Explain about WeakHashMap

WeakHashMap is an implementation of Map interface, where Key is a weak reference to an object. If an object has only weak references associated with it, then Garbage collector permitted to reclaim the memory used by the object.

If you would like to know more about weak reference, I would recommend you to go through my below post.

https://self-learning-java-tutorial.blogspot.com/2014/07/weak-reference-in-java.html

App.java

package com.sample.app;

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

public class App {

	public static class Employee {

	}

	public static class EmployeeInfo {

	}

	public static void main(String args[]) throws InterruptedException {
		Map<Employee, EmployeeInfo> map = new WeakHashMap<>();

		Employee emp = new Employee();

		map.put(emp, new EmployeeInfo());

		EmployeeInfo empInfo = map.get(emp);

		if (map.containsValue(empInfo)) {
			System.out.println("Map contains Employee information");
		}

		emp = null;
		System.gc();

		TimeUnit.SECONDS.sleep(5);

		if (map.containsValue(empInfo)) {
			System.out.println("Map contains Employee information");
		} else {
			System.out.println("Map do not contain Employee information");
		}

	}
}

Output

Map contains Employee information

Map do not contain Employee information

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