ClassCastException In Java

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ClassCastException in java is a run time error it occurs when an object can not be casted to another type.

An object is automatically upcasted to its super class type. You need not to mention class type explicitly. But, when an object is supposed to be downcasted to its sub class type, then you have to mention class type explicitly. In such case, there is a possibility of occurring class cast exception. In most of time, it occurs when you are trying to downcast an object explicitly to its sub class type.

Try to run below program.

package com;
class A
{
	int i = 10;
}

class B extends A
{
	int j = 20;
}

class C extends B
{
	int k = 30;
}

public class ClassCastExceptionDemo
{
	public static void main(String[] args)
	{
		A a = new B();   //B type is auto up casted to A type
		B b = (B) a;     //A type is explicitly down casted to B type.
		C c = (C) b;    //Here, you will get class cast exception
        System.out.println(c.k);
	}
}

You will get ClassCastException. Below is the sample of the error.

Exception in thread “main” java.lang.ClassCastException: com.B cannot be cast to com.C
at com.ClassCastExceptionDemo.main(ClassCastExceptionDemo.java:23)

In the above example, Class B extends Class A and Class C extends Class B. In the main method, Class B-type object is created (Line 21). It will be having two non-static fields. one field (int i) is inherited from class A and another one is its own field (int j). ‘a’ is Class A-type reference variable which will be pointing to this newly created object. In the next statement (Line 22), reference variable ‘a’ is assigned to ‘b’ which is Class B-type reference variable. After execution of this statement, ‘b’ will also be pointing to the same object to which ‘a’ is pointing. In the third statement, ‘b’ is assigned to ‘c’ which is Class C-type reference variable. So, ‘c’ will also be pointing to same object to which ‘a’ and ‘b’ are pointing. While executing this statement, you will get run time exception called Class Cast Exception.

The memory allocation of above program can be diagrammatically represented as,

Why you got this exception?

Every sub class extends its super class. i.e every child class will have some additional properties along with some inherited properties from its parent class. In the above example, Class A has one property (int i). Class B has two properties, one is it’s own and another one is inherited. Class C has three properties. one is it’s own and two are inherited. In this example, Class C-type reference variable is referring to Class B-type object. Class B-type object will be having only two properties. But, through Class C-type reference variable, you can access Class C’s own property (int k) like in the line 24. But, actually this property does not exist in Class B-type object. This creates the confusion. Class B-type can not be casted to Class C-type. That’s why, you will get class cast exception.

Put ClassCastException in simple terms. ClassCastException occurs when code has attempted to cast an object to a type of which it is not an object. In the above example, Class B is a Class A type but Class B is not a Class C type. Therefore, you are getting ClassCastException.

Consider one more case of ClassCastException.

public class ClassCastExceptionDemo
{
	public static void main(String[] args)
	{
		Object o = new String();
		Integer i = (Integer) o;
	}
}

We all know that every class in java is a sub class of java.lang.Object class. String is also a subclass of Obeject class and Integer is also a subclass of Object class. In the above example, String object is created and it is automatically up casted to Object type. Further, this object is explicitly downcasted to Integer type. This causes ClassCastException, because, String object is not an Integer type.

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