Java lang classcastexception как исправить

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.

1. ClassCastException(): It is used to create an instance of the ClassCastException class.
2. 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

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. 

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:

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. 

1. public static String valueOf(boolean boolValue)
2. public static String valueOf(char charValue)
3. public static String valueOf(char[] charArrayValue)
4. public static String valueOf(int intValue)
5. public static String valueOf(long longValue)
6. public static String valueOf(float floatValue)
7. public static String valueOf(double doubleValue)
8. 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.

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

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.

Harley-Davidson
From bike  class...0.0

Fixing ClassCastException in an upcasting way and at the same time, loss of data also occurs

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.

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.

1. ClassCastException(): It is used to create an instance of the ClassCastException class.
2. 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

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. 

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:

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. 

1. public static String valueOf(boolean boolValue)
2. public static String valueOf(char charValue)
3. public static String valueOf(char[] charArrayValue)
4. public static String valueOf(int intValue)
5. public static String valueOf(long longValue)
6. public static String valueOf(float floatValue)
7. public static String valueOf(double doubleValue)
8. 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.

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

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.

Harley-Davidson
From bike  class...0.0

Fixing ClassCastException in an upcasting way and at the same time, loss of data also occurs

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.

An unexcepted, unwanted event that disturbed the normal flow of a program is called Exception. Most of the time exceptions are caused by our program and these are recoverable. Example: If our program requirement is to read data from the remote file locating at the U.S.A. At runtime, if the remote file is not available then we will get RuntimeException saying fileNotFoundException. If fileNotFoundException occurs we can provide the local file to the program to read and continue the rest of the program normally.

There are mainly two types of exception in java as follows:

1. Checked Exception: The exception which is checked by the compiler for the smooth execution of the program at runtime is called a checked exception. In our program, if there is a chance of rising checked exceptions then compulsory we should handle that checked exception (either by try-catch or throws keyword) otherwise we will get a compile-time error.

Examples of checked exceptions are classNotFoundException, IOException, SQLException etc.

2. Unchecked Exception: The exceptions which are not checked by the compiler, whether programmer handling or not such type of exception are called an unchecked exception.

Examples of unchecked exceptions are ArithmeticException, ArrayStoreException etc.

Whether the exception is checked or unchecked every exception occurs at run time only if there is no chance of occurring any exception at compile time.

ClassCastException: It is the child class of RuntimeException and hence it is an unchecked exception. This exception is rise automatically by JVM whenever we try to improperly typecast a class from one type to another i.e when we’re trying to typecast parent object to child type or when we try to typecast an object to a subclass of which it is not an instance.

In the below program we create an object o of type Object and typecasting that object o to a String object s. As we know that Object class is the parent class of all classes in java and as we’re trying to typecast a parent object to its child type then ultimately we get java.lang.ClassCastException

java.lang.ClassCastException: class java.lang.Object cannot be cast to class java.lang.String (java.lang.Object and java.lang.String are in module java.base of loader 'bootstrap')

In order to deal with ClassCastException be careful that when you’re trying to typecast an object of a class into another class ensure that the new type belongs to one of its parent classes or do not try to typecast a parent object to its child type. While using Collections we can prevent ClassCastException by using generics because generics provide the compile-time checking.

Below is the implementation of the problem statement:

A quick guide and fix to java lang ClassCastException in java with examples.

1. Overview

In this article, we will learn what is java.lang.ClassCastException in java and how to fix it.

We will look at the meaning of ClassCastException and a few examples of it.

2. What is the meaning of ClassCastException

From API, ClassCastException is thrown to indicate that the code has attempted to cast an object to a subclass of which it is not an instance. 

If you are not clear what is ClassCastException then look at the below example.

For example, the following code generates a ClassCastException:

Object x = new Integer(0);
System.out.println((String)x);

In this example, we are trying to cast an Integer object into a String. That means converting the Integer object into a String. This operation produced the class cast exception.

The reason behind this error is there is no direct relation between the Integer and String classes.

If there is a Has-a or Is-a relationship between the two classes then the casting is possible, if there is no relation between the classes and if we try to cast it to another class or interface then we get the ClassCastException.

3. Java ClassCastException Examples

A few examples to see in which scenarios we get the class cast exception at runtime.

