Java 中 ClassCastException 异常的解释

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.

It's really pretty simple: if you are trying to typecast an object of class A into an object of class B, and they aren't compatible, you get a class cast exception.

Let's think of a collection of classes.

class A {...}
class B extends A {...}
class C extends A {...}

1. You can cast any of these things to Object, because all Java classes inherit from Object.
2. You can cast either B or C to A, because they're both "kinds of" A
3. You can cast a reference to an A object to B only if the real object is a B.
4. You can't cast a B to a C even though they're both A's.

Do you understand the concept of casting? Casting is the process of type conversion, which is in Java very common because its a statically typed language. Some examples:

Cast the String "1" to an int, via Integer.parseInt("1") -> no problem

Cast the String "abc" to an int -> raises a ClassCastException

Or think of a class diagram with Animal.class, Dog.class and Cat.class

Animal a = new Dog();
Dog d = (Dog) a; // No problem, the type animal can be casted to a dog, because it's a dog.
Cat c = (Dog) a; // Will cause a compiler error for type mismatch; you can't cast a dog to a cat.

You are trying to treat an object as an instance of a class that it is not. It's roughly analogous to trying to press the damper pedal on a guitar (pianos have damper pedals, guitars don't).

It is an Exception which occurs if you attempt to downcast a class, but in fact the class is not of that type.

Consider this heirarchy:

Object -> Animal -> Dog

You might have a method called:

 public void manipulate(Object o) {
Dog d = (Dog) o;
}

If called with this code:

 Animal a = new Animal();
manipulate(a);

It will compile just fine, but at runtime you will get a ClassCastException because o was in fact an Animal, not a Dog.

In later versions of Java you do get a compiler warning unless you do:

 Dog d;
if(o instanceof Dog) {
d = (Dog) o;
} else {
//what you need to do if not
}

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.

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.

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

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.

A Java ClassCastException is an Exception that can occur when you try to improperly convert a class from one type to another.

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


public class ClassCastExceptionExample {


public ClassCastExceptionExample() {


List list = new ArrayList();
list.add("one");
list.add("two");
Iterator it = list.iterator();
while (it.hasNext()) {
// intentionally throw a ClassCastException by trying to cast a String to an
// Integer (technically this is casting an Object to an Integer, where the Object
// is really a reference to a String:
Integer i = (Integer)it.next();
}
}
public static void main(String[] args) {
new ClassCastExceptionExample();
}
}

If you try to run this Java program you’ll see that it will throw the following ClassCastException:

Exception in thread "main" java.lang.ClassCastException: java.lang.String
at ClassCastExceptionExample  (ClassCastExceptionExample.java:15)
at ClassCastExceptionExample.main  (ClassCastExceptionExample.java:19)

The reason an exception is thrown here is that when I’m creating my list object, the object I store in the list is the String “one,” but then later when I try to get this object out I intentionally make a mistake by trying to cast it to an Integer. Because a String cannot be directly cast to an Integer — an Integer is not a type of String — a ClassCastException is thrown.

If you want to sort objects but if class didn't implement Comparable or Comparator, then you will get ClassCastException For example

class Animal{
int age;
String type;


public Animal(int age, String type){
this.age = age;
this.type = type;
}
}
public class MainCls{
public static void main(String[] args){
Animal[] arr = {new Animal(2, "Her"), new Animal(3,"Car")};
Arrays.sort(arr);
}
}

Above main method will throw below runtime class cast exception

Exception in thread "main" java.lang.ClassCastException: com.default.Animal cannot be cast to java.lang.Comparable

Exception is not a subclass of RuntimeException -> ClassCastException

    final Object  exception = new Exception();
final Exception data = (RuntimeException)exception ;
System.out.println(data);