Kotlin ClassCastException

Introduction

In Kotlin, ClassCastException is a runtime exception that is thrown when an attempt is made to cast an object to a type with which it is not compatible. This exception is part of the Kotlin (and Java) standard library and usually indicates a programming error where the cast is not valid.

Table of Contents

1. What is ClassCastException?
2. Common Causes of ClassCastException
3. Handling ClassCastException
4. Examples of ClassCastException
5. Real-World Use Case
6. Conclusion

1. What is ClassCastException?

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

Syntax

throw ClassCastException("Exception message")

2. Common Causes of ClassCastException

• Incorrect type casting
• Using unsafe type casts
• Misusing generics

Example

fun main() {
    val obj: Any = "Kotlin"
    val num: Int = obj as Int // This will cause ClassCastException
}

3. Handling ClassCastException

You can handle ClassCastException using a try-catch block to prevent your program from crashing.

Example

fun main() {
    try {
        val obj: Any = "Kotlin"
        val num: Int = obj as Int
    } catch (e: ClassCastException) {
        println("Caught a class cast exception: ${e.message}")
    }
}

4. Examples of ClassCastException

Example 1: Incorrect Type Casting

This example demonstrates handling incorrect type casting.

fun main() {
    try {
        val obj: Any = "Kotlin"
        val num: Int = obj as Int
    } catch (e: ClassCastException) {
        println("Caught a class cast exception: ${e.message}")
    }
}

Output:

Caught a class cast exception: kotlin.String cannot be cast to java.lang.Integer

Explanation:
This example catches and handles a ClassCastException caused by an incorrect type cast.

Example 2: Safe Cast Operator

This example demonstrates using the safe cast operator as? to avoid ClassCastException.

fun main() {
    val obj: Any = "Kotlin"
    val num: Int? = obj as? Int
    println(num) // Output: null
}

Explanation:
This example uses the safe cast operator as? to perform a safe cast that returns null if the cast is not possible, avoiding ClassCastException.

Example 3: Checking Type Before Casting

This example demonstrates checking the type before casting to avoid ClassCastException.

fun main() {
    val obj: Any = "Kotlin"
    if (obj is Int) {
        val num: Int = obj
        println(num)
    } else {
        println("obj is not an Int")
    }
}

Output:

obj is not an Int

Explanation:
This example checks the type of the object before casting to ensure that the cast is valid.

5. Real-World Use Case: Safe Type Casting in Collections

In a real-world scenario, you might need to safely cast objects when working with collections of mixed types.

Example: Safe Type Casting in Collections

fun printNumbers(list: List<Any>) {
    for (item in list) {
        val num: Int? = item as? Int
        if (num != null) {
            println("Number: $num")
        } else {
            println("Not a number: $item")
        }
    }
}

fun main() {
    val mixedList = listOf(1, "two", 3, "four", 5)
    printNumbers(mixedList)
}

Output:

Number: 1
Not a number: two
Number: 3
Not a number: four
Number: 5

Explanation:
This example uses the safe cast operator as? to safely cast elements in a list to Int and prints a message if the cast is not possible.

Conclusion

ClassCastException in Kotlin is a runtime exception that occurs when an attempt is made to cast an object to a type with which it is not compatible. By understanding how to handle ClassCastException using try-catch blocks, safe cast operators, and type checks, you can write more robust and error-resistant code. Proper handling of type casts is crucial in real-world applications to ensure type safety and prevent runtime exceptions.