ClassCastException in java

In this article, we will understand what is ClassCastException in java, what is its cause, how to avoid it and handle it with examples.

Overview
ClassCastException belongs to java.lang package and is thrown when objects are casted to some other types.
It is an unchecked exception and so it extends java.lang.RuntimeException class in java.
Cause
ClassCastException is thrown in any of the below cases

Trying to cast a parent or super class object to child class. This is also called downcasting.
Below is a program example of this case
```
class Parent {

}

class Child extends Parent {
   
}

// below two lines will raise exception
Parent o = new Parent();
Parent o1 = (Child)o;
```
Result is

Exception in thread “main” java.lang.ClassCastException: class Parent cannot be cast to class Child
Trying to cast an object of one type to another when they are no way related to each other.
Example of such a scenario is
```
List l = new ArrayList<>();
l.add(5);
l.add("A");
// cast integer to string
String s = (String)l.get(0);
```
Output will be

Exception in thread “main” java.lang.ClassCastException: class java.lang.Integer cannot be cast to class java.lang.String

Since we are trying to cast an element of type Integer to String, when they do not have any relation.

Java docs for ClassCastException state,

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

Though the official docs state downcasting as the reason, but it is also raised due to casting between incompatible types.

How to handle
There are two ways to handle ClassCastException.

Using instanceof operator
Whenever you need to perform type casting, use instanceof operator with if statement.
It returns true if types are compatible and it is safe to cast, otherwise returns false. Example,
```
Parent o = new Parent(); 
if(o instanceof Child) {
  Parent o1 = (Child)o;
}
```

Java exception handling
Surround program statements which are performing type casting with java try-catch block. Example,

List l1 = new ArrayList<>();
l1.add(1);
l1.add("A");
try {
  String s = (String)l1.get(0);
} catch (ClassCastException e) {
  System.out.println("Incompatible casting");
}

Hope the article was useful in clarifying the reason of ClassCastException in java and how to handle it to make an application safe from this error.