Java - WeakReference and WeakHashMap examples

WeakHashmapExample.java

package net.seibertmedia;

import java.util.Map;
import java.util.WeakHashMap;
import java.util.concurrent.TimeUnit;

public class WeakHashmapExample {
    public static void main(String[] args) throws InterruptedException {
        Map<Person, String> map = new WeakHashMap<Person, String>();
        Person p1 = new Person("Jeff1");
        map.put(p1, "String1");
        Person p2 = new Person("Jeff2");
        map.put(p2, "String2");
        Person p3 = new Person("Jeff3");
        map.put(p3, "String3");
        Person p4 = new Person("Jeff4");
        map.put(p4, "String4");
        System.out.println("map has " + map.size() + " values");
        p2 = null;
        System.gc();
        TimeUnit.SECONDS.sleep(6);
        System.out.println("map has " + map.size() + " values");
        p3 = null;
        System.gc();
        TimeUnit.SECONDS.sleep(6);
        System.out.println("map has " + map.size() + " values");
    }
    static class Person {
        private final String    name;
        public Person(final String name) {
            this.name = name;
        }

        @Override
        public int hashCode() {
            final int prime = 31;
            int result = 1;
            result = prime * result + ((name == null) ? 0 : name.hashCode());
            return result;
        }

        @Override
        public boolean equals(Object obj) {
            if (this == obj)
                return true;
            if (obj == null)
                return false;
            if (getClass() != obj.getClass())
                return false;
            Person other = (Person) obj;
            if (name == null) {
                if (other.name != null)
                    return false;
            } else if (!name.equals(other.name))
                return false;
            return true;
        }
    }
}

WeakReferenceExample.java

package net.seibertmedia;

import java.lang.ref.WeakReference;

public class WeakReferenceExample {
  static class Hard {

  }

  public static void main(String[] args) throws InterruptedException {
    Hard hard = new Hard();

    WeakReference<Hard> ref = new WeakReference<>(hard);
    Runtime.getRuntime().gc();
    Thread.sleep(3000);
    System.out.printf("weak ref is present? %s%n", ref.get() != null);
    hard = null;
    Runtime.getRuntime().gc();
    Thread.sleep(3000);
    System.out.printf("weak ref is present? %s%n", ref.get() != null);

  }
}