Challenge 1: Making an Object Unreachable — Possible Solution
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ReachabilityDemo.java:

public class ReachabilityDemo {
    public static void main(String[] args) {
        Object obj = new Object();   // obj is a GC root (a local variable on the stack)
                                      // reachable: the object is pointed to by obj

        obj = null;

        // At this point, the original Object created by `new Object()`
        // has no live reference pointing to it anywhere -- `obj` itself
        // now points to null instead. Since nothing reachable from any
        // GC root (this stack frame's local variables, active static
        // fields, etc.) leads to that original object anymore, it is
        // unreachable, and therefore eligible for garbage collection
        // the next time the JVM decides to collect. There was no
        // explicit free() call -- reassigning the only reference was
        // enough to make the object collectible.
    }
}

WHY THIS WORKS AS AN ANSWER
------------------------------
This demonstrates the exact reachability transition the chapter
describes -- reassigning an object's only reference to null -- and the
comment correctly uses "reachability" and "GC root" to explain why
that alone is sufficient to make the object eligible for collection,
with no manual deallocation step required.