Finalize

import java.util.HashSet;
class Demonstration {
  
    static HashSet<UsefulObjectClass> immortals = new HashSet<>();  
  
    public static void main( String args[] ) throws InterruptedException {
        UsefulObjectClass obj = new UsefulObjectClass(immortals);
        obj.printName();
        obj = null;
        Runtime.getRuntime().gc();
        // Sleep the main thread so that the garbage collector thread performs finalization
        Thread.sleep(1000);
        if (immortals.size() == 1) {
            System.out.println("Useful object saved from garbage collection.");
        }
        System.out.println("exiting");      
    }
}
class UsefulObjectClass {
    HashSet<UsefulObjectClass> immortals;
    public UsefulObjectClass(HashSet<UsefulObjectClass> immortals) {
        this.immortals = immortals;
    }
    @Override
    public void finalize() {
        System.out.println("I am being finalized.");
        immortals.add(this);
    }
    public void printName() {
        System.out.println("Hi, I am a useful object.");
    }
}