How to Fix java.lang.ClassCastException in TreeSet?

Last Updated : 04 Jan, 2021

TreeSet class in Java implements the Set interface that uses a tree for storing elements which contain unique objects stored in the ascending order. You may come across an exception called java.lang.ClassCastException while working with TreeSet objects. Basically, TreeSet elements are ordered using natural ordering or by using the Comparator defined in the constructor. If both don't happen i.e natural ordering not occurring and also did not provide any comparator then java throws an exception which is java.lang.ClassCastException.

Example

Java

// Java program to demonstrate ClassCastException by TreeSet  import java.util.TreeSet;  // class which is going to assign // student marks class Student {      int marks;      // constructor     public Student(int marks) { this.marks = marks; }      // override toString() method     // for display purpose     public String toString()     {         return "Student marks = " + this.marks;     } }  // Driver class class GFG {     public static void main(String[] args)     {          // Declaring Tree Set         TreeSet<Student> treeSet = new TreeSet<Student>();          // this line will throw java.lang.ClassCastException         treeSet.add(new Student(1));          // Displaying the contents of in treeSet         System.out.println(treeSet);     } }

Output

Exception in thread "main" java.lang.ClassCastException: class Student cannot be cast to class java.lang.Comparable (Student 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:1291)

at java.base/java.util.TreeMap.put(TreeMap.java:536)

at java.base/java.util.TreeSet.add(TreeSet.java:255)

at GFG.main(File.java:31)

We can resolve this exception in two ways:

By implementing the Comparable interface
By defining custom Comparator class

Approach 1(Implementing Comparable Interface)

Java Comparable interface is implemented by a class by which used to compare and sort the objects according to the natural ordering. Natural ordering is possible using compareTo() function. String objects and wrapper class objects are sorted according to the built-in compareTo() function.

If compareTo function returns positive or negative or zero, then the current object is greater, lesser, and equal to the provided object respectively.

Example 1:

Java

// Java program to sort student data // according to marks // using Comparable interface  import java.util.TreeSet;  // class which is going to assign // student marks class Student implements Comparable<Student> {      int id;     String name;     int marks;      // constructor     public Student(int id, String name, int marks)     {          // assigning values         this.id = id;         this.name = name;         this.marks = marks;     }      // compareTo method to sort in     // ascending order     public int compareTo(Student obj)     {         return this.marks - obj.marks;     }      // override toString() method     // for display purpose     public String toString()     {         return "Id: " + this.id + " Name: " + this.name             + " Marks: " + this.marks;     } }  // Driver class class GFG {     public static void main(String[] args)     {          // Declaring Tree Set         TreeSet<Student> treeSet = new TreeSet<Student>();          treeSet.add(new Student(1, "Suresh", 87));         treeSet.add(new Student(2, "Ramesh", 78));         treeSet.add(new Student(3, "Lokesh", 95));          // Displaying the contents of in treeSet         System.out.println(treeSet);     } }

Output

[Id: 2 Name: Ramesh Marks: 78, Id: 1 Name: Suresh Marks: 87, Id: 3 Name: Lokesh Marks: 95]

Approach 2(Using Custom Comparator class)

A comparator is an interface that has to implemented by the class by which we can sort the objects of the user-defined class. It has 2 main methods that are used widely, compare(T o1, T o2) and equals(Object obj) which returns an int and boolean respectively. Let us implement the same example using a comparator.