Question: What is a ClassCastException?

A ClassCastException is an Exception that can occur in a Java program when you try to convert a class from one type to another. Here's an example Java program which throws a ClassCastException intentionally:

package com.devdaily.javasamples;

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

public class ClassCastExceptionExample
{

  public ClassCastExceptionExample()
  {
    List list = new ArrayList();
    list.add("one");
    
    // ... some time later
    
    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 com.devdaily.javasamples.ClassCastExceptionExample.(ClassCastExceptionExample.java:25)
   at com.devdaily.javasamples.ClassCastExceptionExample.main(ClassCastExceptionExample.java:31)

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.