Example 1

package com.javaprogramto.exception;

public class ClassCastExceptionExample {

	public static void main(String[] args) {

		Object s = "hello";
		Integer i = (Integer) s;
	}
}

Output

Exception in thread "main" java.lang.ClassCastException: class java.lang.String cannot be cast to class java.lang.Integer (java.lang.String and java.lang.Integer are in module java.base of loader 'bootstrap')
	at com.javaprogramto.exception.ClassCastExceptionExample.main(ClassCastExceptionExample.java:8)

Example 2

In this example, class B extends class B that means B is a child of A. Child class object can be assinged to the parent class object but the parenet class object can not be assinged to the child class.

So here we have casted class A instance to class B. This code is resulted in classcastexception.

package com.javaprogramto.exception.classcaseexception;

public class ClassCastExceptionExample {

	public static void main(String[] args) {
		A a = new A();
		B b = (B) a;
	}
}

class A {

}

class B extends A {

}

Output

Exception in thread "main" java.lang.ClassCastException: class com.javaprogramto.exception.classcaseexception.A cannot be cast to class com.javaprogramto.exception.classcaseexception.B (com.javaprogramto.exception.classcaseexception.A and com.javaprogramto.exception.classcaseexception.B are in unnamed module of loader 'app')
	at com.javaprogramto.exception.classcaseexception.ClassCastExceptionExample.main(ClassCastExceptionExample.java:7)

Example 3

package com.javaprogramto.exception.classcaseexception;

import java.util.ArrayList;
import java.util.List;

public class ClassCastExceptionExample2 {

	public static void main(String[] args) {

		List list = new ArrayList<>();

		list.add(10);
		list.add(20);
		list.add("String");
		list.add(30);

		for (int i = 0; i < list.size(); i++) {
			Integer value = (Integer) list.get(i);
			System.out.println(value);
		}
	}
}

Ouptut

10
20
Exception in thread "main" java.lang.ClassCastException: class java.lang.String cannot be cast to class java.lang.Integer (java.lang.String and java.lang.Integer are in module java.base of loader 'bootstrap')
	at com.javaprogramto.exception.classcaseexception.ClassCastExceptionExample2.main(ClassCastExceptionExample2.java:18)

This example is resulted in error becaues of list has string in it and we casted each value of list into Integer. Once it found the string then it is not able to cast into the integer. So, ended up in exception.

4. How to fix ClassCastException

ClassCastException is a unchecked exception that is thrown by the JVM at runtime when you do the invalid or illegal casting is done. 

Finding the ClassCastException at the compile time can not be done as similar to the checked exceptions. 

So, we must be careful when we do the explicit casting to the different types.

To fix ClassCastException, we need to follow the strict generics when working with the collections API.

In the above section example 3, a list is created without generic type which did not show any error at the compile time but it produces the casting exception at the runtime.

Look at the below code how ClassCastException is eliminated at the compile time.

Example 4

package com.javaprogramto.exception.classcaseexception;

import java.util.ArrayList;
import java.util.List;

public class ClassCastExceptionExample2 {

	public static void main(String[] args) {

		List<Integer> list = new ArrayList<>();

		list.add(10);
		list.add(20);
		list.add("String");
		list.add(30);

		for (int i = 0; i < list.size(); i++) {
			Integer value = (Integer) list.get(i);
			System.out.println(value);
		}
	}
}

Output

Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
	The method add(Integer) in the type List<Integer> is not applicable for the arguments (String)

	at com.javaprogramto.exception.classcaseexception.ClassCastExceptionExample2.main(ClassCastExceptionExample2.java:14)

In the above example, the class cast exception is suppressed and caught at the compile time. So we can take out the string from the list.

Another way is to fix class cast exception is with instanceof oeprator.

Example 5

package com.javaprogramto.exception.classcaseexception;

public class ClassCastExceptionExample3 {

	public static void main(String[] args) {
		C c = new C();

		if (c instanceof D) {
			D d = (D) c;
		} else {
			System.out.println("c is not instance of d. skipping..");
		}
	}
}

class C {

}

class D extends C {

}

Output

c is not instance of d. skipping..

If you know any other ways, please post in the comments we will update the post.

5. Conclusion

In this tutorial, We’ve seen understood the meaning of Java ClassCastException which is present in the java.lang package.

how to fix java.lang.classcaseexception in java.

In this article we’ll see what is java.lang.ClassCastException and how to resolve ClassCastException in Java.

ClassCastException in Java

ClassCastException is thrown in a Java application when you try to cast an object to a class of which the original object is not an instance i.e. if the class
of which you have an object and the class you are trying to cast that object to are not having a parent-child relationship then casting to that class will result
in ClassCastException.

