
Approach
Create a class 'TrMap' that implements Comparable Interface. Inside it, declare two variables and define a constructor of this class along with two parameters 'item' and 'price' of type string and double respectively.
Moving further we will convert the data of object into string using 'toString()' method. Then, define 'compareTo' method along with an object of class 'TrMap' as a parameter to compare 'this' object with the newly created object.
Now, in the main() method, declare an object named 'obj' of class 'TrMap' of TreeMap and using an inbuilt method named 'put()' store the details of object to it. The 'item' is key and its corresponding value is 'price'.
In the end, use the 'keySet()' method inside a for-each loop to retrieve and print the values associated with keys.
Example
import java.util.*;
import java.lang.*;
public class TrMap implements Comparable<TrMap> {
String item;
int price;
TrMap(String item, int price) {
// this keyword shows these variables belong 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() {
return this.item;
}
// overriding method
public int compareTo(TrMap comp) {
return this.item.compareTo(comp.item);
}
public static void main(String[] args) {
// Declaring collection TreeMap
TreeMap<String, TrMap> obj = new TreeMap<>();
// Adding object to the obj map
TrMap obj1 = new TrMap("TShirt", 495);
obj.put(obj1.getName(), obj1);
TrMap obj2 = new TrMap("Shirt", 660);
obj.put(obj2.getName(), obj2);
TrMap obj3 = new TrMap("Kurti", 455);
obj.put(obj3.getName(), obj3);
// printing details obj map
System.out.println("Elements of the map: ");
for (String unKey : obj.keySet()) {
System.out.println(obj.get(unKey));
}
}
}
Output
Elements of the map: Item: Kurti, Price: 455 Item: Shirt, Price: 660 Item: TShirt, Price: 495
In the above output, the elements are sorted based on the Item name.
Conclusion
We started this article by defining the TreeMap class of Java Collection Framework and in later sections, we introduced the Comparable Interface and its in-built method named compareTo. In the end, we discussed a Java program to get a better understanding of the use of Comparable Interface in creating TreeMap.
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