How to fix java.lang.ClassCastException while using the TreeMap in Java?
The java.lang.ClassCastException is one of the unchecked exception in Java. It can occur in our program when we tried to convert an object of one class type into an object of another class type.
When we use custom class objects as keys in the TreeMap and neither implements the comparable interface nor comparator interface, then the java.lang.ClassCastException arises.
So there are two ways to fix java.lang.ClassCastException while using the TreeMap in Java:
Method 1: Using Comparable
We can fix java.lang.ClassCastException by the objects used as keys of the TreeMap implement the Comparable interface.
Pseudo Code:
// Custom class Student implements comparable interface
class Student implements Comparable<Student> {
String name;
Integer marks;
public Student(String name, Integer marks) {
this.name = name;
this.marks = marks;
}
// Override toString method
public String toString() {
return this.name + " : " + this.marks;
}
public int getMarks() {
return this.marks;
}
// Override compareTo method that sort treemap in the ascending order of the marks
public int compareTo(Student stu) {
return this.getMarks() - stu.getMarks();
}
}
Implementation:
Java
import java.util.*;
class Student implements Comparable<Student> {
String name;
Integer marks;
public Student(String name, Integer marks)
{
this .name = name;
this .marks = marks;
}
public String toString()
{
return this .name + " : " + this .marks;
}
public int getMarks() { return this .marks; }
public int compareTo(Student stu)
{
return this .getMarks() - stu.getMarks();
}
}
public class GFG {
public static void main(String[] args)
{
TreeMap<Student, Integer> map = new TreeMap<>();
map.put( new Student( "Akshay" , 500 ), 1 );
map.put( new Student( "Bhanu" , 600 ), 2 );
map.put( new Student( "Chetan" , 300 ), 3 );
System.out.println( "The Treemap : " + map);
}
}
|
Output
The Treemap : {Chetan : 300=3, Akshay : 500=1, Bhanu : 600=2}
Method 2: Using Comparator
We can fix java.lang.ClassCastException by provide a custom comparator to the constructor at the creation time of the TreeMap.
Pseudo Code:
// Custom comparator
class MyComparator implements Comparator<Student> {
// Compare method that sort TreeMap in the ascending order of the marks
public int compare(Student stu1, Student stu2) {
return stu1.getMarks() - stu2.getMarks();
}
}
Implementation:
Java
import java.util.*;
class Student {
String name;
Integer marks;
public Student(String name, Integer marks)
{
this .name = name;
this .marks = marks;
}
public String toString()
{
return this .name + " : " + this .marks;
}
public int getMarks() { return this .marks; }
}
class MyComparator implements Comparator<Student> {
public int compare(Student stu1, Student stu2)
{
return stu1.getMarks() - stu2.getMarks();
}
}
public class GFG {
public static void main(String[] args)
{
TreeMap<Student, Integer> map
= new TreeMap<>( new MyComparator());
map.put( new Student( "Akshay" , 500 ), 1 );
map.put( new Student( "Bhanu" , 600 ), 2 );
map.put( new Student( "Chetan" , 300 ), 3 );
System.out.println( "The Treemap : " + map);
}
}
|
Output
The Treemap : {Chetan : 300=3, Akshay : 500=1, Bhanu : 600=2}
Last Updated :
01 Oct, 2021
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