How to solve java.lang.ClassCastException issue while using TreeMap in Java?

TreeMap is a class of Java Collection Framework that implements the NavigableMap interface. It stores the map's elements in a tree structure and provides an efficient alternative to storing key-value pairs in sorted order. Please note that while creating a TreeMap object, one must use the Comparable interface or the Comparator interface so that we can maintain the sort order of its elements, otherwise, we will encounter a java.lang.ClassCastException. In this article, we will explain how to use Comparable and Comparator interfaces to resolve ClassCastException issues in TreeMap

Fix the java.lang.ClassCastException problem in TreeMap

Let us start the discussion with a sample program that will show us ClassCastException in TreeMap.

The Chinese translation of

Example 1

is:

Example 1

In the following example, we will try to add a custom class object to the TreeMap without using the Comparable and Comparator interfaces to demonstrate the situation when the Java compiler throws java.lang.ClassCastException.

import java.util.*;
public class TrMap {
   String item;
   int price;
   TrMap(int price, String item) {
	// 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 static void main(String[] args) {
      // Declaring collection TreeMap
     TreeMap<TrMap, Integer> obj = new TreeMap<>();
      // Adding object to the obj map
      obj.put(new TrMap(495, "TShirt"), 1);
      obj.put(new TrMap(660, "Shirt"), 2);
      // printing details obj map
      System.out.println("Elements of the map: " + obj);
   }
}

The Chinese translation of

Output

is:

Output

Exception in thread "main" java.lang.ClassCastException: class TrMap cannot be cast to class java.lang.Comparable (TrMap is in unnamed module of loader 'app'; java.lang.Comparable is in module java.base of loader 'bootstrap')
	at java.base/java.util.TreeMap.compare(TreeMap.java:1569)
	at java.base/java.util.TreeMap.addEntryToEmptyMap(TreeMap.java:776)
	at java.base/java.util.TreeMap.put(TreeMap.java:785)
	at java.base/java.util.TreeMap.put(TreeMap.java:534)
	at TrMap.main(TrMap.java:18)

How to use Comparable interface to fix java.lang.ClassCastException error

Let us start the discussion by introducing the Comparable interface

Comparable interface

This interface is very useful when we want to sort custom objects in natural order. For example, it sorts strings in lexicographic order and numbers in numerical order. This interface is available in the 'java.lang' package. Generally, the classes and interfaces defined in this package are available to us by default, so there is no need to import this package explicitly.

Grammar

class nameOfclass implements Comparable<nameOfclass>

Here, class is the keyword to create a class, and implements is the keyword to enable the features provided by the interface.

compareTo() is translated as:

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.

Grammar

compareTo(nameOfclass nameOfobject);

The Chinese translation of

Example 2

is:

Example 2

The following example demonstrates the use of Comparable in repairing ClassCastException.

method

• Create a class 'TrMap' that implements the Comparable interface. Inside the class, declare two variables and define a constructor with two parameters of type string and double 'item' and 'price'.

• Moving further, we will use the 'toString()' method to convert the object's data to a string. Then, define the 'compareTo' method and take an object of class 'TrMap' as a parameter to compare the 'this' object with the newly created object

• Now, in the main() method, declare an object of TreeMap class named 'obj' and store the object details into it using the inbuilt method named 'put()'. 'item' is the key and its corresponding value is 'price'.

• Finally, use the 'keySet()' method in a for-each loop to retrieve and print the value associated with the key.

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));
      }
   }
}

The Chinese translation of

Output

is:

Output

Elements of the map: 
Item: Kurti, Price: 455
Item: Shirt, Price: 660
Item: TShirt, Price: 495

How to use Comparator to fix java.lang.ClassCastException problem

First, let us introduce the Comparator interface.

Comparators

As its name suggests, it is used to compare something. In Java, Comparator is an interface for sorting custom objects. We can write our own logic to sort the specified objects in its built-in method named 'compare()'. This method accepts two objects as parameters and returns an integer value. Through this integer value, the Comparator determines which object is larger

Grammar

class nameOfComparator implements Comparator< TypeOfComparator >() {
	compare( type object1, type object2 ) {
		// logic for comparison
	}
}    

The Chinese translation of

Example 3

is:

Example 3

The following example demonstrates the use of Comparator when repairing ClassCastException.

method

• First, import the 'java.util' package so that we can use TreeSet

• Create a class named 'TrMap'. Inside the class, declare two variables and define a constructor that has two parameters 'item' and 'price', which are string type and integer type respectively.

• Moving further, we will use the 'toString()' method to convert the object's data to a string

• Then, define another class 'Comp' that implements the Comparator interface, in which the 'compare()' method is used to sort the TreeMap in ascending order.

• In the 'main()' method, create a TreeMap collection by passing an instance of the 'Comp' class for sorting

• Finally, use the 'put()' method to store some elements into the TreeMap collection and then print the result.

import java.util.*;
class TrMap {
   String item;
   int price;
   TrMap(int price, String item) {
	// 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;
   }
}
// use of comparator interface
class Comp implements Comparator<TrMap> {
   // logic to sort
   public int compare(TrMap i, TrMap j) {
      if(i.price > j.price) {
         return 1;
      } else {
         return -1;
      }
   }
}
public class Example2 {
   public static void main(String[] args) {
      // Declaring collection TreeMap
      TreeMap<TrMap, Integer> obj = new TreeMap<>(new Comp());
      // Adding object to the obj map
      obj.put(new TrMap(495, "TShirt"), 1);
      obj.put(new TrMap(660, "Shirt"), 2);
       // printing details obj map
      System.out.println("Elements of the map: " + obj);
   }
}

Output

的中文翻译为：

输出

Elements of the map: {Item: TShirt, Price: 495=1, Item: Shirt, Price: 660=2}

结论

在本文中，我们首先定义了TreeMap类，然后介绍了TreeMap中的ClassCastException。在下一部分中，我们讨论了可以帮助解决这个ClassCastException的Comparator和Comparable接口。然后，我们看到了三个示例程序，展示了ClassCastException以及如何修复这个异常。

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