Explanation of “ClassCastException” in Java

Question

I read some articles written on "ClassCastException", but I couldn't get a good idea on that. Is there a good article or what would be a brief explanation?

Answer 1

A very good example that I can give you for classcastException in Java is while using "Collection"

List list = new ArrayList();
list.add("Java");
list.add(new Integer(5));

for(Object obj:list) {
    String str = (String)obj;
}

This above code will give you ClassCastException on runtime. Because you are trying to cast Integer to String, that will throw the exception.

Answer 2

You can better understand ClassCastException and casting once you realize that the JVM cannot guess the unknown. If B is an instance of A it has more class members and methods on the heap than A. The JVM cannot guess how to cast A to B since the mapping target is larger, and the JVM will not know how to fill the additional members.

But if A was an instance of B, it would be possible, because A is a reference to a complete instance of B, so the mapping will be one-to-one.

Answer 3

Straight from the API Specifications for the ClassCastException:

Thrown to indicate that the code has attempted to cast an object to a subclass of which it is not an instance.

So, for example, when one tries to cast an Integer to a String, String is not an subclass of Integer, so a ClassCastException will be thrown.

Object i = Integer.valueOf(42);
String s = (String)i;            // ClassCastException thrown here.

Answer 4

A class cast exception is thrown by Java when you try to cast an Object of one data type to another.

Java allows us to cast variables of one type to another as long as the casting happens between compatible data types.

For example you can cast a String as an Object and similarly an Object that contains String values can be cast to a String.

Example

Let us assume we have an HashMap that holds a number of ArrayList objects.

If we write code like this:

String obj = (String) hmp.get(key);

it would throw a class cast exception, because the value returned by the get method of the hash map would be an Array list, but we are trying to cast it to a String. This would cause the exception.

Answer 5

Consider an example,

class Animal {
    public void eat(String str) {
        System.out.println("Eating for grass");
    }
}

class Goat extends Animal {
    public void eat(String str) {
        System.out.println("blank");
    }
}

class Another extends Goat{
  public void eat(String str) {
        System.out.println("another");
  }
}

public class InheritanceSample {
    public static void main(String[] args) {
        Animal a = new Animal();
        Another t5 = (Another) new Goat();
    }
}

At Another t5 = (Another) new Goat(): you will get a ClassCastException because you cannot create an instance of the Another class using Goat.

Note: The conversion is valid only in cases where a class extends a parent class and the child class is casted to its parent class.

How to deal with the ClassCastException:

1. Be careful when trying to cast an object of a class into another class. Ensure that the new type belongs to one of its parent classes.
2. You can prevent the ClassCastException by using Generics, because Generics provide compile time checks and can be used to develop type-safe applications.

Source of the Note and the Rest