Exception hierarchy for the ClassCastException in Java is as follows-

Since ClassCastException derives from RuntimeException that means it is an unchecked exception and there is no need to explicitly handle it using try-catch block.

ClassCastException is closely associated to how type casting is done. You can refer this post Type Casting in Java to get better understanding of type casting in Java.

ClassCastException was quite frequently seen before Java generics came into the picture. When collections were not type safe and you needed to explicit cast the elements when retrieving them from the
collection was the fertile ground for the ClassCastException.

ClassCastException Java example

This example shows a scenario where ClassCastException is thrown. Here I have a class hierarchy where Payment is an interface and there are two classes CashPayment
and CardPayment implementing the Payment interface.

Payment interface

public interface Payment {
 public boolean proceessPayment(double amount);
}

CashPayment class

import org.netjs.examples.interfaces.Payment;

public class CashPayment implements Payment {
 @Override
 public boolean proceessPayment(double amount) {
  System.out.println("Cash payment done for Rs. " + amount);
  return true;
 }
}

CardPayment class

import org.netjs.examples.interfaces.Payment;

public class CardPayment implements Payment {

 @Override
 public boolean proceessPayment(double amount) {
  System.out.println("Card Payment done for Rs. " + amount);
  return true;
 }
 
 public void printSlip(){
  System.out.println("Printing slip for payment" );
 }
}

Note that in CardPayment class there is an extra method printSlip().

You do want to follow proper object oriented programming so you refer the instances of child classes CardPayment and CashPayment through the super type instance
Payment. Only to call printSlip() method you will need downcasting to the child class.

import org.netjs.examples.interfaces.Payment;

public class PaymentDemo {

 public static void main(String[] args) {
  PaymentDemo pd = new PaymentDemo();
  Payment payment = new CashPayment();
  pd.doPayment(payment, 100);
  payment = new CardPayment();
  pd.doPayment(payment, 300);
 }
 
 public void doPayment(Payment pd, int amt){
  pd.proceessPayment(amt);
  //downcasting
  CardPayment cardPay = (CardPayment)pd;
  cardPay.printSlip();
 }
}

Running this will throw ClassCastException at run time-

Cash payment done for Rs. 100.0
Exception in thread "main" java.lang.ClassCastException: org.netjs.examples.impl.CashPayment cannot be cast to org.netjs.examples.impl.CardPayment
 at org.netjs.examples.impl.PaymentDemo.doPayment(PaymentDemo.java:17)
 at org.netjs.examples.impl.PaymentDemo.main(PaymentDemo.java:10)

When instance of CardPayment is passed to Payment reference there is no problem in downcasting it. There is a problem when instance of CashPayment is passed,
widening the reference to Payment is ok but attempting to cast it to CardPayment is not possible as there is no parent-child relationship between these two classes.
Both of them are child classes of Payment and instances of both of these classes are of type Payment but not interchangeable with each other.

Resolving ClassCastException in Java

To ensure that ClassCastException is not thrown you should check for the type of instance using instanceof operator in Java.

public class PaymentDemo {

 public static void main(String[] args) {
  PaymentDemo pd = new PaymentDemo();
  Payment payment = new CashPayment();
  pd.doPayment(payment, 100);
  payment = new CardPayment();
  pd.doPayment(payment, 300);
 }
 
 public void doPayment(Payment pd, int amt){
  pd.proceessPayment(amt);
  if (pd instanceof CardPayment){
   CardPayment cardPay = (CardPayment)pd;
   cardPay.printSlip();
  }
 }
}

Output

Cash payment done for Rs. 100.0
Card Payment done for Rs. 300.0
Printing slip for payment

Here note how instanceof operator is used to check the type and casting is done only if correct instance type is found thus avoiding ClassCastException.

That’s all for this topic java.lang.ClassCastException — Resolving ClassCastException in Java. If you have any doubt or any suggestions to make please drop a comment. Thanks!

Related Topics

1. java.lang.ClassNotFoundException — Resolving ClassNotFoundException in Java
2. Exception Propagation in Java Exception Handling
3. finally Block in Java Exception Handling
4. final Vs finally Vs finalize in Java
5. Java Exception Handling Interview Questions And Answers

You may also like-

1. matches() method in Java String
2. Inheritance in Java
3. AutoBoxing And UnBoxing in Java
4. Reflection in Java — Getting Constructor Information
5. Enum Type in Java
6. Matrix Addition Java Program
7. Spring NamedParameterJdbcTemplate Select Query Example
8. Magic Methods in Python With Examples