Java example showing how a proxied methods return type may cause runtime class cast errors

ExampleProxyMethodReturnTypeMismatch.java

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

class DynamicInvocationHandler implements InvocationHandler {

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        return 42;
    }
}

interface Thing {
    public String getIt();
}

class ExampleProxyMethodReturnTypeMismatch {


    public static void main(String[] args) {

        Thing t =
                (Thing) Proxy.newProxyInstance(
                        ExampleProxyMethodReturnTypeMismatch.class.getClassLoader(),
                        new Class[]{Thing.class},
                        new DynamicInvocationHandler());

        System.out.println(t.getIt());
    }
}

output.log

/Library/Java/JavaVirtualMachines/temurin-17.jdk/Contents/Home/bin/java ....
Exception in thread "main" java.lang.ClassCastException: class java.lang.Integer cannot be cast to class java.lang.String (java.lang.Integer and java.lang.String are in module java.base of loader 'bootstrap')
	at $Proxy0.getIt(Unknown Source)
	at ExampleProxyMethodReturnTypeMismatch.main(scratch_9.java:28)

Process finished with exit code 1