Garbage collection basics

Key points

• You never free memory yourself. The collector finds objects that no running code can reach and reclaims the space.
• Unreachable is the only rule. Assigning null helps only if that was genuinely the last way in.
• System.gc() is a suggestion, not a command. Production code should almost never call it.
• A WeakReference does not keep its target alive, which makes it the honest way to watch a collection happen.
• Collection is not free. It costs CPU, and most collectors still stop your threads briefly while they work.

Example

import java.lang.ref.WeakReference;

public class Main {
    public static void main(String[] args) throws InterruptedException {
        Object strong = new Object();
        WeakReference<Object> watch = new WeakReference<>(strong);

        System.out.println("while a variable points at it -> alive? " + (watch.get() != null));

        strong = null;                              // the last strong reference is gone
        for (int i = 0; i < 5 && watch.get() != null; i++) {
            System.gc();                            // a hint, not a command
            Thread.sleep(50);
        }

        System.out.println("after the reference is dropped -> alive? " + (watch.get() != null));
        System.out.println(watch.get() == null
                ? "collected: nothing in the program could reach it any more"
                : "not collected yet - System.gc() really is only a hint");
    }
}

Reachable means alive. Everything else is fair game.