The Dog POJO representing a Dog. The class has a faulty logic in the equals method which can lead to memory leaks if the object is used as key in Sets/Maps.

Dog.java

package com.cleantutorials.jconsole.memory;

/**
 * Dog represents a dog which can be uniquely identified by the MicroChip ID and
 * can optionally have a name.
 */
public class Dog {

	/** The Unique MicroChip ID of the dog. */
	private int microChipID;
	/** The name of the dog. */
	private String name;
	/** Extra memory space for each instance to speed up the memory leak. */
	private byte[] toExpediteLeak;

	Dog(int microChipID) {
		this.microChipID = microChipID;
	}

	Dog(int microChipID, String name) {
		/**
		 * On Creating a Dog object we allocate a 10Mb storage with each instance to
		 * expedite the memory leak.
		 */
		this.toExpediteLeak = new byte[10000000];
		this.microChipID = microChipID;
		this.name = name;
	}

	@Override
	public boolean equals(Object obj) {
		if (obj == this)
			return true;
		if (!(obj instanceof Dog))
			return false;

		Dog dog = (Dog) obj;
		return dog.microChipID == microChipID && dog.name.equals(name);
	}

	@Override
	public int hashCode() {
		return microChipID;
	}

	/**
	 * @return MicroChip ID of the dog.
	 */

	public int getMicroChipID() {
		return microChipID;
	}

	/**
	 * @return name of the dog.
	 */
	public String getName() {
		return name;
	}

	/**
	 * @param name of the dog.
	 */
	public void setName(String name) {
		this.name = name;
	}
}

DogShelter.java

package com.cleantutorials.jconsole.memory;

import java.util.HashSet;
import java.util.Random;
import java.util.Set;

/**
 * DogShelter class represents a Dog Shelter. Each instance maintains the list
 * of dogs that are currently present in the shelter. This class is designed to
 * demonstrate memory leak in Java due to design flaws and human errors.
 */
public class DogShelter {

	/** In Memory Store containing the dogs present in the shelter. */
	private Set<Dog> shelterDogs = new HashSet<Dog>();

	public static void main(String[] args) {
		DogShelter shelter = new DogShelter();
		shelter.addAndRemoveRandomEntries(1000);
	}

	/**
	 * Adds the <code>Dog</code> object entry to the HashSet.
	 * 
	 * @param microChipID An ID used to uniquely identify a dog.
	 * @param name        Name of the dog.
	 */
	public void addEntry(int microChipID, String name) {
		Dog dog = new Dog(microChipID, name);
		shelterDogs.add(dog);
	}

	/**
	 * Removes the <code>Dog</code> object from the HashSet
	 * 
	 * @param microChipID An ID used to uniquely identify a dog.
	 */
	public void removeEntry(int microChipID) {
		Dog dog = new Dog(microChipID);
		shelterDogs.remove(dog);
	}

	/**
	 * Generates random Dog Objects and add and remove the dog object serially.
	 * 
	 * @param entriesCount Number of random entries to be added and removed from
	 *                     HashSet
	 */
	public void addAndRemoveRandomEntries(int entriesCount) {
		Random rand = new Random();
		String[] commonDogNames = { "Buddy", "Coco", "Charlie", "Cooper", "Maggie" };

		for (int i = 0; i <= entriesCount; i++) {
			/** Generate a random dog name from the list of common dog names. */
			String randomDogName = commonDogNames[rand.nextInt(commonDogNames.length)];
			/** First add and then remove the entry from the HashSet. */
			addEntry(i, randomDogName);
			removeEntry(i);
			System.out.printf("Successfuly removed entry for %s with unique id %d.\n", randomDogName, i);
			try {
				/**
				 * Sleep before adding & removing new entry so that we can see the memory grow
				 * in JConsole.
				 */
				Thread.sleep(500);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}
}