How to Fix java.lang.ClassCastException in TreeSet By Using Custom Comparator in Java?

Last Updated : 07 Jan, 2021

This error is thrown by TreeSet because TreeSet is used to store elements in sorted order and if the specified element cannot be compared with the elements currently in the set then TreeSet won’t be able to store elements in sorted order and hence, TreeSet throws class cast exception.

How to fix this error?

By providing a custom Comparator in TreeSet constructor.

In wrapper class like Integer, Double, etc Comparator interface is already implemented, so we don’t need to worry. But whenever we use a user-defined class in TreeSet we have to do either of the above-mentioned ways.

Approach: By providing a custom Comparator in TreeSet constructor

Unlike Comparable, Comparator is external to the element type we are comparing. It’s a separate class. We create multiple separate classes (that implement Comparator) to compare by different members.

Create a class that implements Comparator (and thus the compare() method that does the work previously done by compareTo()).
Make an instance of the Comparator class.
Call the overloaded sort() method, giving it both the list and the instance of the class that implements Comparator.

Java

// Java program to fix java.lang.ClassCastException 
// in TreeSet using Comparator 
  
import java.util.Comparator; 
import java.util.TreeSet; 
  
class friendsMarks { 
    // class field 
    String name; 
    String nickName; 
    int marks; 
  
    // parameterised constructor 
    public friendsMarks(String name, String nickName, 
                        int marks) 
    { 
        this.name = name; 
        this.nickName = nickName; 
        this.marks = marks; 
    } 
  
    // getter for name 
    public String getName() { return name; } 
  
    // setter for name 
    public void setName(String name) { this.name = name; } 
  
    // getter for marks 
    public int getMarks() { return marks; } 
  
    // setter for marks 
    public void setMarks(int marks) { this.marks = marks; } 
  
    // getter for nickname 
    public String getNickName() { return nickName; } 
  
    // setter for nickname 
    public void setNickName(String nickName) 
    { 
        this.nickName = nickName; 
    } 
  
    //@Override toString method 
    public String toString() 
    { 
        return "friendsMarks{"
            + "name='" + name + '\'' + ", nickName='"
            + nickName + '\'' + ", marks=" + marks + '}'; 
    } 
} 
  
// for comparing names 
class nameCompare implements Comparator<friendsMarks> { 
  
    @Override
    public int compare(friendsMarks friend1, 
                       friendsMarks friend2) 
    { 
        return friend1.getName().compareTo( 
            friend2.getName()); 
    } 
} 
  
// for comparing marks 
class marksCompare implements Comparator<friendsMarks> { 
  
    @Override
    public int compare(friendsMarks f1, friendsMarks f2) 
    { 
        if (f1.getMarks() > f2.getMarks()) { 
            return 1; 
        } 
        else { 
            return -1; 
        } 
    } 
} 
  
// for comparing nick name 
class nickNameCompare implements Comparator<friendsMarks> { 
  
    @Override
    public int compare(friendsMarks obj1, friendsMarks obj2) 
    { 
        return obj1.getNickName().compareTo( 
            obj2.getNickName()); 
    } 
} 
public class Main { 
  
    public static void main(String[] args) 
    { 
  
        // Creating TreeSet 
        // and we have to pass Comparator object 
        // of marksCompare class 
        // in the TreeSet constructor 
        // so that we can sort according to the marks 
        TreeSet<friendsMarks> treeSet 
            = new TreeSet<>(new marksCompare()); 
  
        // adding elements to TreeSet 
        treeSet.add(new friendsMarks("Raushan", "Chamgader", 99)); 
        treeSet.add(new friendsMarks("Yashdeep", "Dopa", 95)); 
        treeSet.add(new friendsMarks("Rupesh", "Gian", 92)); 
        treeSet.add(new friendsMarks("Shivam", "Gorilla", 47)); 
        treeSet.add(new friendsMarks("Sarthak", "Nagin", 78)); 
        treeSet.add(new friendsMarks("Sonika", "Chipkali", 67)); 
        treeSet.add(new friendsMarks("Himanshu", "Lalten", 57)); 
  
