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

User · Answer

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.

User · Answer

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

User · Answer

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

User · Answer

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          You can cast any of these things to Object  because all Java classes inherit from Object  You can cast either B or C to A  because they re both  kinds of  A You can cast a reference to an A object to B only if the real object is a B  You can t cast a B to a C even though they re both A s

User · Answer

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 -  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 its a dog  Cat c    Dog  a     Raises class cast exception  you can t cast a dog to a cat

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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

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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

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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    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  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

User · Answer

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

User · Answer

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

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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

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Exception is not a subclass of RuntimeException - gt   ClassCastException     final Object  exception   new Exception        final Exception data    RuntimeException exception       System out println data

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