How will you go about creating a memory leak in Java | Eklavya Online

How will you go about creating a memory leak in Java

In Java, memory leaks are possible under a number of scenarios. Here is a typical example where hashCode( ) and equals( ) methods are not implemented for the Key class that is used to store key/value pairs in a HashMap.

This will end up creating a large number of duplicate objects. All memory leaks in Java end up with java.lang.OutOfMemoryError, and it is a matter of time. The following code agressively creates the OutOfMemoryError via an endless loop for demonstration purpose.

If you are not familiar with the significance of equals( ) and hashCode ( ) methods in Java, then learn with this example how to define proper key class in Java.

import java.util.HashMap;

import java.util.Map;


public class MemoryLeak {


public static void main(String[] args) {

Map<Key, String> map = new HashMap<Key, String>(1000);


int counter = 0;

while (true) {       // creates duplicate objects due to bad Key class

map.put(new Key(“dummyKey”), “value”);


if (counter % 1000 == 0) {

System.out.println(“map size: ” + map.size());

System.out.println(“Free memory after count ” + counter + ” is ” + getFreeMemory() + “MB”);








// inner class key without hashcode() or equals() — bad implementation


static class Key {

private String key;


public Key(String key) {

this.key = key;





//delay for a given period in milli seconds


public static void sleep(long sleepFor) {

try {


} catch (InterruptedException e) {





//get available memory in MB

public static long getFreeMemory() {

return Runtime.getRuntime().freeMemory() / (1024 * 1024);





If you run the above code, you will get the ouput as shown below

map size: 1000

Free memory after count 1000 is 4MB

map size: 2000

Free memory after count 2000 is 4MB

map size: 1396000

Free memory after count 1396000 is 2MB

map size: 1397000

Free memory after count 1397000 is 2MB

map size: 1398000

Free memory after count 1398000 is 2MB

map size: 1399000

Free memory after count 1399000 is 1MB

map size: 1400000

Free memory after count 1400000 is 1MB

map size: 1401000

Free memory after count 1401000 is 1MB



map size: 1452000

Free memory after count 1452000 is 0MB

map size: 1453000

Free memory after count 1453000 is 0MB

Exception in thread “main” java.lang.OutOfMemoryError: Java heap space

 at java.util.HashMap.addEntry(

 at java.util.HashMap.put(

 at MemoryLeak.main(

As you could see, the size of the map keeps growing with the same objects and the available memory keeps coming down from 4MB to 0MB. At the end, the program dies with an OutOfMemoryError.