        System.out.println("Sorting on the basis of marks"); 
  
        // Displaying using loop 
        for (friendsMarks tree : treeSet) 
            System.out.println(tree); 
  
        // Creating TreeSet 
        // and we have to pass Comparator object 
        // of nameCompare class 
        // in the TreeSet constructor 
        // so that we can sort according to the name 
        TreeSet<friendsMarks> treeSet1 
            = new TreeSet<>(new nameCompare()); 
  
        // adding elements to TreeSet 
        treeSet1.add(new friendsMarks("Raushan", "Chamgader", 99)); 
        treeSet1.add(new friendsMarks("Yashdeep", "Dopa", 95)); 
        treeSet1.add(new friendsMarks("Rupesh", "Gian", 92)); 
        treeSet1.add(new friendsMarks("Shivam", "Gorilla", 47)); 
        treeSet1.add(new friendsMarks("Sarthak", "Nagin", 78)); 
        treeSet1.add(new friendsMarks("Sonika", "Chipkali", 67)); 
        treeSet1.add(new friendsMarks("Himanshu", "Lalten", 57)); 
  
        // Displaying using loop 
        System.out.println("\n\nSorting on the basis of name"); 
  
        for (friendsMarks tree : treeSet1) 
            System.out.println(tree); 
  
        // Creating TreeSet 
        // and we have to pass Comparator object 
        // of nickNameCompare class 
        // in the TreeSet constructor 
        // so that we can sort according to the nickname 
        TreeSet<friendsMarks> treeSet2 
            = new TreeSet<>(new nickNameCompare()); 
  
        // adding elements to TreeSet 
        treeSet2.add(new friendsMarks("Raushan", "Chamgader", 99)); 
        treeSet2.add(new friendsMarks("Yashdeep", "Dopa", 95)); 
        treeSet2.add(new friendsMarks("Rupesh", "Gian", 92)); 
        treeSet2.add(new friendsMarks("Shivam", "Gorilla", 47)); 
        treeSet2.add(new friendsMarks("Sarthak", "Nagin", 78)); 
        treeSet2.add(new friendsMarks("Sonika", "Chipkali", 67)); 
        treeSet2.add(new friendsMarks("Himanshu", "Lalten", 57)); 
  
        // Displaying using loop 
        System.out.println("\n\nSorting on the basis of nick-name"); 
  
        for (friendsMarks tree : treeSet2) 
            System.out.println(tree); 
    } 
}

Output

Sorting on the basis of marks
friendsMarks{name='Shivam', nickName='Gorilla', marks=47}
friendsMarks{name='Himanshu', nickName='Lalten', marks=57}
friendsMarks{name='Sonika', nickName='Chipkali', marks=67}
friendsMarks{name='Sarthak', nickName='Nagin', marks=78}
friendsMarks{name='Rupesh', nickName='Gian', marks=92}
friendsMarks{name='Yashdeep', nickName='Dopa', marks=95}
friendsMarks{name='Raushan', nickName='Chamgader', marks=99}


Sorting on the basis of name
friendsMarks{name='Himanshu', nickName='Lalten', marks=57}
friendsMarks{name='Raushan', nickName='Chamgader', marks=99}
friendsMarks{name='Rupesh', nickName='Gian', marks=92}
friendsMarks{name='Sarthak', nickName='Nagin', marks=78}
friendsMarks{name='Shivam', nickName='Gorilla', marks=47}
friendsMarks{name='Sonika', nickName='Chipkali', marks=67}
friendsMarks{name='Yashdeep', nickName='Dopa', marks=95}


Sorting on the basis of nick-name
friendsMarks{name='Raushan', nickName='Chamgader', marks=99}
friendsMarks{name='Sonika', nickName='Chipkali', marks=67}
friendsMarks{name='Yashdeep', nickName='Dopa', marks=95}
friendsMarks{name='Rupesh', nickName='Gian', marks=92}
friendsMarks{name='Shivam', nickName='Gorilla', marks=47}
friendsMarks{name='Himanshu', nickName='Lalten', marks=57}
friendsMarks{name='Sarthak', nickName='Nagin', marks=78}

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