Unchecked.java

import java.util.*;
class Unchecked {
    static class Foo<A> {
        Optional<A> doIt(Object o) { return Optional.of((A) o);} 
    }

    public static void main(String[] args) {
        System.out.println(new Foo<String>().doIt(3));
    }
}
/* compile it */

//javac Unchecked.java
//Note: Unchecked.java uses unchecked or unsafe operations.
//Note: Recompile with -Xlint:unchecked for details.

//javac -Xlint:unchecked Unchecked.java
//Unchecked.java:4: warning: [unchecked] unchecked cast
//        Optional<A> doIt(Object o) { return Optional.of((A) o);}
//                                                            ^
//  required: A
//  found:    Object
//  where A is a type-variable:
//    A extends Object declared in class Foo
//1 warning

/* Run it */
//java Unchecked
//Optional[3]