ClassCastException

Question about the task

Generics for creating a proxy object

Java Collections, Level 2, Lesson 8

Resolved

I can't seem to figure out a fix for the ClassCastException that my code throws.

Create a public getProxy() method in the Solution class:
1) The getProxy() method must return a proxy for any interface that inherits Item.
2) The getProxy() method must have two parameters. The first is the class of the return value, and the second is additional interface classes (use a variable-length argument ...).
3) Use ItemInvocationHandler to create a proxy.
The main() method is not tested.

Requirements:

The Solution class must have a getProxy() method.
The getProxy() method must have two parameters. The first is the class of the return value, and the second is additional interface classes.
The getProxy() method must return a proxy for any interface that inherits Item.
When calling Proxy.newProxyInstance() in the getProxy() method, pass this.getClass().GetClassLoader() as the first argument.
When calling Proxy.newProxyInstance() in the getProxy() method, pass as the second argument all of the interfaces that the proxy should implement.
When calling Proxy.newProxyInstance() in the getProxy() method, pass new ItemInvocationHandler() as the third argument.
The getProxy() method must function as outlined in the task conditions.

package com.codegym.task.task32.task3206; import java.lang.reflect.Proxy; import java.util.List; import java.util.ArrayList; import java.util.Arrays; /* Generics for creating a proxy object */ public class Solution { public static void main(String[] args) { Solution solution = new Solution(); test(solution.getProxy(Item.class)); // true false false test(solution.getProxy(Item.class, Small.class)); // true false true test(solution.getProxy(Item.class, Big.class, Small.class));// true true true test(solution.getProxy(Big.class, Small.class)); // true true true, because Big inherits Item test(solution.getProxy(Big.class)); // true true false, because Big inherits Item } private static void test(Object proxy) { boolean isItem = proxy instanceof Item; boolean isBig = proxy instanceof Big; boolean isSmall = proxy instanceof Small; System.out.format("%b %b %b\n", isItem, isBig, isSmall); } public <T extends Item> T getProxy(Class<T> first, Class<?>... second) { Class[] interfaces = first.getClass().getInterfaces(); List<Class> list = new ArrayList<>(); for (Class c: second) { list.add(c); } for (Class s: interfaces) { list.add(s); } Class[] allInterfaces = list.toArray(new Class[0]); return (T)(Proxy.newProxyInstance(this.getClass().getClassLoader(), allInterfaces, new ItemInvocationHandler())); } }

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Lisa L

Level 47 , Nuremberg, Germany

10 May 2022, 18:51

useful

A proxy class only has the interfaces available you specify. Eg. for the first and the last test case second is null and so your proxy class has no interface available. Means no method you can invoke on the proxy object. But the first test case should have the Item interface and the last test case the Big interface. Means you just need to create an array of Class objects of all types that can hold all the interfaces you passed to getProxy method and the type (the return value)

Jomar Tayactac

Level 5 , Winnipeg, Canada

Expert

11 May 2022, 15:44

Thanks for the help Lisa. I made changes but it won't pass the 3rd requirement (thus it won't pass the last requirement as well). My getProxy() method looks like below: public <T extends Item>T getProxy(Class<T> first, Class<?>... second) { List<Class> list = new ArrayList<>(); for (Class c: second) { list.add(c); } if (first.getName().equals("com.codegym.task.task32.task3206.Item")) { list.add(Item.class); } else if (first.getName().equals("com.codegym.task.task32.task3206.Big")) { list.add(Big.class); } Class[] allInterfaces = list.toArray(new Class[0]); ClassLoader classLoader = this.getClass().getClassLoader(); return (T) (Proxy.newProxyInstance(classLoader, allInterfaces, new ItemInvocationHandler())); }

Lisa L

Level 47 , Nuremberg, Germany

11 May 2022, 16:35

solution

You could try two things... get rid of all checks validating the base type and just add it to the interfaces list/ array no matter what (worked for me). Or... try with instanceof. That would really be able to add any interface inheriting Item. Currently you just add Item and Big and I'm sure the validator tests an additional interface not listed here in this task.

Guadalupe Gagnon

Level 37 , Tampa, United States

11 May 2022, 17:12

solution

I agree with Lisa. The validation system tests your solutions with multiple values that correspond with what is outlined in the conditions/requirements. Reading those it does not say that the getProxy() must only work with the Item and Big interfaces; it specifically states that "The getProxy() method must return a proxy for any interface that inherits Item.". The first parameter of the method is type T, which is anything that extends Item. You will need to ensure this gets added to the Class[] array or the proxy created will not be type of T. Take, for example, if you added another interface that extends Item and then used that in main() as the first argument of the get proxy method:

interface Test extends Item {}

then in main:

public static void main(String[] args) {
   Solution solution = new Solution();
   test(solution.getProxy(Test.class, Small.class));
}

This will result in a ClassCastException on the return line of the getProxy() method because the first argument is never added to the Class[] array in the code (the "if-else if" in your code prevents this) which then results in the object created by the Proxy.newProxyInstance() not being of that class. Therefore it isn't able to cast it to T and the Exception is thrown.

Guadalupe Gagnon

Level 37 , Tampa, United States

11 May 2022, 17:18

It's not an easy subject but hopefully my example helps understand what I am trying to explain.

Jomar Tayactac

Level 5 , Winnipeg, Canada

Expert

11 May 2022, 17:31

Got rid of the checks within my getProxy() method and replaced it with "list.add(first)" and it passed. Big thanks to both of you!!. 😃