How to Fix java.lang.ClassCastException in TreeSet? - GeeksforGeeks

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 marksclass 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 classclass 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 interfaceBy defining custom Comparator class
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 marksclass 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 classclass 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);    }}
[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.
Java
// Java program to sort student data// according to marks using custom class// which implements Comparator  // comparator interface present in// java.util packageimport java.util.*;// class which is going to assign// student marksclass 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;    }      // method to return    // current marks of student    public int getMarks() { return this.marks; }      // override toString() method    // for display purpose    public String toString()    {        return "Id: " + this.id + " Name: " + this.name            + " Marks: " + this.marks;    }}  // StuComparator class will compare// objects ans sorts in// ascending orderclass StuComparator implements Comparator<Student> {      // defining compare method    public int compare(Student obj1, Student obj2)    {        return obj1.getMarks() - obj2.getMarks();    }}  // Driver classclass GFG {    public static void main(String[] args)    {          // Declaring Tree Set        TreeSet<Student> treeSet            = new TreeSet<Student>(new StuComparator());          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);    }}
[Id: 2 Name: Ramesh Marks: 78, Id: 1 Name: Suresh Marks: 87, Id: 3 Name: Lokesh Marks: 95]
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