ClassCastException in Java occurs when we try to convert the data type of entry into another. This is related to the type conversion feature and data type conversion is successful only where a class extends a parent class and the child class is cast to its parent class.
Here we can consider parent class as vehicle and child class may be car, bike, cycle etc., Parent class as shape and child class may be 2d shapes or 3d shapes, etc.
Two different kinds of constructors available for ClassCastException.
- ClassCastException(): It is used to create an instance of the ClassCastException class.
- ClassCastException(String s): It is used to create an instance of the ClassCastException class, by accepting the specified string as a message.
Let us see in details
Java
import java.math.BigDecimal;public class ClassCastExceptionExample { public static void main(String[] args) { // Creating a BigDecimal object Object sampleObject = new BigDecimal(10000000.45); System.out.println(sampleObject); }} |
10000000.4499999992549419403076171875
If we try to print this value by casting to different data type like String or Integer etc., we will be getting ClassCastException.
Java
import java.math.BigDecimal;public class Main { public static void main(String[] args) { // Creating a BigDecimal object Object sampleObject = new BigDecimal(10000000.45); // Trying to display the object by casting to String // As the object is created as BigDecimal but tried // to display by casting to String, // ClassCastException is thrown System.out.println((String)sampleObject); }} |
Output :
Exception in thread “main” java.lang.ClassCastException: class java.math.BigDecimal cannot be cast to class java.lang.String (java.math.BigDecimal and java.lang.String are in module java.base of loader ‘bootstrap’)
at Main.main(Main.java:11)
We can fix the exception printing by means of converting the code in the below format:
Java
import java.math.BigDecimal;public class ClassCastExceptionExample { public static void main(String[] args) { // Creating a BigDecimal object Object sampleObject = new BigDecimal(10000000.45); // We can avoid ClassCastException by this way System.out.println(String.valueOf(sampleObject)); }} |
10000000.4499999992549419403076171875
So, in any instances when we try to convert data type of object, we cannot directly downcast or upcast to a specified data type. Direct casting will not work and it throws ClassCastException. Instead, we can use
String.valueOf() method. It converts different types of values like int, long, boolean, character, float etc., into the string.
- public static String valueOf(boolean boolValue)
- public static String valueOf(char charValue)
- public static String valueOf(char[] charArrayValue)
- public static String valueOf(int intValue)
- public static String valueOf(long longValue)
- public static String valueOf(float floatValue)
- public static String valueOf(double doubleValue)
- public static String valueOf(Object objectValue)
are the different methods available and in our above example, the last method is used.
Between Parent and Child class. Example shows that the instance of the parent class cannot be cast to an instance of the child class.
Java
class Vehicle { public Vehicle() { System.out.println( "An example instance of the Vehicle class to proceed for showing ClassCastException"); }}final class Bike extends Vehicle { public Bike() { super(); System.out.println( "An example instance of the Bike class that extends Vehicle as parent class to proceed for showing ClassCastException"); }}public class ClassCastExceptionExample { public static void main(String[] args) { Vehicle vehicle = new Vehicle(); Bike bike = new Bike(); Bike bike1 = new Vehicle(); // Check out for this statement. Tried to convert // parent(vehicle) object to child object(bike). // Here compiler error is thrown bike = vehicle; }} |
Compiler error :
In order to overcome Compile-time errors, we need to downcast explicitly. i.e. downcasting means the typecasting of a parent object to a child object. That means features of parent object lost and hence there is no implicit downcasting possible and hence we need to do explicitly as below way
Giving the snippet where the change requires. In the downcasting Explicit way
Java
// An easier way to understand Downcastingclass Vehicle { String vehicleName; // Method in parent class void displayData() { System.out.println("From Vehicle class"); }}class Bike extends Vehicle { double cost; // Overriding the parent class method and we can // additionally mention about the child class @Override void displayData() { System.out.println("From bike class" + cost); }}public class ClassCastExceptionExample { public static void main(String[] args) { Vehicle vehicle = new Bike(); vehicle.vehicleName = "BMW"; // Downcasting Explicitly Bike bike = (Bike)vehicle; bike.cost = 1000000; // Though vehiclename is not assigned, it takes BMW // as it is System.out.println(bike.vehicleName); System.out.println(bike.cost); bike.displayData(); }} |
BMW 1000000.0 From bike class1000000.0
Upcasting implicit way
An example for upcasting of the child object to the parent object. It can be done implicitly. This facility gives us the flexibility to access the parent class members.
Java
// An easier way to understand Upcastingclass Vehicle { String vehicleName; // Method in parent class void displayData() { System.out.println("From Vehicle class"); }}class Bike extends Vehicle { double cost; // Overriding the parent class method and we can // additionally mention about the child class @Override void displayData() { System.out.println("From bike class..." + cost); }}public class ClassCastExceptionExample { public static void main(String[] args) { // Upcasting Vehicle vehicle = new Bike(); vehicle.vehicleName = "Harley-Davidson"; // vehicle.cost //not available as upcasting done // but originally Vehicle class dont have cost // attribute System.out.println(vehicle.vehicleName); vehicle.displayData(); // Hence here we will get // output as 0.0 }} |
Harley-Davidson From bike class...0.0
Fixing ClassCastException in an upcasting way and at the same time, loss of data also occurs
Java
class Vehicle { public Vehicle() { System.out.println( "An example instance of the Vehicle class to proceed for showing ClassCast Exception"); } public String display() { return "Vehicle class display method"; }}class Bike extends Vehicle { public Bike() { super(); // Vehicle class constructor msg display as // super() is nothing but calling parent // method System.out.println( "An example instance of the Bike class that extends \nVehicle as parent class to proceed for showing ClassCast Exception"); } public String display() { return "Bike class display method"; }}public class ClassCastExceptionExample { public static void main(String[] args) { Vehicle vehicle = new Vehicle(); Bike bike = new Bike(); // But we can make bike point to vehicle, i.e. // pointing child object to parent object This is an // example for upcasting of child object to parent // object. It can be done implicitly This facility // gives us the flexibility to access the parent // class members vehicle = bike; // As upcasted here, vehicle.display() will provide // "Bike class display method" as output It has lost // its parent properties as now vehicle is nothing // but bike only System.out.println( "After upcasting bike(child) to vehicle(parent).." + vehicle.display()); }} |
An example instance of the Vehicle class to proceed for showing ClassCast Exception An example instance of the Vehicle class to proceed for showing ClassCast Exception An example instance of the Bike class that extends Vehicle as parent class to proceed for showing ClassCast Exception After upcasting bike(child) to vehicle(parent)..Bike class display method
Conclusion: By means of either upcasting or downcasting, we can overcome ClassCastException if we are following Parent-child relationships. String.valueOf() methods help to convert the different data type to String and in that way also we can overcome.
